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SynC · Editorial revision

Generator Paralleling Switchgear

Revision1
EditedOct 17, 2026
StatusCurrent
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Current revision. This is editorial revision 1, the current text of this standard. Read it on the standard's page.

SynC Standards Team — Specifier (Platform Standards) ✓ OfficialDiscloses employment SynC (SynC Platform Team)
Initial publication

1 Scope

NOTE This standard governs paralleling switchgear rated 1000 V and below as one listed assembly: the generator breaker sections, the master and per-set control sections, the bus tie section where one is furnished, the distribution sections that feed the transfer switches or the emergency distribution, the bus that joins them, the controls that synchronize the sets and share the load among them, the protective functions that guard each set and the bus, and the field work that puts the assembly into service. (1.1)
NOTE The scheme is decided elsewhere and implemented here. Emergency And Standby PowerEmergency and Standby Power SystemsResolves to the current adopted revision.sync/emergency-and-standby-power decides the number of sets, the source redundancy, the paralleling scheme, whether the sets parallel with the utility, and the NFPA 110 level, type, and class of the system; the one-line diagram shows the scheme so decided, and this standard specifies the equipment that implements it. (1.2)
NOTE The construction basis is a decision made once in this standard. The assembly is built either as a switchboard listed to UL 891 or as metal-enclosed low-voltage power circuit breaker switchgear listed to UL 1558, and Low Voltage SwitchboardsLow Voltage SwitchboardsResolves to the current adopted revision.sync/low-voltage-switchboards or Low Voltage SwitchgearLow Voltage SwitchgearResolves to the current adopted revision.sync/low-voltage-switchgear governs the structure, the bus construction, the barriers, the breakers, and their trip units for the basis selected. This standard states only what paralleling adds: electrically operated breakers with a defined operating and tripping duty, the breaker control switches and position indication, the synchronizing and its permissive, the control and protection sections, and the sequences that run them. (1.3)
NOTE The following are governed elsewhere and are outside this standard: (1.4)
  • the classification of loads, the number of sets, the source redundancy, the paralleling scheme, the transfer scheme, and the NFPA 110 level, type, and class (Emergency And Standby PowerEmergency and Standby Power SystemsResolves to the current adopted revision.sync/emergency-and-standby-power)
  • the engine-generator set, its controller, its governor, its voltage regulator, its starting batteries, and its remote annunciator (GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators)
  • the transfer switches on the load side of the switchgear, their time delays, and their neutral treatment (Automatic Transfer SwitchesAutomatic Transfer SwitchesResolves to the current adopted revision.sync/automatic-transfer-switches)
  • the branch assignment and topology of a health care essential electrical system (Healthcare Essential Electrical SystemsHealthcare Essential Electrical SystemsResolves to the current adopted revision.sync/healthcare-essential-electrical-systems)
  • the structure, the bus construction and plating, the barriers, the feeder devices, and the trip units of the construction basis (Low Voltage SwitchboardsLow Voltage SwitchboardsResolves to the current adopted revision.sync/low-voltage-switchboards, Low Voltage SwitchgearLow Voltage SwitchgearResolves to the current adopted revision.sync/low-voltage-switchgear)
  • the paralleling and load sharing of UPS modules (Uninterruptible Power Supply SystemsUninterruptible Power Supply SystemsResolves to the current adopted revision.sync/uninterruptible-power-supply-systems)
  • the station battery, its charger, and its DC distribution as products (DC Battery SystemsDC Battery SystemsResolves to the current adopted revision.sync/dc-battery-systems)
  • the feeders from the sets and to the transfer switches, and their raceways or busway (Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables, Raceways And ConduitRaceways and ConduitResolves to the current adopted revision.sync/raceways-and-conduit, BuswayBuswayResolves to the current adopted revision.sync/busway)
  • the control and signal cable between the sets and the switchgear (Instrumentation And Control CableInstrumentation and Control CableResolves to the current adopted revision.sync/instrumentation-and-control-cable)
  • the grounding electrode system and the bonding of a separately derived system (Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current adopted revision.sync/grounding-and-bonding)
  • the meters as products and the monitoring system they report to (Electrical Power MonitoringElectrical Power Monitoring and MeteringResolves to the current adopted revision.sync/electrical-power-monitoring)
  • the short-circuit, coordination, and arc-flash studies that establish the ratings and settings (Short Circuit StudyShort-Circuit StudyResolves to the current adopted revision.sync/short-circuit-study, Protective Coordination StudyProtective Device Coordination StudyResolves to the current adopted revision.sync/protective-coordination-study, Arc Flash StudyArc-Flash Hazard AnalysisResolves to the current adopted revision.sync/arc-flash-study)
  • the room the switchgear occupies (Electrical RoomsElectrical RoomsResolves to the current adopted revision.sync/electrical-rooms) and the pad it stands on (Concrete PadsConcrete Equipment PadsResolves to the current adopted revision.sync/concrete-pads)
  • the test program under which the field tests named here are performed (Electrical Acceptance TestingElectrical Acceptance TestingResolves to the current adopted revision.sync/electrical-acceptance-testing)
  • paralleling switchgear and alternators rated above 1000 V, which no SynC standard yet owns
  • economic-dispatch and microgrid generation controllers that run the sets for energy management rather than to restore a backup function, which no SynC standard yet owns
1.5 The location of the switchgear shall be as indicated on the electrical plans.
1.6 The number of sets, the bus arrangement, the tie, the utility connection, and the distribution devices shall be as indicated on the one-line diagram.
1.7 The Contractor shall coordinate the ratings, the set-side interface, the transfer switch interface, the control power source, and the utility interconnection of the switchgear with the standards named in this section before the switchgear is released for manufacture.

2 Referenced Standards

2.1 Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
2.2 Where the contract documents, the serving utility's requirements, the Authority Having Jurisdiction, or two referenced standards impose conflicting requirements, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
Standard Title
UL 891 Switchboards
UL 1558 Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear
UL 489 Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures
UL 1066 Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures
UL 508A Industrial Control Panels
UL 1741 Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources
UL 50 Enclosures for Electrical Equipment, Non-Environmental Considerations
UL 50E Enclosures for Electrical Equipment, Environmental Considerations
NEMA PB 2 Deadfront Distribution Switchboards
NEMA 250 Enclosures for Electrical Equipment (1000 Volts Maximum)
IEEE C37.13 Low-Voltage AC Power Circuit Breakers Used in Enclosures
IEEE C37.20.1 Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear
IEEE C37.20.7 Guide for Testing Switchgear Rated Up to 52 kV for Internal Arcing Faults
IEEE C37.2 Standard Electrical Power System Device Function Numbers, Acronyms, and Contact Designations
IEEE C37.90 Standard for Relays and Relay Systems Associated with Electric Power Apparatus
IEEE C37.90.1 Standard for Surge Withstand Capability Tests for Relays and Relay Systems
IEEE C37.102 Guide for AC Generator Protection
IEEE C57.13 Requirements for Instrument Transformers
IEEE 1547 Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces
IEEE 1547.1 Standard Conformance Test Procedures for Equipment Interconnecting Distributed Energy Resources with Electric Power Systems
IEEE 1584 Guide for Performing Arc-Flash Hazard Calculations
NFPA 70 National Electrical Code (Articles 110, 240, 250, 445, 517, 700, 701, 702, 705, and 708)
NFPA 70E Standard for Electrical Safety in the Workplace
NFPA 99 Health Care Facilities Code
NFPA 110 Standard for Emergency and Standby Power Systems
ANSI/NETA ATS Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems
ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures (Chapter 13)
ICC-ES AC156 Acceptance Criteria for Seismic Certification by Shake-Table Testing of Nonstructural Components

3 Submittals

3.1 Action Submittals

3.1.1 The Contractor shall submit the following for review before the switchgear is released for manufacture:
  • shop drawings giving the front and side elevations, the section widths, depths, and heights, the shipping splits, the weight and lifting provisions of each shipping section, the conduit entry areas, and the anchorage pattern
  • an assembly one-line diagram showing every set connection, the generator breakers, the tie, the utility connection where one is furnished, the distribution devices, the bus with its continuous and short-circuit ratings, the neutral and ground bus, and the instrument transformers
  • three-line and control schematic diagrams showing every protective function, the synchronizing and sync-check circuits, the dead-bus arbitration, the breaker close and trip circuits, the lockout relays, the load-shed outputs, and the control power distribution
  • a written sequence of operation covering automatic operation, manual operation, test operation, each utility paralleling mode selected, load add and load shed, load-demand operation where furnished, controller failover, and the maintenance bypass where furnished
  • a description of the control architecture naming each controller, its function, its redundancy, and the behavior of the system on the loss of each controller and of the network between them
  • a load priority table assigning every transfer switch or distribution breaker to a priority level, with the load at each level
  • a device schedule giving the frame, the trip rating, the trip unit, the interrupting rating, the short-time rating, and the accessories of every breaker
  • a protective function list stating each function furnished for each set and for the bus, the device it acts on, and the proposed initial setting of every element and every synchronizing window, on a settings record the Engineer of Record will review
  • the bus material, plating, and sizing basis, with the bus bracing rating
  • the assembly short-circuit current rating and the basis on which it is established, including the fault contribution of every set paralleled and of the utility where paralleled
  • the operator interface screen layouts and the alarm list
  • the heat release of each section at rated load, for coordination of the room ventilation
  • the seismic certification for the switchgear as furnished, where the datasheet requires one
  • the utility interconnection package, including the utility-interactive relay data and the IEEE 1547.1 certification, where the datasheet selects a closed-transition or extended parallel mode
  • the nameplate schedule listing the text to appear on each assembly, section, and device nameplate
Action Submittalscheckbox
☑ Shop drawings
☑ Assembly one-line diagram
☑ Three-line and control schematic diagrams
☑ Written sequence of operation
☑ Control architecture description
☑ Load priority table
☑ Device schedule
☑ Protective function list and proposed settings record
☑ Bus data and bracing rating
☑ Short-circuit current rating basis
☑ Operator interface screen layouts and alarm list
☑ Heat release per section
☐ Seismic certification
☐ Utility interconnection package
☑ Nameplate schedule
3.1.2 The switchgear shall not be released for manufacture until the action submittals covering it have been reviewed and returned.
NOTE The sequence of operation is the submittal that most often comes back wrong, because the manufacturer's standard sequence describes a generic lineup, and the sequence that applies on the project is the one that names this project's priority levels, this project's transfer switches, and the utility mode this project selected; a sequence that is reviewed as a generic document is discovered to be wrong at the integrated test. (3.1.3)

3.2 Informational Submittals

3.2.1 The Contractor shall submit the following with or before the action submittals:
  • the manufacturer's qualification statement required under Quality Assurance
  • the qualifications of the firm and the technicians who will perform field acceptance testing
  • the generator manufacturer's written confirmation that the governor and the voltage regulator of every set accept the load-sharing and synchronizing signals the switchgear controls furnish
  • the factory integrated test procedure, where the datasheet selects witnessed testing
  • the serving utility's written approval of parallel operation and the executed interconnection agreement, where the datasheet selects a closed-transition or extended parallel mode
  • the time-current characteristic curves and the setting ranges of every trip unit and protective function furnished, for use in the study performed under Protective Coordination StudyProtective Device Coordination StudyResolves to the current adopted revision.sync/protective-coordination-study
  • the manufacturer's published derating factors applied for ambient temperature and altitude, where either exceeds the rating basis of the listing
Informational Submittalscheckbox
☑ Manufacturer's qualification statement
☑ Field testing firm and technician qualifications
☑ Generator manufacturer's interface confirmation
☐ Factory integrated test procedure
☐ Utility approval of parallel operation and interconnection agreement
☑ Trip unit and protective function characteristic data for the coordination study
☐ Ambient and altitude derating factors

