This specification covers metal-enclosed, low voltage power circuit breaker switchgear assemblies rated 600V and below. Equipment shall comply with UL 1558 and IEEE C37.20.1 and shall be listed and labeled by a Nationally Recognized Testing Laboratory (NRTL).
Switchgear covered by this standard utilizes individually mounted drawout power circuit breakers as the primary switching and protective devices. For applications using molded case or insulated case breakers in a non-drawout configuration, see Low Voltage Switchboards.
For medium voltage switchgear rated above 600V, see Medium Voltage Switchgear. Grounding and bonding per Grounding And Bonding.
Equipment and installation shall comply with the latest edition of the following. Where conflicts exist between referenced standards, the more stringent requirement shall govern unless otherwise directed by the Engineer of Record.
| Standard | Title |
|---|---|
| UL 1558 | Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear |
| IEEE C37.20.1 | Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear |
| IEEE C37.13 | Low-Voltage AC Power Circuit Breakers Used in Enclosures |
| IEEE C37.20.7 | Arc-Resistance Testing of Metal-Enclosed Switchgear |
| IEEE C37.51 | Conformance Testing of Metal-Enclosed LV AC Power Circuit Breaker Switchgear |
| IEEE C37.90 | Relays and Relay Systems Associated with Electric Power Apparatus |
| IEEE C57.13 | Requirements for Instrument Transformers |
| IEEE 1584 | Guide for Performing Arc-Flash Hazard Calculations |
| NFPA 70 | National Electrical Code |
| NEMA ICS 6 | Industrial Control and Systems: Enclosures |
| NETA ATS | Acceptance Testing Specifications for Electrical Power Equipment and Systems |
| ASCE 7 | Minimum Design Loads and Associated Criteria for Buildings |
| ASHRAE 90.1 | Energy Standard for Buildings (submetering requirements) |
| ANSI/NEMA SG 3 | Low-Voltage Power Circuit Breakers |
Contractor shall submit the following for review prior to fabrication:
Contractor shall provide at substantial completion:
Switchgear shall be manufactured by a single company responsible for the assembly, bus, and all integral protective devices. The manufacturer shall have a minimum of five years documented experience producing UL 1558 listed switchgear and shall maintain an ISO 9001 certified quality management system.
The manufacturer shall maintain a 24/7 service organization with factory-trained field representatives available within 24 hours of notification.
All switchgear sections, bus, circuit breakers, protective relays, and metering equipment shall be produced or furnished by the switchgear manufacturer as an integrated assembly. Third-party components assembled by others are not acceptable.
Acceptance testing shall be performed by a firm regularly engaged in testing electrical power equipment, employing technicians certified by NETA or equivalent. Testing personnel shall have a minimum of three years experience testing low voltage switchgear assemblies.
Switchgear shall be suitable for continuous operation under the following ambient conditions. Where site conditions exceed these parameters, notify the manufacturer and derate equipment accordingly.
Equipment rated for standard conditions (40°C, below 6,600 ft) shall be derated per IEEE C37.20.1 when installed outside these parameters. The Engineer shall document derating calculations on the contract drawings.
For installations classified C4 or C5, bus bars shall be copper (tin or silver-plated), and enclosure paint systems shall provide enhanced corrosion protection per IEEE C57.12.29. See Electrical Rooms for room environmental classification requirements.
Where required by the applicable building code, switchgear shall be seismically certified by shake-table testing per ICC ES AC156 or by analysis per ASCE 7.
Seismic certification shall be by an independent third-party testing laboratory. Certification shall cover the complete assembly including all circuit breakers, bus, and ancillary components as installed. Certification of individual components in isolation is not acceptable.
Where switchgear serves as main service entrance equipment, it shall comply with NFPA 70 Article 230 including provisions for metering, service disconnect, and available fault current labeling. Coordinate metering requirements with the serving utility prior to submittal.
Main bus continuous current rating shall be as indicated on the one-line diagram. Bus shall be braced for the available short-circuit current at the point of installation as determined by a short-circuit analysis per IEEE 551.
Short-circuit current rating shall equal or exceed the available fault current at the switchgear terminals. Verify available fault current with the utility provider and perform a short-circuit study per Short Circuit Study prior to equipment procurement.
