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title: Uninterruptible Power Supply Systems
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description: >
When to use: Static (solid-state) uninterruptible power supply (UPS) systems that protect critical AC electrical loads — data processing and IT equipment, network and communications gear, life-safety and process controls, medical equipment, and other loads that cannot tolerate a power interruption or a sag — from utility disturbances, sags, surges, and outages. Covers single-phase and three-phase units of small (point-of-use), medium, and large (data center and facility) ratings; topology selection per IEC 62040-3 (double-conversion VFI, line-interactive VI, standby VFD); transformer-free and transformer-based designs; VRLA and lithium-ion battery energy storage with runtime sizing per IEEE 1184; internal static bypass and external maintenance (wraparound) bypass; redundancy from a single module through N+1, parallel, and dual-bus/2N; input harmonics and output performance for nonlinear loads; efficiency and energy-saver/eco modes; monitoring, communications, and emergency power off; seismic anchorage; and the room and code requirements the unit imposes.
− Not intended for: DC battery and charger plants that supply switchgear control power ([[syncs/dc-battery-systems]]); engine generators that provide the long-duration standby source behind a UPS ([[syncs/generators]]); automatic transfer switches that select between sources ([[syncs/automatic-transfer-switches]]); surge protective devices ([[syncs/surge-protective-devices]]); rotary and flywheel (kinetic energy storage) UPS, which are a separate technology; and stationary energy storage systems intended for load shifting, peak shaving, or grid support, which are specified to NFPA 855 and UL 9540 as energy storage rather than as a UPS.
+ Not intended for: DC battery and charger plants that supply switchgear control power ([[sync/dc-battery-systems]]); engine generators that provide the long-duration standby source behind a UPS ([[sync/generators]]); automatic transfer switches that select between sources ([[sync/automatic-transfer-switches]]); surge protective devices ([[sync/surge-protective-devices]]); rotary and flywheel (kinetic energy storage) UPS, which are a separate technology; and stationary energy storage systems intended for load shifting, peak shaving, or grid support, which are specified to NFPA 855 and UL 9540 as energy storage rather than as a UPS.
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## Related Standards {toc}
−### The generator or alternate source that backs the UPS for outages longer than the battery autonomy shall be coordinated with [[syncs/generators]].
+### The generator or alternate source that backs the UPS for outages longer than the battery autonomy shall be coordinated with [[sync/generators]].
−### The transfer of the UPS input between the normal source and the alternate source shall be coordinated with [[syncs/automatic-transfer-switches]].
+### The transfer of the UPS input between the normal source and the alternate source shall be coordinated with [[sync/automatic-transfer-switches]].
−### Surge protection ahead of the UPS input shall be coordinated with [[syncs/surge-protective-devices]].
+### Surge protection ahead of the UPS input shall be coordinated with [[sync/surge-protective-devices]].
−### Grounding and bonding of the UPS, its output neutral where the output is a separately derived system, and the battery enclosure shall be coordinated with [[syncs/grounding-and-bonding]].
+### Grounding and bonding of the UPS, its output neutral where the output is a separately derived system, and the battery enclosure shall be coordinated with [[sync/grounding-and-bonding]].
−### Input and output feeders, battery conductors, and their ampacity, voltage drop, and derating shall be coordinated with [[syncs/conductors-and-cables]].
+### Input and output feeders, battery conductors, and their ampacity, voltage drop, and derating shall be coordinated with [[sync/conductors-and-cables]].
−### The room or space that houses the UPS and its battery, including clearances, ventilation, and environmental control, shall be coordinated with [[syncs/electrical-rooms]].
+### The room or space that houses the UPS and its battery, including clearances, ventilation, and environmental control, shall be coordinated with [[sync/electrical-rooms]].
−### A DC control-power battery plant for switchgear is a different system from a UPS and shall be specified to [[syncs/dc-battery-systems]]. {note}
+### A DC control-power battery plant for switchgear is a different system from a UPS and shall be specified to [[sync/dc-battery-systems]]. {note}
# Referenced Standards {toc}
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### Transformer-free designs are the predominant modern construction, offering higher efficiency, lower weight, and a smaller footprint, while transformer-based designs provide galvanic isolation and a derived output neutral that some installations require for grounding or for fault isolation between input and output. {note}
−### Where the output is to be a separately derived system with its own neutral-to-ground bond, the need for an output isolation transformer shall be confirmed with the grounding design per [[syncs/grounding-and-bonding]].
+### Where the output is to be a separately derived system with its own neutral-to-ground bond, the need for an output isolation transformer shall be confirmed with the grounding design per [[sync/grounding-and-bonding]].
# Capacity and Load {toc}
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### A modern active power-factor-corrected rectifier holds input THDi below about 3 percent and input power factor near unity across the load range, which limits the harmonic burden the UPS places on the upstream system and on any generator; older rectifiers draw highly distorted current that can force upstream equipment and generators to be oversized. {note}
−### The input THDi limit shall be coordinated with the harmonic limits of [[syncs/generators]] for the generator that backs the UPS, and with IEEE 519 at the point of common coupling.
+### The input THDi limit shall be coordinated with the harmonic limits of [[sync/generators]] for the generator that backs the UPS, and with IEEE 519 at the point of common coupling.
## Input Power Factor {toc}
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### The rectifier shall include a soft-start (walk-in) that ramps the input current over a settable interval on source restoration, so that the simultaneous return of several UPS units does not overload the generator or trip the upstream breaker.
−### The walk-in interval shall be coordinated with the generator's load-acceptance capability per [[syncs/generators]].
+### The walk-in interval shall be coordinated with the generator's load-acceptance capability per [[sync/generators]].
# Energy Storage and Autonomy {toc}
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### The UPS shall provide the specified network communications interface(s) for monitoring and for orderly shutdown signaling to the protected equipment.
−### BACnet or Modbus points reported to the building management system shall be coordinated with [[syncs/electrical-rooms]] and the facility monitoring scope.
+### BACnet or Modbus points reported to the building management system shall be coordinated with [[sync/electrical-rooms]] and the facility monitoring scope.
## Dry-Contact Indications {toc}
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# Grounding {toc}
−## The UPS enclosure, the battery enclosure, and the input and output equipment grounding conductors shall be bonded in accordance with NFPA 70 and IEEE 1100, coordinated with [[syncs/grounding-and-bonding]]. {note}
+## The UPS enclosure, the battery enclosure, and the input and output equipment grounding conductors shall be bonded in accordance with NFPA 70 and IEEE 1100, coordinated with [[sync/grounding-and-bonding]]. {note}
## Where the UPS output is a separately derived system, the output neutral shall be bonded to ground at a single point and the grounding electrode connection shall be made in accordance with NFPA 70 for separately derived systems.
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## Heat Rejection {toc}
−### The UPS heat rejection at the expected operating load shall be coordinated with the room cooling so that the space stays within the UPS and battery rated temperature, per [[syncs/electrical-rooms]].
+### The UPS heat rejection at the expected operating load shall be coordinated with the room cooling so that the space stays within the UPS and battery rated temperature, per [[sync/electrical-rooms]].
### UPS losses appear as heat in the room, and a UPS that is efficient at full load still rejects meaningful heat at part load; the room cooling shall be sized to the actual heat rejection, not assumed negligible. {note}
## Conductor Connections {toc}
−### Input, output, bypass, and battery conductors shall be sized for ampacity, voltage drop, and the applicable derating, and terminated per the manufacturer's instructions, coordinated with [[syncs/conductors-and-cables]].
+### Input, output, bypass, and battery conductors shall be sized for ampacity, voltage drop, and the applicable derating, and terminated per the manufacturer's instructions, coordinated with [[sync/conductors-and-cables]].
### The output neutral conductor of a three-phase UPS serving line-to-neutral nonlinear loads shall be sized for the harmonic neutral current, which can exceed the phase current.
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### Where the UPS is backed by a generator, an integrated test shall demonstrate the full sequence: loss of normal power, transfer to UPS battery, generator start and stabilization, transfer of the UPS input to the generator, and return to normal, with the critical load energized throughout.
−### The integrated test shall confirm that the generator accepts the UPS rectifier load, including the rectifier walk-in, without instability, coordinated with [[syncs/generators]] and [[syncs/automatic-transfer-switches]].
+### The integrated test shall confirm that the generator accepts the UPS rectifier load, including the rectifier walk-in, without instability, coordinated with [[sync/generators]] and [[sync/automatic-transfer-switches]].
# Delivery, Storage, and Handling {toc}
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## A maintenance and replacement contract should be considered for mission-critical UPS systems, because the battery is a consumable with a defined service life and the system's value depends on the battery being replaced before it fails an in-service discharge. {note}