3.3 Closeout Submittals

3.3.1 The Contractor shall submit the following before the switchgear is accepted:
  • record drawings reflecting every field change to the device arrangement, the wiring, the priority table, and the network addressing
  • the certified factory test report, including the integrated test with the sets
  • the field acceptance test report for the switchgear, giving every measured value, the acceptance criterion applied, and the tester's determination
  • the installation acceptance test record required by NFPA 110 Chapter 7, where the switchgear serves a system classified under NFPA 110
  • the settings record as commissioned, giving the final value of every protective element, every synchronizing window, every load-shed threshold and delay, every time delay, and every utility-interface setting
  • a backup of the program and configuration of every controller and relay, in the native file format and in a printed listing, together with the software and the cables needed to restore it
  • the network address list and the point list for every networked device, where a network is furnished
  • operation and maintenance data covering the structure, the bus, each breaker, each controller and relay, the manual synchronizing procedure, the maintenance bypass procedure where furnished, and the inspection and re-torque intervals
  • the warranty certificate identifying the switchgear by designation and the warranty commencement date
  • a signed receipt from the Owner for the spare parts and keys delivered
  • a record of the operator training delivered, signed by the attendees
Closeout Submittalscheckbox
☑ Record drawings
☑ Certified factory test report including the integrated test
☑ Field acceptance test report
☐ NFPA 110 installation acceptance test record
☑ Settings record as commissioned
☑ Controller and relay program backup with restore tools
☐ Network address and point list
☑ Operation and maintenance data
☑ Warranty certificate
☑ Spare parts and keys receipt
☑ Operator training record
NOTE A paralleling system whose program exists only in its own controllers loses it with the first controller replacement, and the replacement arrives from the factory with a generic program; the backup and the tools to restore it are what let a successor technician put the system back the way it was commissioned. (3.3.2)

4 Quality Assurance

4.1 Manufacturer Qualifications

4.1.1 The switchgear shall be produced by a manufacturer that has furnished paralleling switchgear of the construction basis and the control architecture selected for not less than the period indicated in the datasheet.
Manufacturer Production Experiencerange
years
351020
Default: Not less than 5 years
4.1.2 The manufacturer shall maintain a factory-trained field service organization able to reach the site, with technicians trained on the controller family furnished, and shall state in the qualification statement the location of the nearest service center, the parts stocked there, and the period, measured from the date of manufacture, for which replacement controllers and breakers for the platform will remain available.

4.2 Source Responsibility

4.2.1 The source arrangement for the sets and the switchgear shall be as indicated in the datasheet.
Source Arrangementradio
○ Generator manufacturer furnishes the sets and the paralleling switchgear
○ Switchgear manufacturer furnishes the paralleling switchgear and integrates sets of another make
4.2.2 One party shall be responsible for the operation of the sets and the switchgear as one system, including the matching of the governor and voltage regulator signals to the controls, the synchronizing, the load sharing, and the integrated test, and that party shall be named in the action submittals.
NOTE A generator manufacturer that furnishes both the sets and the switchgear controls every signal between them from its own drawings and tests the two together as a matter of course; a switchgear manufacturer that integrates sets of another make draws on a wider choice of sets and prices and carries the matching of governor and regulator interfaces it did not design, which is where the interface confirmation under Informational Submittals earns its place. In either arrangement the Owner has one party to call when the sets will not share load, which is what the responsibility clause secures. (4.2.3)

4.3 Listing of the Assembly

4.3.1 The switchgear shall be listed and labeled as a complete assembly by a Nationally Recognized Testing Laboratory to UL 891 where switchboard construction is selected and to UL 1558 where metal-enclosed switchgear construction is selected, including the generator breaker sections, the control sections, the tie and distribution sections, and the bus.
4.3.2 The control sections shall be covered by the assembly listing, or shall be listed to UL 508A as industrial control panels installed within the assembly under the assembly listing.
4.3.3 Protective relays and controllers performing protective functions shall comply with IEEE C37.90 and shall have passed the surge withstand tests of IEEE C37.90.1.
4.3.4 A device from another maker shall be installed in the switchgear only where the switchgear manufacturer has evaluated it for the assembly under the assembly listing, and the assembly short-circuit current rating with that device installed shall be stated in the action submittals.

4.4 Field Testing Firm

4.4.1 Field acceptance testing shall be performed by the firm indicated in the datasheet.
Field Acceptance Testing Firmradio
○ Independent testing firm accredited to ANSI/NETA ATS
○ Manufacturer's field service organization
○ Installing contractor's own technicians
4.4.2 The field testing firm shall be qualified in accordance with Electrical Acceptance TestingElectrical Acceptance TestingResolves to the current adopted revision.sync/electrical-acceptance-testing, and the technician who performs the synchronizing, load-sharing, and load-shed tests shall have documented experience commissioning paralleling systems of the controller family furnished.
NOTE An independent firm has no stake in whether the switchgear passes and reports against a published acceptance standard; the manufacturer's organization knows the controls best and is already on site for start-up; the installing contractor's own technicians cost least and are available on the installer's schedule. Which of these the Owner wants depends on how the results will be used and on whether a report from the party who built or installed the equipment would satisfy the Owner's insurer and the Authority Having Jurisdiction. (4.4.3)

5 Environmental and Service Conditions

5.1 Ambient Temperature

5.1.1 The switchgear shall carry its nameplate ratings continuously at the design ambient temperature of the space it is installed in.
5.1.2 The design ambient temperature for switchgear installed indoors shall be as indicated in the datasheet.
Indoor Design Ambient Temperaturerange
°C
2530405055
Default: Not less than 40 °C
NOTE UL 891, UL 1558, and IEEE C37.20.1 establish the assembly ratings at a 40 °C ambient, so a 40 °C design ambient is the condition at which every rating applies without correction; a higher value covers a generator room or an electrical room whose ventilation cannot hold 40 °C with the sets at full load, at the cost of a larger bus or a larger breaker frame for the same load. (5.1.3)
5.1.4 Switchgear installed outdoors or in a space that follows the outdoor temperature shall be rated for Site Ambient Temperature MaximumSite Ambient Temperature MaximumParameterEach project supplies its own value.site-ambient-temperature-maximum where that temperature exceeds 40 °C, and the manufacturer shall state the derating applied in the action submittals.

5.2 Altitude

5.2.1 The switchgear and its devices shall be derated for AltitudeAltitudeParameterEach project supplies its own value.altitude in accordance with the manufacturer's published correction factors where the altitude exceeds the rating basis of the listing.
NOTE Air thins with altitude, which reduces both the convective cooling that sets the continuous current rating of the bus and the breakers and the dielectric strength of the clearances inside the structure, so switchgear rated at sea level carries less current and less insulation margin at elevation than its nameplate states. (5.2.2)
5.2.3 Ambient temperature derating and altitude derating shall be applied cumulatively where both conditions apply.

5.3 Condensation Control

5.3.1 Where the switchgear is installed outdoors or in a space that is not conditioned, a thermostatically controlled space heater shall be furnished in each vertical section.
5.3.2 Where space heaters are furnished, they shall be sized to hold the interior of each section above the dew point at Site Ambient Temperature MinimumSite Ambient Temperature MinimumParameterEach project supplies its own value.site-ambient-temperature-minimum, and shall be supplied from a circuit that stays energized while the paralleling bus is dead.
NOTE Paralleling switchgear stands de-energized for weeks between exercises, and the condensation that forms on the bus, the breaker contacts, and the controller terminals through that idle period is the mechanism behind most tracking failures and most controller faults on the first start after a long idle; a heater supplied only from the paralleling bus is off exactly when it is needed. (5.3.3)

5.4 Seismic Qualification

5.4.1 The seismic certification required for the switchgear shall be as indicated in the datasheet.
Seismic Certification Requirementselect
Not required
Anchorage design only per ASCE 7 Chapter 13
Special seismic certification by shake-table test per ICC-ES AC156 or by analysis accepted by the Authority Having Jurisdiction
Derived — Seismic Design CategorySeismic Design CategoryParameterEach project supplies its own value.seismic-design-category and the component importance factor assigned to the switchgear under ASCE 7 Chapter 13 (by default)
NOTE Anchorage keeps the switchgear on its pad; special certification demonstrates that the breakers and the controls still operate after the design earthquake, which ASCE 7 requires of a component whose importance factor is above 1.0. Switchgear that parallels the alternate source for egress lighting, fire pumps, smoke control, or another life-safety load carries an importance factor of 1.5 under ASCE 7, and so does switchgear needed for the continued operation of a Risk Category IV facility. (5.4.2)
5.4.3 Where seismic certification is required, it shall be performed in accordance with the seismic provisions of Adopted Building CodeAdopted Building CodeParameterEach project supplies its own value.adopted-building-code and ASCE 7 Chapter 13, and the certificate shall cover the switchgear as furnished, including the structure, the bus, every breaker, and the control sections, in the heaviest section arrangement furnished.
5.4.4 A field change to the devices installed or to the anchorage that departs from the certified configuration shall be re-evaluated by the manufacturer against the certification before the switchgear is energized.

6 System Interface and Classification

6.1 NFPA 110 Classification

6.1.1 The NFPA 110 level of the system the switchgear serves shall be as indicated in the datasheet.
NFPA 110 Levelselect
Level 1
Level 2
Not classified under NFPA 110
Derived — the NEC article the switchgear serves and the level the adopted code assigns to that system under NFPA 110 Chapter 4, NFPA 101, and NFPA 99, as established under the emergency and standby power standard (by default)
6.1.2 The NFPA 110 type of the system the switchgear serves shall be as indicated in the datasheet.
NFPA 110 Typeselect
Type 10
Type 60
Type 120
Type M
Not classified under NFPA 110
Derived — the NEC article the switchgear serves, with the maximum restoration time of 10 seconds under NEC 700.12 and 60 seconds under NEC 701.12, and the type the adopted code assigns under NFPA 110 Chapter 4 (by default)
NOTE The level fixes the alarms the switchgear annunciates, the control power source NFPA 110 accepts, and the acceptance test it receives; the type fixes the time within which the first set must be on the bus with the priority loads connected. Both are properties of the system the switchgear serves, so they are stated here as derived values rather than decided at the switchgear. (6.1.3)
6.1.4 Where the system is Type 10, the first set to reach rated voltage and frequency shall be closed to the dead bus and the transfer switches serving the Article 700 loads shall be signaled that the bus is available within 10 seconds of the loss of the normal source, with the start and stabilization time of the set included, in accordance with NEC 700.12.
6.1.5 Where the system is Type 60, the same sequence shall be completed within 60 seconds, in accordance with NEC 701.12.
NOTE A paralleled source cannot wait for every set before it serves the emergency loads, because synchronizing the second and later sets takes seconds that the type time does not allow; the first set to reach its windows takes the dead bus alone and carries the first priority level, and each later set adds its capacity as it synchronizes, which is why the restoration time is measured to the first set and the load add sequence is measured from it. (6.1.6)

6.2 Set-Side Interface

6.2.1 Every set paralleled by the switchgear shall be furnished under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators with an electronic governor that accepts a speed bias signal from the load-sharing controls, a voltage regulator that includes the paralleling provision and accepts a voltage bias signal from the reactive load-sharing controls, a two-wire remote start interface, and auxiliary contacts reporting the set running and the set-mounted breaker position where one is furnished.
6.2.2 The sets paralleled on one bus shall have alternators of the same rated voltage, the same winding pitch, and the same number of poles, so that no circulating harmonic current flows between them on the common bus.
6.2.3 The switchgear manufacturer shall confirm in writing, before the switchgear is released for manufacture, that the governor and the voltage regulator of every set accept the load-sharing and synchronizing signals the controls furnished generate, and that confirmation shall be the interface confirmation submitted under Informational Submittals.
6.2.4 The switchgear shall start every set on any start signal it receives, shall stop the sets after the cool-down run the set controllers require, and shall carry the engine-start signal of every transfer switch furnished under Automatic Transfer SwitchesAutomatic Transfer SwitchesResolves to the current adopted revision.sync/automatic-transfer-switches to the sets through its own start logic rather than through wiring from the switches to the sets directly.
6.2.5 Where the sets are started by the switchgear rather than by the transfer switches directly, loss of the start wiring between the switchgear and any set, or loss of control power at the switchgear, shall call for every set to start.
NOTE A set whose governor runs in droop while its neighbor runs isochronous takes none of a load increase until the frequency has fallen, and two sets whose regulators have no reactive droop or cross-current provision push reactive current back and forth between them until one trips on overcurrent; the interface clauses exist because the signals that prevent both conditions are furnished on the set and read by the switchgear, and neither party sees the mismatch until the integrated test. (6.2.6)

6.3 Transfer Switch Interface

6.3.1 The switchgear shall provide a bus-available signal to each transfer switch, wired as a dry contact that closes when the paralleling bus is within its voltage and frequency windows and opens when the bus is dead or shed, and the transfer switches shall sense the bus as their alternate source through that signal or through their own sensing as Automatic Transfer SwitchesAutomatic Transfer SwitchesResolves to the current adopted revision.sync/automatic-transfer-switches selects.
6.3.2 The load-shed output for each priority level shall be wired to the load-shed inputs of the transfer switches assigned to that level under the load priority table, where the datasheet selects shedding through the transfer switches.
6.3.3 Where the switchgear serves an Article 700 emergency system, the control and signal wiring between the switchgear and the transfer switches shall be installed and separated in accordance with NEC 700.10, and where an Article 701 system, in accordance with NEC 701.10.