Oversized neutral bus is required where nonlinear loads exceed 30% of the total connected load to accommodate triplen harmonic currents. The Engineer shall evaluate harmonic loading per IEEE 519 when selecting neutral bus sizing.
Ground bus shall be bare copper, minimum 1/4 in. x 2 in. cross-section, extending the full length of the switchgear assembly. Ground bus shall be accessible without removing any covers or barriers required for safe operation.
Enclosure shall be fabricated from galvanized steel with a minimum thickness of 12 gauge (2.66 mm) for structural members and 14 gauge (1.90 mm) for covers and doors. All joints shall be continuously welded or bolted with gaskets as required for the specified NEMA rating.
For outdoor installations (NEMA 3R), enclosure shall include sun shields, rain hoods over ventilation openings, and condensation heaters in each section. Heater sizing shall be a minimum of 250W per vertical section. Heaters shall be thermostat controlled and wired to remain energized whenever the switchgear is de-energized.
Rear-accessible configurations require minimum 36 in. working clearance at rear per NFPA 70 Article 110.26. Rear compartments shall have individual hinged doors with padlockable latches.
Switchgear overall dimensions and arrangement shall be per the equipment room layout drawings.
Bus bar joints shall be bolted with Belleville washers to maintain contact pressure under thermal cycling. All joint contact surfaces shall be silver-plated. Bus connections shall be designed to be accessible for re-torquing during maintenance without de-energizing adjacent sections.
Phase bus shall be fully insulated with flame-retardant material rated for the operating temperature of the bus. Bus insulation shall be continuous through barriers and bus transitions between sections.
Switchgear shall provide metal barriers between all functional units and between the bus compartment and cable compartment. Barriers shall prevent propagation of an arc fault from one compartment to adjacent compartments.
Individual circuit breaker compartments shall allow insertion, withdrawal, and testing of any breaker without exposing personnel to energized components in adjacent compartments. Interlocks shall prevent opening compartment doors while the breaker is in the connected position.
Where arc-resistant construction is specified, switchgear shall be tested per IEEE C37.20.7 at the specified short-circuit current rating and duration. Arc-resistant design shall redirect arc gases through plenums or ducts to areas where personnel are not present. Pressure relief flaps and reinforced enclosure construction shall contain the arc event.
Arc-resistant construction does not eliminate the need for arc flash hazard analysis per IEEE 1584. See Arc Flash Study for arc flash study requirements and labeling.
Enclosure shall include removable infrared inspection windows or cover plates at each bus joint, main breaker connections, and feeder breaker connections. Windows shall allow thermographic inspection of energized connections without removing covers or de-energizing equipment.
Circuit breakers shall comply with IEEE C37.13 and shall be drawout, stored-energy type with manual charging and electrical close/trip capability. Each breaker shall have three distinct positions: connected, test, and disconnected. It shall not be possible to move a breaker between positions with the door closed unless motor operators are provided.
Main breaker frame size and trip rating shall be as indicated on the one-line diagram.
LSIG trip units are recommended for all applications. Ground fault protection is required by NFPA 70 Article 230.95 for solidly grounded wye services of more than 150V to ground and 1000A or more. Even where not code-required, ground fault protection significantly reduces equipment damage from arcing faults.
NFPA 70 Article 240.87 requires that circuit breakers rated 1200A or more have one of the following arc energy reduction methods. Specify the method appropriate for the project's operating requirements.
Zone-selective interlocking allows upstream breakers to delay tripping only when a downstream breaker detects the fault, reducing arc flash energy at the upstream equipment while maintaining coordination. This is the most common method and does not require any mode switching for maintenance activities.
Feeder breaker sizes, quantities, and trip ratings are as indicated on the one-line diagram and panel/breaker schedules.
Auxiliary contacts shall be provided on every breaker regardless of other accessory selections. Contacts shall indicate breaker open/close status and shall be wired to terminal blocks accessible without removing the breaker from its compartment.
All breakers shall be fully drawout and interchangeable within a given frame size. A manual racking mechanism shall allow movement between connected, test, and disconnected positions. The racking mechanism shall include a mechanical position indicator visible from the front of the switchgear and a means to padlock the breaker in any of the three positions.
Self-aligning primary disconnects shall engage automatically when the breaker is racked to the connected position. Secondary disconnects for control wiring shall engage at the test position. It shall be possible to operate the breaker electrically in the test position for trip testing without energizing the primary circuit.
