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Temporary Backup Power for Critical Operations Power Systems

When this document was written, Hurricane Laura had just traversed the southern USA. The storm caused extensive damages, a sobering reminder that facilities providing emergency response services are needed most just when they are most likely to experience utility power outages. This article surveys certain requirements to assure that backup power can be available for facilities that support public safety and security even when equipment is taken off line for service.

COPS Requirement Summary

Article 708 of the National Electrical Code® (NEC®) sets forth requirements for Critical Operations Power Systems (COPS). These are power systems in facilities needed to maintain public safety or national security. Air traffic control towers and police, fire, and ambulance stations are just a few examples of the facilities affected by Article 708.

In addition to other NEC requirements for Life-Safety, Legally Required and Optional loads (respectively contained in Articles 700, 701, and 702), Article 708 sets forth certain additional requirements for COPS. These include completing a risk assessment of hazards that could impact operations as well as the development of a plan to mitigate those risks, plus requirements for performing and documenting maintenance and service. Mitigating the risks and effects of electrical power outages is central to providing continuous safety and security services. Figure 1 below shows a configuration for a backup power system at a COPS facility.
Maintaining Backup Capability During Service Events

Backup power equipment needs periodic service to ensure equipment condition and readiness for use. This typically requires taking equipment such as generators and transfer switches out-of-service. Where that equipment is the only pathway to supplying COPS loads, a service event would make backup power unavailable for mission-critical functions until the equipment is returned to service.

If a utility power outage occurred during a service event, then a facility will be without electrical power needed to fulfil its mission until either the utility service or the backup power system is functionally restored. When such outages occur at an ordinary business, such as at a commercial dry goods warehouse during a severe storm, the impact to the public is typically indirect. If the same outage occurs at a COPS facility, it could interrupt essential operations that place the health, safety, and security of people at risk.

In the figure above, a facility’s COPS loads are served by a single generator and a single automatic transfer switch. Both units require periodic maintenance that will render them unavailable during their respective service events. Either unit could also require unscheduled repairs that produce the same circumstance.

Requirement for Bypass Transfer Switches at COPS Facilities

The design of Bypass Isolation Automatic Transfer Switches enables operators to direct current through a secondary transfer mechanism while the primary mechanism is isolated for inspection, service, and repair. When placed in bypass mode, transfer functionality is maintained during service events. If a utility outage occurs during equipment service, loads can still be transferred to and served by an alternate power source. A diagram of a bypass isolation transfer switch is shown in Figure 2.
Because of the critical nature of their missions, COPS loads served by a single transfer switch must use a bypass isolation transfer switch. This need is reflected in Article 708 of the 2020 NEC:

708.24(D) Bypass Isolation Automatic Transfer Switches.

Where loads are supplied by only one automatic transfer switch, the automatic transfer switch shall include a bypass isolation switch to facilitate maintenance as required in 708.6(C) without jeopardizing continuity of power. When the bypass isolation transfer switch is in the bypass mode, either it shall automatically initiate transfer between power sources upon loss of the connected power source or it shall remain actively supervised by a qualified person who can manually initiate a transfer between power sources.

Requirement for a Temporary Power Source at COPS Facilities

Generators also require periodic maintenance that makes them unavailable during service events. For example, if a facility regulated by NEC Article 700 provides backup power to life safety loads from a single generator, connections for a temporary power source must be provided according to Article 700.3(F):

700.3(F) Temporary Source of Power for Maintenance or Repair of the Alternate Source of Power.

If the emergency system relies on a single alternate source of power, which will be disabled for maintenance or repair, the emergency system shall include permanent switching means to connect a portable or temporary alternate source of power, which shall be available for the duration of the maintenance or repair.

Similarly, COPS loads require a “backup-for-the-backup”. If a COPS facility cannot be supplied with sufficient electrical power when one of its generators (often its only generator) is taken offline for service, essential operations would be disrupted, placing the health, safety or security of people at risk. For this reason, NEC Article 708 requires providing COPS with capability to provide adequate backup power if an outage were to occur during generator service:

708.20(F)(6) Means for Connecting Portable or Vehicle-Mounted Generator.

Where the COPS is supplied by a single generator, a means to connect a portable or vehicle-mounted generator shall be provided.

Article 708 effectively requires that facilities have either (1) sufficient redundant generators to supply COPS loads with one generator offline, or (2) a means of temporarily connecting sufficient generation capacity. The latter can be accomplished by equipping backup power systems with an additional manual transfer switch and provisions for temporary connection. Connectors can be incorporated either into a dedicated panel or integrated with a manual transfer switch. An example configuration is shown within Figure 1. An ASCO Power Technologies Manual Transfer Switch with Integrated Quick Connects is shown in Figure 3.
The cost of this solution is typically lower than providing a permanent redundant generator.


Mitigating the risks and effects of electrical power outages is central to providing continuous safety and security services. NEC Article 708 sets forth requirements for COPS in facilities that support public safety or national security, including a requirements to ensure that backup power is available when backup power equipment is taken offline for service. Using Bypass Isolation Transfer Switches provides concurrently maintainability during transfer switch service events. Adding a manual transfer switch and an integrated or separate connection panel for a temporary or portable generator provides concurrent maintainability for backup power sources. For additional information, contact an ASCO Power Technologies representative.

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