7 Construction Basis

7.1 Assembly Construction

7.1.1 The construction basis of the switchgear shall be as indicated in the datasheet.
Construction Basisradio
○ Switchboard construction listed to UL 891
○ Metal-enclosed low-voltage power circuit breaker switchgear listed to UL 1558
NOTE A switchboard built to UL 891 carries fixed-mounted or drawout insulated-case breakers in a structure with a smaller footprint and a lower cost, and its breakers are maintained with the bus de-energized unless drawout mounting is selected; metal-enclosed switchgear built to UL 1558 and IEEE C37.20.1 carries drawout power circuit breakers in individual compartments, each rated to carry and interrupt the full bus fault current for 30 cycles, each withdrawable with the bus energized, at the largest footprint and the highest cost. Which basis a project uses follows from how the switchgear will be maintained, from the short-time rating the coordination with the transfer switch feeders needs, and from the construction of the other distribution equipment on the project, so neither is the norm. (7.1.2)
7.1.3 Where switchboard construction is selected, the structure, the bus construction and plating, the barriers, the feeder devices, the trip units, the section accessibility, the enclosure material and finish, and the conduit entry and wiring space shall comply with Low Voltage SwitchboardsLow Voltage SwitchboardsResolves to the current adopted revision.sync/low-voltage-switchboards as requirements of this standard, except as this standard states otherwise.
7.1.4 Where metal-enclosed switchgear construction is selected, the same elements shall comply with Low Voltage SwitchgearLow Voltage SwitchgearResolves to the current adopted revision.sync/low-voltage-switchgear as requirements of this standard, except as this standard states otherwise.
7.1.5 The control sections shall be of the same construction, height, depth, and finish as the breaker sections, bolted in line with them, and shall be covered by the same listing.
7.1.6 The bus shall be continuous through the generator breaker sections, the tie section where furnished, and the distribution sections as one listed assembly, and a shipping split shall be made only at a section joint with a bolted splice furnished by the manufacturer.

7.2 Generator Breaker Construction

7.2.1 The generator breakers shall be of the type indicated in the datasheet.
Generator Breaker Typeselect
Molded-case circuit breaker, electrically operated, fixed-mounted
Insulated-case circuit breaker, electrically operated, fixed-mounted
Insulated-case circuit breaker, electrically operated, drawout-mounted
Low-voltage power circuit breaker, electrically operated, drawout-mounted
7.2.2 Where metal-enclosed switchgear construction is selected, the generator breakers, the tie breaker, and the utility main breaker where one is furnished shall be low-voltage power circuit breakers complying with IEEE C37.13 and listed to UL 1066, drawout-mounted.
7.2.3 Where switchboard construction is selected, the generator breakers shall be listed to UL 489 for a molded-case or insulated-case breaker, and the drawout cradle where selected shall comply with the drawout provisions of Low Voltage SwitchboardsLow Voltage SwitchboardsResolves to the current adopted revision.sync/low-voltage-switchboards.
NOTE A molded-case generator breaker is the smallest and least expensive and clears every fault instantaneously in the larger frames, which limits how the transfer switch feeders can be coordinated below it; an insulated-case breaker adds a stored-energy mechanism and a short-time rating, which lets its instantaneous element be set above the arcing current or defeated so that a feeder clears first; drawout mounting lets a generator breaker be racked out for inspection or exchange with the other sets still on the bus, at the cost of the cradle and the section depth; a power circuit breaker carries the 30-cycle rating and the maintainable contacts of switchgear construction and is the breaker that construction is built around. (7.2.4)

7.3 Bus Arrangement

7.3.1 The bus arrangement shall be as indicated in the datasheet.
Bus Arrangementradio
○ Single paralleling bus without a tie
○ Paralleling bus divided by a tie breaker
Per drawings — the one-line diagram (deferred by default)
NOTE A single bus is the arrangement most lineups are built with; a tie divides the bus so that a bus fault, a bus maintenance outage, or a failed section drops only the sets and the loads on one side, and it is the arrangement a scheme that assigns sets to two load groups or that must keep one bus energized through maintenance on the other is drawn with. The arrangement is decided with the scheme under the emergency and standby power standard, so the field records what the one-line diagram shows. (7.3.2)
7.3.3 Where a tie breaker is furnished, it shall be of the same type and frame as the generator breakers, electrically operated, with a synchronizing permissive across it as this standard requires for a generator breaker.
7.3.4 The distribution devices feeding the transfer switches or the emergency distribution, their frame and trip ratings, and the loads they serve shall be as indicated on the one-line diagram.
7.3.5 Provisions for future sets, including bussed and equipped generator breaker positions and the control section capacity to add a set, shall be as indicated on the one-line diagram.
7.3.6 Where a future set position is shown, the bus, the control power distribution, and the controller input and output capacity shall be furnished so that the set can be added without replacing a section or a controller.

8 Ratings

8.1 System Voltage and Frequency

8.1.1 The system voltage of the paralleling bus shall be as indicated in the datasheet.
System Voltageselect
208Y/120 V 3Φ 4-wire
240 V 3Φ 3-wire delta
480Y/277 V 3Φ 4-wire
480 V 3Φ 3-wire delta
600Y/347 V 3Φ 4-wire
600 V 3Φ 3-wire delta
Per drawings — the one-line diagram (deferred by default)
8.1.2 The system voltage shall match the rated voltage and winding configuration of every set selected under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators and the alternate-source terminals of every transfer switch selected under Automatic Transfer SwitchesAutomatic Transfer SwitchesResolves to the current adopted revision.sync/automatic-transfer-switches.
8.1.3 The rated frequency of the switchgear shall be as indicated in the datasheet.
Rated Frequencyradio
● 60 Hz
○ 50 Hz

8.2 Number of Sets and Bus Continuous Current

8.2.1 The number of generator sets the switchgear parallels, including future positions, shall be as indicated in the datasheet.
Number of Generator Setsrange
sets
216
Per drawings — the one-line diagram (deferred by default)
8.2.2 The main bus continuous current rating shall be as indicated in the datasheet.
Main Bus Continuous Currentrange
A
4006008001000120016002000250030003200400050006000
Per drawings — the one-line diagram (deferred by default)
8.2.3 The main bus rating shall be not less than the sum of the rated currents of every set the bus can carry at the site-rated output and the rated power factor, including every future set position, and not less than the sum of the ratings of the distribution devices where that sum is larger.
NOTE The bus is sized for every set that will ever be on it, because a bus cannot be uprated after manufacture without replacing the sections that carry it, and a paralleling lineup is the equipment most often bought with a set position left for the future. (8.2.4)
8.2.5 Each generator breaker shall have a continuous current rating not less than the rated current of its set at the site-rated output and the rated power factor, and its frame and trip rating shall be as indicated on the one-line diagram.

8.3 Short-Circuit Current Rating

8.3.1 The available fault current at the paralleling bus, with every set paralleled and with the utility paralleled where the datasheet selects a closed-transition or extended parallel mode, shall be as established under Short Circuit StudyShort-Circuit StudyResolves to the current adopted revision.sync/short-circuit-study with the reactances and the decrement curves of the alternators furnished.
8.3.2 The assembly short-circuit current rating marked on the nameplate shall equal or exceed the available fault current at the paralleling bus so established, and shall be as indicated in the datasheet.
Assembly Short-Circuit Current Ratingrange
kA
22253542506585100150200
Per drawings — the one-line diagram (deferred by default)
NOTE Every set on the bus contributes its subtransient fault current at once, so the fault duty at the paralleling bus is the sum over every set that can be paralleled, plus the contribution of the running motors, and during a closed-transition or extended parallel it includes the utility contribution, which is usually the largest term; a lineup rated for the duty of one set is under-rated the day the second set closes. (8.3.3)
8.3.4 The bus short-circuit bracing shall be as indicated in the datasheet.
Bus Short-Circuit Bracingrange
kA
22253542506585100150200
Derived — the assembly short-circuit current rating; the bus is braced to not less than the rating marked on the switchgear (by default)
8.3.5 Every generator breaker, the tie breaker, the utility main breaker where furnished, and every distribution device shall carry an interrupting rating not less than the assembly short-circuit current rating, and a series-rated combination shall not be relied on anywhere in the switchgear.
8.3.6 The short-time withstand rating of the assembly shall be as indicated in the datasheet.
Short-Time Withstand Ratingradio
○ 3-cycle short-circuit rating only
○ 30-cycle short-time withstand rating
Derived — the construction basis, with a 30-cycle rating inherent to UL 1558 switchgear, and the short-time delay settings the coordination study assigns to the generator breakers and the tie breaker in switchboard construction (by default)
8.3.7 Where a 30-cycle short-time withstand rating applies, the generator breakers, the tie breaker, and the utility main breaker where furnished shall each carry a short-time rating not less than the assembly rating for 30 cycles, and the bus bracing shall be established for that duration.
NOTE An alternator's fault current falls from its subtransient value toward a sustained value within a few cycles, and the sustained value the excitation holds under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators is roughly three times rated current for ten seconds; a generator breaker set with a short-time delay to let a transfer switch feeder clear first must therefore be rated to carry the current it will see through that delay, and the sustained current must stay above the pickup of every device expected to clear, which is what the coordination study checks and what the voltage-restrained overcurrent function under this standard exists for. (8.3.8)

8.4 Neutral and Ground Bus

8.4.1 Where the system voltage is a four-wire system, a neutral bus rated not less than 100 percent of the phase bus shall be furnished, insulated from the structure, extending the full length of the switchgear, with a neutral terminal for every set connection and every distribution device.
8.4.2 The neutral of the paralleled source shall be bonded to ground at one point only, and that point shall be as indicated in the datasheet.
System Bonding Jumper Locationradio
○ At the switchgear neutral bus, with every set neutral carried to the switchgear unbonded
○ No bond at the switchgear, the neutral solid through the transfer switches and bonded at the service
Derived — the neutral treatment of the transfer switches selected under the automatic transfer switch standard, with the switchgear neutral bus as the bonding point where the sets are shipped with their neutrals isolated under the generator standard (by default)
8.4.3 Where the bond is at the switchgear neutral bus, the system bonding jumper and the grounding electrode conductor connection shall be installed there and at no other point, in accordance with NEC 250.30 and Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current adopted revision.sync/grounding-and-bonding, and every set shall be shipped with its neutral isolated from its frame as GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators provides.
8.4.4 Where no bond is made at the switchgear, the neutral bus shall remain insulated from the structure, and the Contractor shall confirm the absence of a neutral-to-ground bond at every set and at the switchgear before the first set is energized.
NOTE A set bonded at its own neutral while its neighbor is bonded at its neutral too puts two bonds on one system, and the neutral current then circulates between the sets through the equipment grounding path and defeats every ground-fault sensor on the bus; bonding once at the switchgear neutral bus is the arrangement nearly every lineup with switched-neutral transfer switches is built with, and the field exists so that a lineup behind solid-neutral switches records that no bond is made here. (8.4.5)
8.4.6 Where the switchgear serves an Article 700 emergency system on a solidly grounded system of more than 150 V to ground with a generator breaker rated 1000 A or more, ground-fault indication shall be furnished at the switchgear in accordance with NEC 700.31, shall indicate and shall not trip, and its sensor shall be located where the bonding selected under this standard places the fault current.
8.4.7 Ground-fault sensing on the distribution devices, where furnished, shall be arranged so that it reads correctly with any combination of sets on the bus and with the tie open or closed.
8.4.8 A full-length copper ground bus shall be furnished, bonded to each section of the structure, with a terminal for the equipment grounding conductor of every set feeder and every distribution device, and grounding and bonding of the switchgear shall comply with Grounding And BondingGrounding and Bonding for Electrical SystemsResolves to the current adopted revision.sync/grounding-and-bonding.