Protective relays shall comply with IEEE C37.90 and shall be microprocessor-based with digital displays, self-diagnostics, and event recording. Relays shall be tested per IEEE C37.90.1 for surge withstand capability.
Protection coordination study shall be performed per Protective Coordination Study. Relay settings are as indicated on the relay setting schedule.
Current transformers shall comply with IEEE C57.13. Accuracy class shall be appropriate for the connected metering:
Current transformers for metering and relaying shall be provided on separate cores. Do not share CTs between metering and protection functions.
Where building energy code requires submetering (ASHRAE 90.1 Section 8.4.3 or local amendments), meter data shall be accessible via the selected communication protocol for integration with the building management system. Coordinate communication wiring and protocols with Building Automation.
Enclosure shall receive a minimum two-coat paint system: corrosion-resistant primer and manufacturer's standard polyester powder coat finish. Minimum total dry film thickness shall be 3 mils (75 microns).
For outdoor installations or environments classified C3 or higher, an enhanced paint system with a minimum of 5 mils dry film thickness shall be applied. All internal structural members shall receive the same corrosion treatment as exterior surfaces.
Manufacturer shall provide engraved phenolic nameplates for the switchgear assembly and each individual device. Nameplates shall identify:
Mimic bus diagram shall be provided on the front of the switchgear showing the bus configuration, breaker positions, and instrument transformer locations. Equipment labeling shall follow Equipment Labeling conventions.
The manufacturer shall perform the following production tests on the completed switchgear assembly per IEEE C37.51:
Where witnessed factory testing is specified, the manufacturer shall provide a minimum of two weeks advance notice of test readiness. Test procedures shall be submitted for review prior to testing.
Contractor shall engage a qualified independent testing firm to perform acceptance testing per NETA ATS. Testing shall occur after installation is complete and before the equipment is energized.
Field acceptance tests shall include as a minimum:
Infrared scanning shall be performed under normal operating load conditions (minimum 40% of rated load). All connections exceeding 10°C rise above ambient shall be reported and corrected. Follow-up scan at 11 months captures connections that may loosen during the initial thermal cycling period.
Switchgear shall be mounted on a reinforced concrete housekeeping pad extending a minimum of 3 in. beyond the base of the equipment on all sides. Pad shall be a minimum of 4 in. above finished floor for indoor installations and 6 in. above finished grade for outdoor installations.
Coordinate pad dimensions, conduit penetrations, and anchor bolt locations with equipment shop drawings prior to concrete placement. See Concrete Pads for construction requirements.
Contractor shall comply with manufacturer's installation instructions and applicable rigging requirements. Remove all temporary shipping braces, blocking, and desiccants prior to final assembly. Verify section alignment and bolt all sections together per manufacturer's torque specifications.
After assembly, verify the following before energizing:
Minimum working space shall be maintained per NFPA 70 Article 110.26 based on the nominal voltage and accessibility configuration:
| Voltage | Condition 1 (exposed on one side) | Condition 2 (exposed on both sides) |
|---|---|---|
| 0-150V | 36 in. | 36 in. |
| 151-600V | 36 in. | 48 in. |
Working space shall not be used for storage. Dedicated electrical space above and below the equipment shall be maintained per NFPA 70 Article 110.26(E).
Switchgear shall be shipped in the largest factory-assembled sections that can be transported to and within the installation site. Verify all pathway dimensions (doors, hallways, elevator shafts) between the delivery point and the final installation location prior to ordering.
Equipment shall be stored indoors in a clean, dry location. Where indoor storage is not available, the manufacturer shall provide weatherproof packaging. Condensation heaters shall be connected and energized during storage if the equipment will be stored for more than 30 days or if the storage environment is not climate-controlled.
Warranty shall cover defects in materials and workmanship under normal use and service conditions. The manufacturer shall maintain a service organization capable of providing emergency replacement parts and field service within 24 hours during the warranty period.
Spare breakers shall be of the same type, rating, and configuration as the installed breakers and shall be fully interchangeable. Spare breakers shall be stored in a manufacturer-provided storage cabinet or on a breaker storage truck, located in the electrical room.
Manufacturer shall provide the following additional spare parts: