Standby Power Systems, based on the 2023 NEC

Figure 01

Note: This article is based on the 2023 NEC.

Standby power systems kick in when the normal power source is interrupted. But which type do you have?

The NEC recognizes three types of standby power systems:

• Emergency systems [700]. These are generally used to provide power so occupants can safely exit in the event of fire or other disaster. Other uses include providing ventilation essential to maintain life and running fire pumps.
• Legally required [701]. A government agency requires the facility to have a standby power system, usually due to public safety concerns.
• Optional [702]. This is a backup the facility owner/operator chooses to have, usually due to financial concerns.
  

The requirements for each differ, and a single facility could have all three types. Let's look more closely at legally required systems.

Legally required
Article 701 covers the installation, operation, and maintenance of legally required standby systems consisting of circuits and equipment intended to supply illumination or power when the normal electrical supply is interrupted. The goal is to aid in firefighting, rescue operations, control of health hazards, and similar operations. Figure 01

Legally required standby systems typically supply loads such as heating and refrigeration systems, ventilation and smoke removal systems, sewage disposal, lighting systems, and industrial processes that, when stopped, could create hazards or hamper rescue or firefighting operations [701.1 Note 4].

Commissioning and maintenance
To ensure the legally required standby system meets or exceeds the original installation specifications, the authority having jurisdiction must conduct or witness the commissioning of the legally required standby system upon completion of the installation [701.3(A)].

Legally required standby systems must be periodically tested in a manner approved by the authority having jurisdiction to ensure adequate maintenance has been performed and the systems are in proper operating condition [701.3(B)]. Running the legally required standby system under the loads of the facility to make sure power transfers within 60 seconds is often considered an acceptable method of operational testing.

Legally required standby system equipment must be maintained per the manufacturer's instructions and industry standards [701.3(C)]. Part of the NEC-required maintenance is performing OCPD maintenance (lubrication, testing, etc.) per the applicable standard. This will greatly reduce the chances of malfunction that can result in massive legal and financial liability. This maintenance must be performed by qualified personnel, which usually means outsourcing the work to a qualified testing firm that has both the personnel and the equipment.

A written record must be kept of all required tests and maintenance [701.3(D)]. The NEC does not specify the required record retention period, but it is typical to keep such records in a CMMS where they can reside indefinitely if desired.

Capacity and rating
Equipment for legally required standby system must be suitable for the available fault current at its terminals [701.4(A)].

The alternate power supply must have adequate capacity per Parts I through IV of Article 220 or by another approved method. The system capacity must be sufficient for the rapid load changes, and transient power and energy requirements associated with any expected loads [701.4(B)].

The legally required standby alternate power supply can supply legally required standby and (also) optional standby system loads if there is adequate capacity (for the extra loads), or where a load management system includes automatic selective load pickup and load shedding; this is to ensure adequate power to the legally required standby system circuits [701.4(C)].

Transfer switches
Transfer switches must be automatic, listed, and marked for emergency system or legally required standby system use. Meter-mounted transfer switches are not permitted for legally required standby system use [701.5(A)]. Automatic transfer switches must be able to be electrically operated and mechanically held [701.5(C)]. The short-circuit current rating of the transfer switch must be field marked on the exterior of the transfer switch [701.5(D)]

Signals and signs
Audible and visual signal devices must be installed where practicable to indicate [701.6(A) through (D)]:

• A malfunction of the standby source of power.
• Whether the standby source is carrying load.
• If the battery charger is not functioning.
• A ground fault in a 4-wire, three-phase, 277/480V wye-connected system rated 1000A or more.
  

A sign must be placed at the service-entrance equipment indicating the type and location of on-site legally required standby power systems [701.7].

Circuit wiring
Unlike wiring for emergency systems, which must be kept entirely independent of other wiring, the wiring for legally required standby systems may be installed with other (normal) wiring because legally required standby system loads are not essential for life safety [701.10].