9 Generator Breaker Sections

9.1 Breaker Operation and Duty

9.1.1 Each set shall be connected to the bus through its own generator breaker in its own section or compartment, with the controls, the metering, and the protective functions for that set in or adjacent to that section.
9.1.2 Each generator breaker shall be electrically operated with a stored-energy mechanism, shall close and trip on command from the controls and from its local control switch, shall be trip-free and anti-pump, and shall be rated for the duty of closing onto the live paralleling bus at the synchronizing windows this standard sets and of interrupting the assembly short-circuit current rating.
9.1.3 Each generator breaker shall be mechanically latched in the closed position, so that loss of control power does not change its position, and shall be tripped only by its trip coil, its trip unit, or its manual trip.
9.1.4 The stored-energy mechanism of each generator breaker shall be charged from the control power source selected under this standard, so that every generator breaker can be closed with the paralleling bus dead and the normal source lost.
NOTE A charging motor fed only from the paralleling bus cannot charge the first breaker that has to close onto a dead bus, and a lineup so wired is started by hand-charging the first breaker in the dark; the clause exists because this is a common field discovery. (9.1.5)
9.1.6 Each generator breaker shall be furnished with auxiliary contacts reporting its position to its controller, to the master controller where one is furnished, and to the operator interface, and with a bell alarm contact reporting a protective trip as distinct from a controlled opening.

9.2 Local Control and Indication

9.2.1 Each generator breaker section shall carry, on its door and operable with the door closed, a breaker control switch with close and trip positions, red and green position indicators, and a selector that places the set in automatic, manual, or off.
9.2.2 The close circuit of each generator breaker shall pass through the sync-check permissive of this standard in every mode, including a manual close from the control switch, and no close path other than the dead-bus close shall exist that bypasses the permissive.
9.2.3 A protective trip of a generator breaker shall operate a lockout relay, device 86, that trips the breaker and blocks its closing until the relay is reset by hand at the section, and the lockout shall be annunciated at the operator interface.
9.2.4 Where a drawout generator breaker is furnished, the cradle shall carry connected, test, and disconnected positions with a position indicator visible with the door closed, shall interlock so that the breaker cannot be racked between the connected and disconnected positions while closed, and shall permit the controls to be exercised with the breaker in the test position.

9.3 Metering and Instrument Transformers

9.3.1 Each set shall be metered at its section for voltage on each phase, current on each phase, frequency, kilowatts, kilovolt-amperes reactive, power factor, and kilowatt-hours, and the paralleling bus shall be metered for voltage on each phase, frequency, total kilowatts, and total kilovolt-amperes reactive, displayed at the operator interface.
9.3.2 Current transformers shall comply with IEEE C57.13, and current transformers serving a protective function shall be on cores separate from those serving metering, with a relaying accuracy class the protective function manufacturer states as adequate at the assembly short-circuit current rating.
9.3.3 Potential transformers or direct voltage sensing shall be furnished for each set on the set side of its generator breaker and for the bus, so that the synchronizing controls read the incoming set and the running bus independently, and a bus potential shall be furnished on each side of the tie breaker where one is furnished.
9.3.4 Every current transformer secondary shall be wired through shorting-type terminal blocks, and every potential circuit shall be fused at the transformer.

10 Tie and Distribution Sections

10.1 Tie Breaker Operation

10.1.1 Where a tie breaker is furnished, the tie shall open automatically on a bus fault on either side as the protective functions of this standard act, shall close automatically only through the sync-check permissive across the tie, and shall be operable by hand from its control switch through the same permissive.
10.1.2 Where a tie breaker is furnished, the sequence of operation shall state the normal position of the tie, the conditions under which it closes and opens automatically, and the behavior of each bus when the sets on the other bus are lost.

10.2 Distribution Devices

10.2.1 The distribution devices shall be of the feeder device type the construction basis standard selects, and each shall carry the frame, trip rating, and trip unit indicated on the one-line diagram.
10.2.2 Every distribution device that serves as a load-shed point under this standard shall be electrically operated, shall close and trip on command from the load controls, and shall carry auxiliary contacts reporting its position.
10.2.3 Where the switchgear serves loads that NEC 700.32, 701.32, or 708.54 require to be selectively coordinated, the generator breakers, the tie breaker, and the distribution devices shall be furnished with trip units and settings that achieve the selectivity established under Protective Coordination StudyProtective Device Coordination StudyResolves to the current adopted revision.sync/protective-coordination-study, so that a fault on a transfer switch feeder opens that feeder's device and does not open a generator breaker or the tie.
NOTE A feeder fault that opens a generator breaker takes a set off the bus for a fault that was not the set's, and where the bus is lightly populated it drops the emergency loads with it; the generator breaker is set to carry the fault for the feeder's clearing time, which is what the short-time rating and the voltage-restrained overcurrent function of this standard are furnished for. (10.2.4)
10.2.5 The trip units and settings of every distribution device shall be as established under Protective Coordination StudyProtective Device Coordination StudyResolves to the current adopted revision.sync/protective-coordination-study, and the settings as commissioned shall be recorded in the closeout submittals.

11 Control Architecture

11.1 Controller Arrangement

11.1.1 The control architecture of the switchgear shall be as indicated in the datasheet.
Control Architectureradio
○ One master controller with per-set controllers
○ One master controller with a hot-standby redundant master and per-set controllers
○ Distributed per-breaker controllers with no master, peer-to-peer
NOTE A single master holds the system logic, the priority table, and the utility interface in one place, is the simplest to program and to read, and is the one controller whose failure leaves the sets running but the system unable to add load, shed load, or start a set it has not already started; a redundant master removes that single point at the cost of a second controller and the failover logic, and it is the arrangement a Level 1 system that cannot accept a loss of automatic operation is built with; a distributed architecture puts the synchronizing, load sharing, and protection of each set in its own controller and shares the system state over a peer-to-peer network, so that no one controller failure takes the system out of automatic, at the cost of logic that is spread across every controller and a network whose loss every controller must survive. (11.1.2)
11.1.3 Whichever architecture is selected, the failure of any one controller shall not open the breaker of a set carrying load, shall not shut down any set, and shall leave the remaining sets sharing the load on the bus.
11.1.4 Whichever architecture is selected, the failure of any one controller shall be annunciated at the operator interface and shall raise the not-in-automatic alarm of this standard.
11.1.5 Where a redundant master is selected, the standby master shall track the state of the active master continuously, shall assume control automatically on a failure of the active master without operator action and without a change of any breaker position, and the transfer shall be tested as a field acceptance test of this standard.
11.1.6 Where a distributed architecture is selected, each controller shall perform the synchronizing, the load sharing, and the protective functions of its own set, and on loss of the network between controllers each controller shall revert to a mode that keeps every set already on the bus sharing load, and shall annunciate the network loss.
11.1.7 Every controller shall retain its program, its settings, and an event log of not fewer than 500 events in non-volatile memory through loss of control power, and changes to settings shall be protected by a password or a keyed switch so that a setting cannot be changed by an unauthorized person.

11.2 Operating Modes

11.2.1 The switchgear shall provide an automatic mode in which the sets are started, synchronized, loaded, shed, and stopped without operator action; a manual mode in which the operator starts each set, synchronizes it with the manual synchronizing provisions of this standard, and closes its breaker through the sync-check permissive; and a test mode in which the sets are started and paralleled on the bus without transferring any load from the normal source.
11.2.2 The mode shall be selected at the operator interface or at a selector switch at the master control section, and the switchgear shall raise a not-in-automatic alarm at the operator interface, at the remote annunciator furnished under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators where one is furnished, and on a dry contact whenever any set or the system is not in automatic.
11.2.3 Every set shall be started in automatic mode on any of the following: the engine-start signal of any transfer switch, loss of the normal source sensed at the utility main breaker where one is furnished, a start command from the operator interface, the exerciser schedule, and a load-demand call where load-demand operation is furnished.
11.2.4 The exerciser of the switchgear shall start and parallel the sets on a programmed schedule, with or without load transfer as the transfer switches are set under Automatic Transfer SwitchesAutomatic Transfer SwitchesResolves to the current adopted revision.sync/automatic-transfer-switches, and the exercise of the paralleling system shall satisfy NFPA 110 Section 8.4 for the level derived where the system is classified under NFPA 110.

11.3 Dead-Bus Closing

11.3.1 Where the bus is dead, the first set to reach rated voltage and frequency within its windows shall be closed to the bus without synchronizing, and every other set shall be synchronized to the live bus.
11.3.2 The dead-bus close shall be permitted only through a bus undervoltage sensing function that proves the bus dead and an arbitration that permits one set, and one only, to close to the dead bus at a time, so that two sets cannot close to the dead bus together.
NOTE Two sets that each sense a dead bus and each close in the same instant are paralleled unsynchronized, which is the same event as a synchronizer failure, so the arbitration that admits one set at a time is a protective function and not a convenience. (11.3.3)
11.3.4 Where a tie breaker is furnished and both buses are dead, the dead-bus close shall be arranged for each bus with the tie open, and the tie shall close only through its sync-check permissive after each bus is energized.

12 Synchronizing

12.1 Automatic Synchronizing

12.1.1 Each set shall be synchronized to the live bus automatically by a synchronizer that adjusts the speed of the set through the governor bias and the voltage of the set through the regulator bias until the set is within the synchronizing windows, and then closes the generator breaker.
12.1.2 The synchronizing windows for voltage difference, slip frequency, and phase angle shall each be programmable, and the synchronizer shall compensate for the closing time of the breaker so that the contacts make at the smallest phase angle the windows permit.
12.1.3 Where the synchronizer fails to bring a set within its windows within a programmable time, the switchgear shall annunciate a synchronizing failure for that set, shall stop attempting to close it, and shall continue to serve the loads with the sets already on the bus.
12.1.4 Synchronizing of the tie breaker and of the utility main breaker where one is furnished shall be performed by the same means, with the running and incoming sides as the scheme requires.

12.2 Sync-Check Permissive

12.2.1 A synchronism-check function, device 25, independent of the automatic synchronizer shall supervise the close circuit of every generator breaker, the tie breaker, and the utility main breaker where furnished, and shall permit a close only while the incoming and running voltages are within its windows.
12.2.2 The sync-check windows shall be programmable and shall be set initially to not more than 10 electrical degrees of phase angle, 5 percent of rated voltage difference, and 0.2 Hz of slip frequency, with the final values recorded on the settings record.
12.2.3 The sync-check function shall be furnished as a discrete relay or as a function of a controller that is not the controller performing the automatic synchronizing for that breaker, and shall be powered from the control power source selected under this standard.
NOTE A synchronizer that fails or a wiring error that swaps two phases closes a set onto the bus out of phase, and the torque that follows breaks couplings, shifts stator windings, and lifts an alternator off its base; the sync-check function reads the same two voltages through a path the synchronizer does not share and holds the close circuit open, which is why it is independent and why every close path runs through it. (12.2.4)

12.3 Manual Synchronizing

12.3.1 Manual synchronizing provisions shall be furnished at the master control section, comprising a synchroscope or an equivalent display on the operator interface, incoming and running voltmeters and frequency meters, a synchronizing selector that assigns the provisions to one breaker at a time, and raise and lower controls for the speed and the voltage of the selected set.
12.3.2 A manual close shall be made only with the synchronizing selector assigned to that breaker, through the sync-check permissive, and the provisions shall be operable with the operator interface out of service.
NOTE Manual synchronizing is the backup for an automatic synchronizer that has failed, and the instruments that let an operator do it by eye are hardwired for the same reason the sync-check relay is independent: the fault that took the synchronizer down is as likely to have taken the display with it. (12.3.3)

13 Load Sharing and Load Control

13.1 Real and Reactive Load Sharing

13.1.1 The sets on the bus shall share the real load in proportion to their site-rated outputs, within ±5 percent of the rated kilowatts of each set, with the bus frequency held at its rated value from no load to the capacity of the sets on the bus.
13.1.2 The sets on the bus shall share the reactive load in proportion to their ratings, within ±5 percent of the rated kilovolt-amperes reactive of each set, with the bus voltage held within the regulation band selected for the sets under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators.
NOTE Isochronous load sharing holds the bus at rated frequency and rated voltage while dividing the load among the sets by the bias signals the switchgear sends to each governor and regulator, and it is the mode nearly every paralleling system runs in, because a droop scheme lets the frequency and the voltage fall with load and leaves the operator to correct them; droop remains the fallback a distributed architecture drops to when it loses its network. (13.1.3)
13.1.4 Where the sets on the bus differ in rating, the load sharing shall be proportional to the rating of each set, and no set shall be loaded above its site-rated output while another set on the bus has capacity to take load.
13.1.5 Where the sets are paralleled with the utility under this standard, the switchgear shall control the sets in a base-load, import, or export mode as the extended parallel functions selected require, and shall return the sets to isochronous sharing on separation from the utility without a change of any generator breaker position.

13.2 Load Add Sequence

13.2.1 The transfer switches or the distribution devices served by the switchgear shall be grouped into priority levels, and the number of priority levels the switchgear provides shall be not less than the number indicated in the datasheet.
Load Priority Levelsrange
levels
2345678
NOTE The number of levels follows from the load classes the scheme serves: a system that carries emergency, legally required standby, and optional standby loads on one alternate source needs a level for each class at least, because NEC 700.4 permits that arrangement only with automatic load pickup and load shedding that keeps the emergency loads served, and a health care system carries at least the life safety, critical, and equipment branches; a system with a few large motor loads adds levels so that each motor is started on its own. The count is a property of the scheme, so the field carries no default. (13.2.2)
13.2.3 The first priority level shall be connected as soon as the first set is on the bus, and each succeeding level shall be connected only when the capacity of the sets on the bus, less the load already connected, exceeds the load of that level by a programmable margin.
13.2.4 The load add sequence shall be completed without operator action, and the time at which each level is connected shall be recorded in the event log.
13.2.5 The assignment of every transfer switch and every distribution device to a priority level shall be as stated on the load priority table submitted under this standard, and the Contractor shall configure the switchgear to that table.

13.3 Load Shed Sequence

13.3.1 The switchgear shall shed load in reverse priority order when the load on the bus exceeds the capacity of the sets on the bus, when a set trips off the bus, or when the bus frequency falls below a programmable threshold for a programmable time, and shall restore the shed levels in priority order when capacity returns.
13.3.2 An underfrequency load shed, acting through a protective function or a hardwired relay independent of the master controller, shall shed the lowest priority levels on a bus frequency below a programmable threshold, so that the emergency loads are held through a loss of the master.
13.3.3 The means by which load is shed shall be as indicated in the datasheet.
Load Shed Meansradio
○ Shed signals to the load-shed inputs of the transfer switches
○ Opening of the distribution devices at the switchgear
○ Shed signals to the transfer switches for some levels and opening of distribution devices for others, as the priority table assigns
NOTE Shedding through the transfer switch returns the shed load to the normal source where it is available and opens the switch where it is not, and it needs a load-shed input at each switch and a control conductor to it; opening a distribution device at the switchgear sheds the whole feeder at once with no wiring beyond the switchgear and needs an electrically operated device, and it drops the load whether or not the normal source has returned. A lineup that feeds both transfer switches and direct emergency distribution uses each means where it fits. (13.3.4)
13.3.5 The load shed thresholds, the time delays, and the restoration margin shall be programmable, and the values as commissioned shall be recorded on the settings record.

13.4 Load-Demand Operation

13.4.1 Whether load-demand start and stop is furnished shall be as indicated in the datasheet.
Load-Demand Start and Stopradio
○ Not furnished
○ Furnished
NOTE Load-demand operation stops sets when the connected load has fallen below the capacity of fewer sets for a programmed time and restarts them as the load rises, which spares fuel, engine hours, and the wet-stacking of lightly loaded diesels through a long outage, at the cost of a start that must succeed when the load returns; it is furnished where outages are long or the sets are many, and omitted where the Owner wants every set running for the whole outage. (13.4.2)
13.4.3 Where load-demand operation is furnished, the switchgear shall keep on the bus not fewer than the number of sets needed to carry the connected load plus a programmable reserve, shall never stop a set while the bus is carrying the first priority level with no other set available, shall restart a stopped set and add it before the connected load reaches the capacity of the sets on the bus, and shall rotate the order in which sets are stopped so that the run hours of the sets are equalized.

14 Generator Protective Functions

14.1 Protective Function Platform

14.1.1 The platform on which the protective functions are furnished shall be as indicated in the datasheet.
Protective Function Platformradio
○ Discrete multifunction generator protection relay for each set
○ Functions integral to the per-set paralleling controller
NOTE A discrete relay is a device built and tested to IEEE C37.90 for protection alone, with its own current and voltage inputs and its own trip output, and it keeps protecting a set whose controller has failed; functions integral to the controller use the controller's metering inputs and cost a relay less per set, and they are as available as the controller is. Whether the separation is worth the relay depends on the size of the sets, on how an insurer or the Owner's engineering standard treats generator protection, and on whether the controller furnished carries the IEEE C37.90 qualification for its protective functions. (14.1.2)
14.1.3 Reverse-power protection shall trip its generator breaker through a path that does not depend on the master controller, whichever platform is selected.
14.1.4 Protective functions shall be identified by their IEEE C37.2 device function numbers on the schematic diagrams, the settings record, and the operator interface.

14.2 Protective Functions Furnished

14.2.1 Each set shall be furnished with the protective functions indicated in the datasheet, acting on its generator breaker through the lockout relay of this standard.
Generator Protective Functionscheckbox
☐ Reverse power, device 32
☐ Undervoltage, device 27
☐ Overvoltage, device 59
☐ Underfrequency, device 81U
☐ Overfrequency, device 81O
☐ Voltage-restrained or voltage-controlled overcurrent, device 51V
☐ Loss of excitation, device 40
☐ Generator differential, device 87G
☐ Negative-sequence overcurrent, device 46
☐ Phase overcurrent, device 50/51
☐ Ground overcurrent, device 51G
☐ Bus differential, device 87B
NOTE Reverse power is the function every paralleled set needs, because a set whose engine falters is driven as a motor by its neighbors until its breaker opens, and the engine that is being motored is damaged within seconds; the voltage and frequency functions take a set off the bus when its governor or regulator has lost control before it drags the bus with it; voltage-restrained overcurrent is the overcurrent function that still sees a fault after the alternator's current has decayed, because it lowers its pickup as the terminal voltage collapses; loss of excitation matters as the sets grow, because an under-excited set in parallel draws reactive current from its neighbors and slips poles, and the field is listed so that a project with large sets records it; differential protection clears a fault inside the alternator or on the bus without waiting for an overcurrent element, at the cost of current transformers at both ends of the zone; negative-sequence overcurrent protects the rotor from the heating of an unbalanced load or an open phase. Which functions a project furnishes follows from the size of the sets and from IEEE C37.102, so the field carries no default. (14.2.2)
14.2.3 The settings of every protective function shall be as established under Protective Coordination StudyProtective Device Coordination StudyResolves to the current adopted revision.sync/protective-coordination-study, with the proposed initial values submitted on the settings record and the values as commissioned recorded in the closeout submittals.
14.2.4 A protective trip shall open the generator breaker and shall not shut the set down, except that a differential or loss-of-excitation trip shall also signal the set controller to shut the set down and lock it out, and every protective trip shall be annunciated at the operator interface with the function that operated.
14.2.5 Where the switchgear serves a system classified as Level 1 under NFPA 110, no protective function shall shut a set down for a condition NFPA 110 requires to be a pre-alarm only, and the functions furnished shall act on the generator breaker rather than on the engine for every such condition.
14.2.6 Where bus differential protection is furnished, its zone shall include the bus on both sides of the tie where one is furnished, and its operation shall trip every generator breaker on the faulted bus and the tie.

15 Utility Paralleling

15.1 Utility Paralleling Mode

15.1.1 The utility paralleling mode of the switchgear shall be as indicated in the datasheet.
Utility Paralleling Moderadio
○ Open transition only, with no parallel between the sets and the utility
○ Closed-transition momentary parallel of 100 ms or less for load transfer
○ Extended parallel under an interconnection agreement for soft-load transfer, peak shaving, or base load
NOTE Open transition leaves every transfer to the transfer switches and keeps the sets off the utility at all times, so no interconnection review, no utility-interactive relay, and no interconnection agreement is needed, and every transfer back to the utility interrupts the load for the switch's transfer time; a momentary parallel closes the sets onto the utility for a fraction of a second so that a planned transfer in either direction interrupts nothing, and most utilities treat a parallel that short as not an interconnection, though they still review it; an extended parallel holds the sets on the utility for minutes or hours, ramps the load between the two sources so that a transfer disturbs nothing, and lets the sets carry load against a demand setpoint or at a base load, which is an interconnection under IEEE 1547 and the utility's tariff, with the relay, the agreement, the metering, and the engine certification that go with it. The mode is decided under the emergency and standby power standard with the utility, so the field carries no default. (15.1.2)
NOTE Requirements in the articles that follow apply where the datasheet selects a closed-transition momentary parallel or an extended parallel, and an open-transition switchgear carries none of them. (15.1.3)

15.2 Utility Connection and Transfer

15.2.1 Where a closed-transition or extended parallel mode is selected, the normal source shall enter the switchgear through an electrically operated utility main breaker of the same construction as the generator breakers, as indicated on the one-line diagram, and the loads transferred by closed transition shall be fed from the switchgear.
15.2.2 Where a closed-transition or extended parallel mode is selected, the parallel shall be made only through the automatic synchronizer and the sync-check permissive of this standard applied to the utility main breaker, with the utility as the running source.
15.2.3 Where a closed-transition momentary parallel is selected, the duration of the parallel shall not exceed 100 ms, an independent timing function shall trip the utility main breaker or the generator breakers if the parallel exceeds that duration, and the duration measured during the field acceptance tests shall be recorded.
15.2.4 Where a closed-transition or extended parallel mode is selected, the switchgear shall fall back to an open transition when the sources cannot be brought within the synchronizing windows within a programmable time, and shall make an open transition whenever the normal source has failed.
15.2.5 Where a closed-transition or extended parallel mode is selected, the Contractor shall obtain the serving utility's written approval of parallel operation, including the protective relaying, the anti-islanding provisions, the visible disconnect, and the metering the utility requires under its interconnection rules and IEEE 1547, before the switchgear is released for manufacture, and where an extended parallel is selected shall deliver the executed interconnection agreement with the informational submittals.
15.2.6 Where an extended parallel is selected, the Contractor shall confirm under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators that the emissions certification and the air permit of every set permit non-emergency operation for the hours the mode will run, and shall report any limit to the Engineer of Record before the switchgear is released for manufacture.
NOTE An emergency engine is certified on the condition that it runs for emergencies and for readiness testing, and an Owner who runs it for peak shaving under an extended parallel has re-classified it as a non-emergency engine, which it was not certified to be; the confirmation clause exists because the switchgear can be built for a mode the engines are not permitted to run. (15.2.7)