Power sources
If the normal supply fails, legally required standby power must be available within 60 seconds [701.12]. The supply system for the legally required standby system power supply can be one or more of the types of systems described in 701.12 (A) through (I)].

(A) Power Source Considerations. In selecting a legally required standby source of power, consideration must be given to the type of service to be rendered, whether of short-time duration or long duration.

(B) Equipment Design and Location. Consideration must be given to the location or design, or both, of all equipment to minimize the hazards that might cause complete failure due to floods, fires, icing, and vandalism.

Note: For further information, see ANSI/IEEE 493, Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems.

(D) Generator Set.
(1) Prime Mover-Driven. A generator approved by the authority having jurisdiction and sized per 701.4 is permitted as the legally required power source if it has the means to automatically start the prime mover on failure of the normal power source.

(E) Stored-Energy Power Supply Systems.
(1) Types. Stored-energy power supply systems must consist of one the following types:
(1) Uninterruptible power supply
(2) Fuel cell system
(3) Energy storage system
(4) Storage battery
(5) Other approved equivalent stored energy sources that comply with 701.12

(F) Separate Service. An additional service is permitted as the legally required power source where approved by the authority having jurisdiction [230.2(A)] and the following additional requirements:
(1) Separate service conductors are installed from the electric utility.
(2) The legally required service conductors must be electrically and physically remote from other service conductors to minimize the possibility of simultaneous interruption of supply.

To minimize the possibility of simultaneous interruption, the service disconnect for the legally required power system must be remotely located from the other power system's service disconnect [230.72(B)].

(G) Connection Ahead of Service Disconnecting Means. If approved by the authority having jurisdiction, connection ahead of (but not within) the same cabinet, enclosure, or vertical switchboard or switchgear section is permitted as the legally required power source. See 230.82(5) for additional information.

(H) Microgrid Systems. The system shall isolate the legally required standby system from other loads when the normal electric supply is interrupted or it must meet the requirements of 701.4(C).

(I)Battery-Equipped Emergency Luminaires. If used with legally required standby systems, they must comply with 701.12(H). This requirement is new with the 2023 revision.

This list in the 2023 revision differs from the one in the 2020 revision. For example, (J) Unit Equipment was dropped from the list. There was also some renumbering and some editorial change such as DC Microgrid Systems is now Microgrid Systems.

Overcurrent protection
The branch-circuit overcurrent protective devices (OCPDs) for legally required standby circuits must be accessible to authorized persons only [701.30]. Some facility managers consider the job done if they put a sign on the door, but accessibility must be restricted by something more substantial such as a lock on the door.

OCPDs for legally required standby systems must be selectively coordinated with all supply-side and load-side OCPDs. The design must be made by an engineer or similarly qualified person and it must be documented and made available to those authorized to design, install, inspect, maintain, and operate the system [701.32(A)].

Selective Coordination means the overcurrent protection scheme confines the interruption to a specific area rather than to the whole system [Article 100]. For example, if a short circuit or ground fault occurs with selective coordination, the only breaker/fuse that will open is the one protecting just the branch circuit involved. Without selective coordination, an entire floor of a building can go dark.

If legally required standby system OCPDs are replaced, they must be re-evaluated to ensure selective coordination is maintained [701.32(B)].

If modifications, additions, or deletions to the legally required standby systems occur, selective coordination is required of the legally required system(s) [701.32(C)]. Note: See the NEC Note Figure 701.32(C) for an example of how legally required standby system OCPDs (OCPDs) selectively coordinate with all supply-side OCPDs.

Avoiding problems
Be careful to not mix the requirements for the different types of standby power systems. They serve different purposes and thus the NEC requirements differ (which is why we have three standby power system articles not just one). However, the NEC is providing the minimum requirements [90.2] not the optimal requirements. Nothing stops you from using selective coordination in an optional standby system even though that's required only for emergency and legally required systems.

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