15.3 Utility-Interactive Protection

15.3.1 Where a closed-transition or extended parallel mode is selected, a utility-interactive protection relay shall be furnished in the form indicated in the datasheet.
Utility-Interactive Protection Relayradio
○ Not furnished, open transition only
○ Discrete multifunction utility-interactive relay at the utility main breaker
○ Functions integral to the paralleling controller, listed to UL 1741
NOTE A discrete relay at the utility tie is the device most utilities expect to see on their drawings and to test themselves, and it protects the interconnection whatever the controller is doing; functions integral to a controller that carries the UL 1741 listing for them cost a relay less and put the interconnection logic and the paralleling logic in one device, which some utilities accept and others do not. The utility's interconnection rules decide, so the field carries no default beyond the open-transition case. (15.3.2)
15.3.3 Where a utility-interactive relay is furnished, it shall be listed to UL 1741, its functions shall have been tested to IEEE 1547.1, and it shall provide over- and undervoltage, over- and underfrequency, unintentional islanding detection, and the reconnection delay after the utility returns, with the trip settings and clearing times IEEE 1547 assigns for the performance category the serving utility designates, together with any function the utility's interconnection rules add.
15.3.4 Where a utility-interactive relay is furnished, its operation shall trip the utility main breaker, or the generator breakers where the utility requires the sets to be removed instead, through a lockout relay that requires a manual reset, and shall be annunciated at the operator interface.
15.3.5 Where an extended parallel is selected, the point of common coupling metering, its current transformers, and the visible disconnect shall be furnished as the utility's interconnection rules require, and the metering shall comply with Electrical Power MonitoringElectrical Power Monitoring and MeteringResolves to the current adopted revision.sync/electrical-power-monitoring.

15.4 Extended Parallel Functions

15.4.1 Where an extended parallel is selected, the switchgear shall provide the functions indicated in the datasheet.
Extended Parallel Functionscheckbox
☐ Soft-load transfer in both directions with a programmable ramp
☐ Peak shaving against a programmable utility demand setpoint
☐ Base load at a programmable kilowatt setpoint
☐ Import and export control at the point of common coupling
NOTE A soft-load transfer ramps the real and reactive load between the sets and the utility over a programmable period so that neither source sees a step; peak shaving runs the sets to hold the utility demand below a setpoint read from the utility metering; base load runs the sets at a fixed output with the utility taking the difference; import and export control holds the power at the point of common coupling at a setpoint, including zero export where the utility forbids export. Each function the switchgear carries is a function the interconnection agreement has to admit, and economic dispatch beyond these functions is outside this standard. (15.4.2)
15.4.3 Where a soft-load transfer is furnished, the ramp period shall be programmable, the transfer shall be completed without a step change in the bus voltage or frequency, and the parallel shall be ended by opening the utility main breaker or the generator breakers as the direction of transfer requires.
15.4.4 Where peak shaving, base load, or import and export control is furnished, the switchgear shall hold the sets within their site-rated outputs, shall shed none of the emergency loads for the sake of the setpoint, and shall end the parallel and return to isochronous operation on the loss of the utility or on the operation of the utility-interactive relay.

16 Control Power

16.1 Control Power Source

16.1.1 The control power source for the switchgear shall be as indicated in the datasheet.
Control Power Sourceradio
○ Station battery and charger furnished under the DC battery system standard
○ Starting batteries of the generator sets, diode-isolated at the switchgear
○ Uninterruptible power supply dedicated to the switchgear controls
NOTE A station battery gives the switchgear a control source that is its own, sized for the trip and close duty of every breaker and the control load for a stated period, and maintained as a battery under DC Battery SystemsDC Battery SystemsResolves to the current adopted revision.sync/dc-battery-systems, at the cost of a battery, a charger, and a room or cabinet for them; the starting batteries of the sets are already on the site and already maintained for the NFPA 110 cranking duty, and feeding the controls from them through diode isolation gives as many sources as there are sets, at the cost of a control voltage that sags during every crank and a system whose control power depends on the sets' battery chargers; a dedicated UPS gives clean alternating-current control power with a battery of its own, at the cost of a stored-energy device that is itself a product to maintain and whose runtime bounds how long the controls survive a failed start. None of the three is the norm across projects, and the level of the system and the Owner's maintenance practice decide. (16.1.2)
16.1.3 The control power for the breaker trip and close circuits, the controllers, the protective functions, the sync-check permissive, and the operator interface shall remain available with the paralleling bus dead and the normal source lost, for not less than the duration this article requires for the source selected.
16.1.4 Where a station battery is selected, the battery shall be sized under DC Battery SystemsDC Battery SystemsResolves to the current adopted revision.sync/dc-battery-systems for the continuous control load of the switchgear for not less than 8 hours, followed by the closing of every generator breaker and the tripping of every breaker in the switchgear at the end of that period.
16.1.5 Where the starting batteries of the sets are selected, each per-set control section shall be supplied from the battery of its own set, the master controller and the sync-check and protective functions common to the system shall be supplied through diode-isolated feeds from the battery of every set, and the loss of any one battery shall not remove control power from the master or from any other set.
16.1.6 Where a dedicated UPS is selected, the UPS shall be supplied from the normal source and from the paralleling bus with automatic selection, its runtime at the control load of the switchgear shall be not less than the time the Engineer of Record establishes for the longest failed-start sequence of the sets and in no case less than 30 minutes, and the UPS shall comply with Uninterruptible Power Supply SystemsUninterruptible Power Supply SystemsResolves to the current adopted revision.sync/uninterruptible-power-supply-systems.
16.1.7 Where the switchgear serves a system classified as Level 1 under NFPA 110, the control power source shall be one NFPA 110 Chapter 5 accepts for the level, and where the source selected is not named there it shall be accepted by the Authority Having Jurisdiction in writing before the switchgear is released for manufacture.

16.2 Control Power Distribution

16.2.1 Control power shall be distributed within the switchgear through individually fused or breaker-protected circuits for each breaker's trip circuit, each breaker's close circuit, each controller, the protective functions, and the operator interface, so that a fault on one circuit does not remove control power from another.
16.2.2 Loss of control power on any circuit shall be annunciated at the operator interface and on a dry contact, and the trip circuit of every breaker shall be supervised so that an open trip coil or a blown trip fuse is annunciated.
16.2.3 Alternating-current control power for the operator interface, the space heaters, and the breaker charging motors where they are not supplied from the source selected shall be taken from a control power transformer on the paralleling bus and from the normal source where one is available at the switchgear, with automatic selection.

17 Operator Interface and Annunciation

17.1 Operator Interface

17.1.1 The operator interface of the switchgear shall be as indicated in the datasheet.
Operator Interfaceradio
● Local touchscreen at the master control section
○ Remote operator interface only, with hardwired control switches and indicators at the sections
NOTE A touchscreen at the lineup is what nearly every paralleling system is built with, because the operator who is synchronizing by hand or reading a lockout is standing at the switchgear; a remote-only interface is built where the Owner's operating program puts every operator at a control room and the lineup is visited only for maintenance, and it leaves the hardwired switches and indicators of this standard as the only local control. (17.1.2)
17.1.3 The operator interface shall display a one-line mimic of the switchgear with the position of every breaker, the state of every set, the metering of each set and of the bus, the mode of the system and of each set, the priority table with the state of each level, the synchroscope display for manual synchronizing where the datasheet selects a touchscreen, the alarm list, and the event log, and shall provide the start, stop, mode, and breaker commands of this standard under a password.
17.1.4 The event log shall record every breaker operation, every protective trip, every synchronizing attempt and its result, every load add and load shed, every mode change, every alarm, and every controller failover with a time stamp, and the clocks of every controller and relay shall be synchronized so that the sequence of events across the lineup reads in order.
17.1.5 The hardwired control switches, position indicators, selectors, and manual synchronizing provisions of this standard shall be furnished whichever operator interface is selected, so that every set can be started, synchronized, loaded, and stopped with the operator interface out of service.

17.2 Annunciation

17.2.1 The switchgear shall annunciate at the operator interface, and on a dry contact for each, the following: system not in automatic, any set not in automatic, control power failure, any controller failure, any protective trip with the set identified, synchronizing failure, load shed active with the level identified, bus overload, utility parallel active where a parallel mode is selected, and maintenance bypass active where one is furnished.
17.2.2 Where the switchgear serves a system classified under NFPA 110, the conditions of the switchgear that NFPA 110 Chapter 5 requires to be annunciated for the level derived shall be wired to the remote annunciator furnished under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators, or to a system annunciator at the same attended location, and the common alarm of the switchgear shall be wired to it whichever conditions apply.
17.2.3 A common alarm contact and a bus-energized contact shall be furnished in addition to the contacts of this article, rated for the pilot duty of the circuits they serve and wired to terminal blocks that can be reached without exposing energized power conductors.

18 Communications and Remote Monitoring

18.1 Network Protocol

18.1.1 The communications protocol of the switchgear shall be as indicated in the datasheet.
Communications Protocolselect
No network, dry contacts only
Modbus RTU
Modbus TCP
BACnet/IP
BACnet MS/TP
EtherNet/IP
DNP3
IEC 61850
NOTE The protocol has to match the system the switchgear reports to: a lineup whose status and metering go to the building automation system speaks the protocol that system reads under Building Automation SystemBuilding Automation SystemResolves to the current adopted revision.sync/building-automation-system, one reporting to a plant supervisory system speaks what that system reads under SCADA And HMI SystemsSCADA and HMI SystemsResolves to the current adopted revision.sync/scada-and-hmi-systems, and one reporting to a power monitoring system speaks what that system reads under Electrical Power MonitoringElectrical Power Monitoring and MeteringResolves to the current adopted revision.sync/electrical-power-monitoring; the serial protocols cost least and carry the least, the Ethernet protocols carry the full metering and the event log, DNP3 is what a utility or a generation control system reads, and IEC 61850 is the protocol a lineup integrated with protective relaying on a substation bus speaks. Dry contacts only is listed so that a lineup reporting through its contacts alone is recorded as such rather than left blank. (18.1.2)
18.1.3 Where a network is furnished, the switchgear shall report at least the position of every breaker, the state and mode of every set, the metering of every set and of the bus, the state of every priority level, every alarm of this standard, and every event in its log, and the point list shall be coordinated with the system the switchgear reports to.
18.1.4 Where a network is furnished, the network devices, cable, and power supplies within the switchgear shall be furnished and installed by the manufacturer and brought to a terminal or connector in a designated compartment for the field connection, and network cable within the switchgear shall be routed separately from the power conductors and the bus.
18.1.5 A close or trip of any breaker, a change of mode, and a change of any setting over the network shall be disabled as delivered, and shall be enabled only where the Owner authorizes the function in writing and the system the switchgear reports to applies the authentication the Owner's operating program requires.
18.1.6 The dry contacts of this standard shall be furnished whether or not a network is furnished, so that the alarms of the switchgear reach the attended location through a path that does not depend on the network.

19 Maintenance Bypass

19.1 Bypass Arrangement

19.1.1 The maintenance bypass arrangement of the switchgear shall be as indicated in the datasheet.
Maintenance Bypass Arrangementradio
○ Not furnished
○ Bypass connection from one designated set directly to the emergency distribution around the paralleling sections
○ Bypass bus with a manual breaker for each set, carrying the sets to the distribution with the paralleling controls isolated
NOTE A lineup without a bypass is serviced during a planned outage of the alternate source, which is the condition most projects accept and schedule; a bypass from one set lets a single set carry the emergency distribution directly while the paralleling sections and their controls are de-energized for service, with the capacity of that one set as the limit; a bypass bus with a manual breaker for each set lets the sets be run to the distribution one at a time, by hand, with the controls isolated, so that the controls can be replaced while the sets remain available, at the largest cost and footprint. Which arrangement a project needs follows from whether the loads can accept the planned outage, which the emergency and standby power standard settles with the source redundancy. (19.1.2)
19.1.3 Where a bypass is furnished, it shall be interlocked by key or by mechanical means so that a set connected through the bypass cannot be paralleled with the bus, so that the bypass cannot be closed while the paralleling bus is feeding the same distribution, and so that the paralleling controls can be de-energized with the bypass carrying the load.
19.1.4 Where a bypass is furnished, the set carrying the bypass shall run in isochronous mode on its own governor and regulator with no bias from the switchgear, and the switchgear shall annunciate the bypass active condition of this standard.
19.1.5 Where a bypass is furnished, the bypass operating state shall be among the operating states analyzed under Arc Flash StudyArc-Flash Hazard AnalysisResolves to the current adopted revision.sync/arc-flash-study, and the arc-flash label on the bypassed distribution shall carry the value for that state where it governs.
19.1.6 Where a bypass is furnished, the bypass operating sequence shall be placed on a permanently mounted placard at the bypass section and included in the operation and maintenance data.

20 Enclosure

20.1 Enclosure Type

20.1.1 The enclosure type shall be as indicated in the datasheet.
Enclosure Type per NEMA 250select
Type 1
Type 1 gasketed
Type 2
Type 3R
Type 4
Type 4X
Type 12
NOTE A dry, conditioned electrical room is the installed location on most projects, which is why Type 1 is the default; every other location named in the clauses of this article calls for the type those clauses require. (20.1.2)
20.1.3 The enclosure shall comply with UL 50 and, for a type other than Type 1, with UL 50E and NEMA 250 for the type selected, and the type shall be marked on the structure.
20.1.4 Switchgear installed outdoors shall have an enclosure of Type 3R, Type 4, or Type 4X, with rain hoods over ventilation openings and a drip shield over each door, and the outdoor housing arrangement shall be as the construction basis standard selects.
20.1.5 Switchgear installed where it is subject to hose-directed water or wash-down shall have an enclosure of Type 4 or Type 4X.
20.1.6 Switchgear installed where the atmosphere carries salt, chemical vapor, or another corrosive agent, including a coastal site, a wastewater facility, and a food-processing area, shall have a Type 4X enclosure.
20.1.7 Switchgear installed indoors where airborne dust, lint, or fibers accumulate shall have an enclosure of Type 12, Type 1 gasketed, Type 4, or Type 4X.
20.1.8 Switchgear installed indoors where dripping or light splashing of a non-corrosive liquid occurs shall have an enclosure of Type 2, Type 12, Type 4, or Type 4X.
20.1.9 Where a rating higher than the datasheet selection is required by the clauses of this article for the installed location, the higher rating shall be furnished and the conflict reported to the Engineer of Record before the switchgear is released for manufacture.
20.1.10 The enclosure material, the finish, the section accessibility, the internal barriers, the device covers, the conduit entry, and the wiring space shall be as the construction basis standard selects, applied to every section of the lineup including the control sections.

21 Arc-Flash Mitigation and Maintenance Safety

21.1 Arc-Flash Mitigation Provisions

21.1.1 The arc-flash mitigation provisions furnished shall be as indicated in the datasheet.
Arc-Flash Mitigation Provisionscheckbox
☐ Energy-reducing maintenance switch on each generator breaker, the tie breaker, and the utility main breaker
☐ Zone-selective interlocking between the generator breakers and the distribution devices
☐ Remote operation of every electrically operated breaker from the operator interface or a station outside the arc-flash boundary
☐ Remote racking device for drawout breakers
☐ Infrared inspection windows at the bus splices and the breaker connections
☐ Voltage presence indicators at each section
☐ Arc-resistant construction tested to IEEE C37.20.7 in the configuration furnished
NOTE Each provision lets a task be performed with less exposure than the alternative: a maintenance switch lowers the instantaneous pickup for the duration of the work and depends on a worker turning it on; zone-selective interlocking lets the generator breakers trip without delay on a bus fault while still waiting for a feeder device on a feeder fault; remote operation and remote racking put the operator outside the boundary for the operations most often performed on energized gear; an infrared window lets a connection be scanned under load with the cover on; a voltage indicator confirms the state of a section before a cover is removed; arc-resistant construction directs the products of an internal arcing fault away from the worker with the doors closed, at the cost of a plenum and height on every section. (21.1.2)
NOTE The incident energy at the paralleling bus rises with every set that is on it and falls only as slowly as the alternators' fault current decays, so the bus is often the highest-energy location in the facility and the one at which the provisions above earn their cost; the study under Arc Flash StudyArc-Flash Hazard AnalysisResolves to the current adopted revision.sync/arc-flash-study analyzes each number of sets paralleled, the utility parallel where selected, and the maintenance bypass where furnished as separate operating states. (21.1.3)
21.1.4 Where a generator breaker, the tie breaker, the utility main breaker, or a distribution device is rated or can be adjusted to 1200 A or more, arc energy reduction shall be provided in accordance with NFPA 70 240.87 by the method the construction basis standard selects, and the method shall act for every combination of sets on the bus.
21.1.5 Where arc-resistant construction is selected, the switchgear shall have been tested to IEEE C37.20.7 in the configuration furnished at the assembly short-circuit current rating and the short-time withstand duration derived, and the exhaust plenum and its discharge location shall be shown on the shop drawings and coordinated with Electrical RoomsElectrical RoomsResolves to the current adopted revision.sync/electrical-rooms.
21.1.6 Arc-flash warning labels shall be applied to each section in accordance with NFPA 70 110.16 and NFPA 70E, with the values taken from the study performed under Arc Flash StudyArc-Flash Hazard AnalysisResolves to the current adopted revision.sync/arc-flash-study for the worst-case operating state at that section, placed so that each is read before a cover is removed or a door opened.

22 Identification

22.1 Nameplates, arc-flash warning labels, available fault current markings, and source identification shall comply with Equipment LabelingElectrical Equipment LabelingResolves to the current adopted revision.sync/equipment-labeling for material, lettering, attachment, and content, and the text shall be that listed on the nameplate schedule.
22.2 The switchgear shall carry an assembly nameplate stating the switchgear designation, the system voltage with the number of phases and wires, the main bus rating, the assembly short-circuit current rating, the short-time withstand rating where one applies, the number of sets paralleled, and the utility paralleling mode selected.
22.3 Each generator breaker section shall carry a nameplate stating the set designation, the set rating, and the breaker frame and trip rating, and the designation shall match the designation on the one-line diagram.
22.4 Each distribution device shall carry a nameplate stating the device designation, the frame and trip rating, the load served, and the priority level assigned to it.
22.5 The master control section shall carry a permanently mounted sequence-of-operation placard and a permanently mounted load priority table, each revised to the as-commissioned condition before the switchgear is accepted.
22.6 The switchgear shall be marked as a component of the emergency system in accordance with NEC 700.10(A) where it serves an Article 700 system, and the location and the disconnecting means of the control power source shall be marked on the master control section.
22.7 The available fault current at the paralleling bus with every set paralleled, and the date it was calculated, shall be field-marked on the switchgear in accordance with NEC 110.24.
22.8 Whether a mimic bus is furnished shall be as indicated in the datasheet.
Mimic Busradio
● Furnished
○ Not furnished
NOTE A mimic bus traces the sets, the bus, the tie, and the distribution on the front of the lineup as the one-line diagram does, so that an operator synchronizing by hand or resetting a lockout sees which breaker feeds what without reading the nameplates; a multi-source lineup whose operation depends on the operator understanding the arrangement is the equipment a mimic bus was made for, which is why it carries the default, and a lineup whose operator interface carries the same mimic is the one built without it. (22.9)

23 Testing

23.1 Factory Tests

23.1.1 The manufacturer shall perform the production tests of UL 891 or UL 1558 for the construction basis selected on the switchgear before shipment, including a dielectric withstand test of the bus, a continuity check of every control, protection, metering, and communications circuit against the schematic diagrams, a mechanical operation check of every breaker and interlock, an inspection of every bus joint against the listed torque, and an inspection of the assembly against the reviewed shop drawings.
23.1.2 The manufacturer shall perform a functional test of the controls before shipment, with the sets simulated where the integrated test of this article is performed elsewhere, exercising every sequence in the written sequence of operation, every protective function by secondary injection at its proposed setting, every synchronizing window, every load add and load shed level, every controller failover, and every utility paralleling function selected.
23.1.3 The switchgear shall be tested at the factory connected to the generator sets furnished under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators, or to sets of the same model and controls where the project sets cannot be brought to the test, through the automatic start, the dead-bus close, the synchronizing of every set, the load sharing at not fewer than three load points, the load add and load shed sequence, the load-demand operation where furnished, and the stop sequence, with the results recorded in the certified factory test report.
23.1.4 Witnessing of the factory integrated test shall be as indicated in the datasheet.
Factory Integrated Test Witnessingradio
○ Witnessed by the Owner's representative
○ Certified test report without witnessing
NOTE A witnessed integrated test lets the Owner and the Engineer of Record see the sets synchronize, share load, and shed load under the project's sequence before anything ships, and it commits a representative to a trip to the factory for a test that lasts a day or more; a certified report records the same test without the trip, and leaves any discrepancy to be found at field acceptance testing, where a correction costs a controller change on site rather than at the factory. Whether the trip is worth it depends on how far the factory is, on how novel the sequence is, and on what the Owner's commissioning program expects, so the field carries no default. (23.1.5)
23.1.6 Where witnessing is selected, the manufacturer shall give not less than two weeks' notice of test readiness, shall submit the test procedure for review before the test, and shall not ship the switchgear until the witness has accepted the results or has waived attendance in writing.

23.2 Field Acceptance Tests

23.2.1 Field acceptance tests shall be performed on the switchgear after installation is complete and before it is placed in service, in accordance with ANSI/NETA ATS and Electrical Acceptance TestingElectrical Acceptance TestingResolves to the current adopted revision.sync/electrical-acceptance-testing, and shall include the tests indicated in the datasheet.
Field Acceptance Testscheckbox
☑ Switchgear and switchboard assembly inspection and tests per ANSI/NETA ATS Section 7.1, including insulation resistance of the bus and contact resistance of the bus splices and breaker connections
☑ Low-voltage power circuit breaker tests per ANSI/NETA ATS Section 7.6.1.2, or insulated-case and molded-case breaker tests per Section 7.6.1.1, as the construction basis selects
☑ Primary current injection of every electronic trip unit at one point per function
☑ Protective function tests per ANSI/NETA ATS Section 7.9, with every element injected at its applied setting and its trip proven through the lockout relay to the breaker
☑ Instrument transformer ratio, polarity, and burden per ANSI/NETA ATS Section 7.10
☐ Ground-fault sensing performance test per ANSI/NETA ATS Section 7.14
☐ Control power battery and charger tests per ANSI/NETA ATS Section 7.18, where a station battery is furnished
☑ Synchronizing verification, including the sync-check permissive proven to block a deliberately out-of-window close attempt and the dead-bus close arbitration proven to admit one set
☑ Load sharing verification at not fewer than three load points with every set on the bus, one at not less than 75 percent of the paralleled capacity
☑ Load add and load shed sequence through every priority level, including the loss of one set under load and the underfrequency shed
☑ Automatic start, synchronize, and transfer sequence from a simulated loss of the normal source, with measurement of the time to the first set on the bus and to each later set
☐ Closed-transition parallel duration measurement and utility-interactive relay tests per IEEE 1547.1, where a parallel mode is selected
☑ Controller failover to the redundant master, or network loss on distributed controllers, with the sets under load
☑ Control power loss simulation with every breaker holding its position
☐ Maintenance bypass operating sequence, where furnished
☑ Operation of every alarm, annunciator point, dry contact, and communications point against the point list
23.2.2 Where the building load at the time of testing cannot bring the paralleled sets to the load points this standard requires, a load bank shall be furnished for the load sharing and load shed tests, and the load bank duty shall be coordinated with the acceptance load test of the sets under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators.
23.2.3 The measured time to the first set on the bus with the first priority level connected shall be within the type time derived under this standard, and switchgear whose measured time exceeds that limit shall not be placed in service until the cause is corrected and the test repeated.
23.2.4 The polarity of every current transformer and the phase rotation of every set and of the bus shall be verified before any set is closed to a bus with another set on it.
NOTE A current transformer landed with its polarity reversed reads a set's real load as reverse power, and the first time that set takes load its breaker trips on a function that was working exactly as set; the polarity check exists because this is the most common cause of a failed first parallel. (23.2.5)
23.2.6 Where the switchgear serves a system classified under NFPA 110, the field acceptance tests shall be performed as part of the installation acceptance test NFPA 110 Chapter 7 requires, and the record shall be signed and delivered with the closeout submittals.
23.2.7 The field acceptance tests of the switchgear shall be scheduled with the generator field tests required under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators and the transfer switch field tests required under Automatic Transfer SwitchesAutomatic Transfer SwitchesResolves to the current adopted revision.sync/automatic-transfer-switches so that they roll up into the integrated system test required under Emergency And Standby PowerEmergency and Standby Power SystemsResolves to the current adopted revision.sync/emergency-and-standby-power, and the chain from loss of the normal source to the restoration of every priority level is verified as one system.
23.2.8 Switchgear that fails a field acceptance test shall not be placed in service, the Contractor shall report the failure to the Engineer of Record within one working day with the measured values, and the cost of the correction and of every repeated test shall be borne by the Contractor.

23.3 Manufacturer Start-Up

23.3.1 Whether the manufacturer performs a field start-up shall be as indicated in the datasheet.
Manufacturer Field Start-Upradio
● Required, with a start-up report
○ Not required
NOTE A manufacturer's start-up is the condition of the warranty on nearly every paralleling system, which is why it carries the default, and it is the visit at which the controllers are loaded with the project program, the settings record is applied, the sets are first paralleled under the manufacturer's eye, and the warranty commences. (23.3.2)
23.3.3 Where manufacturer start-up is required, the manufacturer's technician shall verify the installation against the shop drawings, shall load and verify the program of every controller and relay, shall apply the settings record, shall witness the first parallel of every set and the load sharing and load shed tests, and shall furnish the start-up report required in the closeout submittals.
23.3.4 Infrared thermographic inspection shall be as indicated in the datasheet.
Infrared Thermographic Inspectionselect
Not required
One inspection during the acceptance load test
One inspection during the acceptance load test and a second at eleven months
23.3.5 Where infrared inspection is indicated, it shall be performed with the sets carrying not less than 40 percent of the paralleled capacity through the scanned connections, and every connection showing a rise of more than 10 °C above its neighbors shall be reported and corrected.
NOTE A lineup that stands idle between exercises carries full load only during the acceptance test and the outage, so the acceptance load test is the one scheduled chance to scan it under load; a second inspection at eleven months finds the connection that loosened through a year of exercise cycles, at the cost of a second load bank visit and a warranty claim filed before the warranty ends. (23.3.6)

24 Installation

24.1 Location and Working Space

24.1.1 The room in which the switchgear is installed shall satisfy Electrical RoomsElectrical RoomsResolves to the current adopted revision.sync/electrical-rooms, and the room ventilation shall carry away the heat release of every section with the sets at full load so that the room ambient does not exceed the indoor design ambient temperature selected in the datasheet.
24.1.2 Working space in front of the switchgear, and behind it where it is rear-accessible, shall comply with NEC 110.26 for the system voltage and the condition of the space, and no door swing shall reduce that space.
24.1.3 Where the switchgear is rated 800 A or more and is over 6 ft wide, the working space shall have an entrance at each end in accordance with NEC 110.26(C)(2), unless the arrangement satisfies one of the exceptions that section permits.
24.1.4 The dedicated electrical space above the switchgear required by NEC 110.26(E) shall be kept clear of piping, ductwork, and equipment not associated with the electrical installation, and working space shall not be used for storage at any time.
24.1.5 The switchgear shall be set on a concrete housekeeping pad constructed under Concrete PadsConcrete Equipment PadsResolves to the current adopted revision.sync/concrete-pads, extending not less than 3 in. beyond the structure on every side and not less than 4 in. above the finished floor indoors or 6 in. above finished grade outdoors, with the conduit stub-ups and the anchor locations coordinated with the reviewed shop drawings before the pad is placed.
24.1.6 The switchgear shall be anchored to the pad as indicated on the seismic anchorage details, and where seismic certification is required the anchorage shall be that on which the certification depends.

24.2 Setting and Assembly

24.2.1 The Contractor shall handle, set, and assemble the switchgear in accordance with the manufacturer's instructions and with NEMA PB 2 or IEEE C37.20.1 for the construction basis selected, lifting each shipping section by the lifting provisions furnished and never by the bus, the doors, or the breaker handles.
24.2.2 Shipping sections shall be set plumb and level, aligned flush at the front, bolted together, and the bus spliced at every shipping split with the splice plates furnished, torqued to the manufacturer's values with a calibrated torque tool and witness-marked.
24.2.3 Every shipping brace, blocking, desiccant, and temporary ground shall be removed before the switchgear is energized, and the manufacturer's shipping checklist shall be completed and included in the field test report.

24.3 Field Wiring

24.3.1 The feeders from the sets and the feeders to the transfer switches shall comply with Conductors And CablesConductors and CablesResolves to the current adopted revision.sync/conductors-and-cables or BuswayBuswayResolves to the current adopted revision.sync/busway as the one-line diagram indicates, and raceways shall comply with Raceways And ConduitRaceways and ConduitResolves to the current adopted revision.sync/raceways-and-conduit.
24.3.2 Every power termination shall be torqued to the device manufacturer's published value with a calibrated torque tool, and the values applied shall be recorded in the field test report.
24.3.3 The governor and regulator bias circuits, the load-sharing lines, and the current and potential transformer circuits between the sets and the switchgear shall be installed in shielded cable complying with Instrumentation And Control CableInstrumentation and Control CableResolves to the current adopted revision.sync/instrumentation-and-control-cable, in raceways separate from the power conductors, with the shields grounded at the switchgear end only.
24.3.4 Where the switchgear serves an Article 700 emergency system, the feeders from the sets, the feeders to the emergency transfer switches, and the control wiring between the sets, the switchgear, and the switches shall be kept entirely independent of all other wiring in accordance with NEC 700.10(B), and the switchgear shall contain no wiring other than the wiring NEC 700.10(B) permits within emergency system equipment.
24.3.5 The system bonding jumper and the grounding electrode connection shall be installed, or omitted, at the switchgear in accordance with the bonding location derived under this standard, and the Contractor shall record the as-installed condition on the record drawings.

24.4 Energization

24.4.1 Before any set is closed to the bus, the Contractor shall load the reviewed program into every controller and relay, shall apply the reviewed settings record, and shall confirm the settings against the record.
24.4.2 The switchgear shall be energized first by one set closed to the dead bus in manual mode with every distribution device open, and a second set shall be synchronized to the bus only after the phase rotation of the bus, the polarity of every current transformer, and the metering of the first set have been verified.
24.4.3 Where the datasheet selects a closed-transition or extended parallel mode, the phase rotation of the utility and of the paralleling bus shall be verified to agree before the first parallel, and the first parallel shall be made in manual mode through the sync-check permissive with the serving utility notified.

25 Delivery, Storage, and Handling

25.1 The switchgear shall be shipped in the largest factory-assembled sections the delivery route and the installation pathway admit, and the Contractor shall verify the door, corridor, elevator, and turning dimensions between the delivery point and the installed location against the shipping section dimensions before the switchgear is released for manufacture.
25.2 Each shipping section shall be delivered in the manufacturer's packaging with the listing marks and the nameplates intact, and shall be inspected for shipping damage at receipt.
25.3 The switchgear shall be stored indoors in a clean, dry space where the temperature is held above the dew point, and where indoor storage is not available it shall be stored under cover on pallets, off the ground, and protected from condensation, precipitation, dust, and physical damage.
25.4 Where the switchgear will stand more than 30 days in an unconditioned space before energization, its space heaters, where furnished, shall be energized from a temporary source, or the structure shall otherwise be protected from condensation.
25.5 Drawout breakers shall be shipped in their compartments or in separate crates as the manufacturer directs, and a breaker shipped separately shall be stored with the switchgear and installed only after the structure is set.
25.6 Switchgear damaged or wetted in transit, in storage, or in handling shall be repaired or replaced as the manufacturer directs and as the Engineer of Record accepts in writing.

26 Warranty

26.1 The manufacturer shall warrant the switchgear, its bus, every breaker, every controller and relay, and every factory-installed device against defects in materials and workmanship for the period indicated in the datasheet.
Warranty Periodrange
years
1235
Default: Not less than 1 years
26.2 The warranty period shall commence on the date indicated in the datasheet.
Warranty Commencementselect
Date of substantial completion
Date of energization
Date of shipment
NOTE Warranty commencement at shipment is the manufacturer's usual term, and it consumes the warranty during storage and construction; commencement at energization or substantial completion moves the coverage to the period the Owner is operating the equipment, and the manufacturer prices the added months into the switchgear. (26.3)
26.4 The warranty coverage scope shall be as indicated in the datasheet.
Warranty Coverage Scoperadio
○ Parts only
○ Parts and labor
26.5 The warranty shall cover the operation of the sets and the switchgear as one system under the source arrangement selected, and a defect in the matching of the governor and regulator signals, the synchronizing, or the load sharing shall be the responsibility of the party named under Source Responsibility regardless of which maker built the set or the switchgear.
26.6 The Contractor shall warrant the installation, including every termination, the bus splices, the anchorage, the grounding, the control wiring between the sets and the switchgear, and the field-applied settings, for the project warranty period.
26.7 A repair or replacement performed under this warranty shall itself be warranted for a full new term equal to the original period from the date the repair is completed, or for the remainder of the original period, whichever ends later.
26.8 The emergency field service response time committed for the warranty period shall be as indicated in the datasheet.
Emergency Field Service Response Timerange
hours
48244872
26.9 Where the switchgear serves a system classified as NFPA 110 Level 1, the response time shall be coordinated with the facility's continuity-of-operations plan and with the response commitment made under GeneratorsEngine GeneratorsResolves to the current adopted revision.sync/generators.

27 Spare Parts

27.1 The Contractor shall deliver the spare parts indicated in the datasheet to the Owner before the switchgear is accepted, and shall obtain a signed receipt for them.
Spare Parts to Be Furnishedcheckbox
☑ One set of control fuses of each type and rating installed
☑ One set of indicating lamps or indicators of each type installed
☐ One per-set controller of the type installed, programmed
☐ One master controller, or one spare input and output module of each type, programmed
☐ One protective relay of each type installed, where discrete relays are furnished
☐ One sync-check relay, where a discrete relay is furnished
☐ One generator breaker of each frame and trip unit type installed
☐ One set of breaker close and trip coils of each type installed
☑ Two sets of keys for every lock and key interlock
☐ One pint of touch-up paint matching the finish
NOTE A spare controller that arrives from the factory with a generic program is not a spare until it carries the project program, which is why the spare controllers are listed as programmed and why the program backup is a closeout submittal. (27.2)
27.3 Spare parts shall be of the same manufacturer and rating as the installed items, shall be delivered in their original packaging, and shall be labeled with the switchgear designation and the section they serve.
27.4 Spare parts shall be stored in a labeled cabinet in the electrical room furnished by the Contractor, and shall not be stored inside an operating section of the switchgear.
27.5 The Owner's operating personnel shall be trained on the switchgear for the period indicated in the datasheet, covering automatic, manual, and test operation, manual synchronizing, the load priority table and the load shed sequence, the lockout reset, the maintenance bypass sequence where furnished, the operator interface, the settings record, the program backup and restore procedure, and the routine inspection and testing NFPA 110 Chapter 8 requires where it applies.
Operator Training Periodrange
hours
481624