Local and national codes will dictate which electrical systems are required to be served by an emergency power system. NFPA 101, Life Safety Code, and NFPA 99, Health Care Facilities, and NFPA 110, Emergency and Standby Power Systems, published by the National Fire Protection Association, contain specific definitions of required emergency power loads but, in general, they include the following:
Emergency systems, including emergency and egress (exit) lighting, essential ventilation systems, fire detection and alarm systems, elevators, fire pumps, public safety communications systems, and industrial processes where interruption could cause life safety risk. Power must be restored to these loads in not less than 10 s (or less, depending on local codes).
Legally-required standby systems, including heating and refrigeration systems, communications systems, ventilation and smoke removal systems, sewage disposal, lighting systems, and industrial processes, where interruption could create hazards or hamper rescue or fire-fighting operations. Power must be restored to these loads in not less than 60 s (or less, depending on local codes).
Optional standby systems, including heating and refrigeration systems, data processing and communications systems, and industrial processes, where interruption could cause discomfort, serious interruption of the process, damage to the product or process, or the like. Power restoration to these loads should occur, as determined by the engineer, in a period that will adequately protect the loads or process.
Small facilities may only need emergency power for emergency and egress lighting, in which case, fixtures with self-contained battery backup may be adequate.
Larger facilities generally require an engine generator to provide emergency power.
Emergency loads are connected to a dedicated panelboard or switchboard fed through an automatic transfer switch (ATS) that will detect loss of utility power, signal the generator to start, and transfer the emergency loads onto the generator, all in 10 s or less. After utility power returns, the ATS will retransfer the emergency loads back to utility power and then stop the generator. To prevent equipment damage and voltage surges, an ATS should be provided with an in-phase monitor that waits until the generator drifts into synchronism with the utility source before allowing retransfer.
Generator fuel source may be gasoline, LP gas, diesel fuel, or (if acceptable to the authority having jurisdiction) public utility gas. Local and federal environmental regulations should be consulted if it will be necessary to store liquid fuels. If an aboveground storage tank will be needed, it must be a double-walled tank equipped with electronic leak detection. A subbase, fuel-storage tank, that is an integral part of the generator frame, may also be used for liquid fuel.
Though normally used only for emergency power, the generator, if large enough, may be used to reduce electric bills through demand peak shaving or as a cogen erator. Opportunities for these possibilities should be reviewed with the utility company.
Where acceptable to the authority having jurisdiction, a second utility service may be used to provide emergency power. This approach should be evaluated carefully to ensure that the separate service is designed to minimize the possibility of simultaneous interruption of both services.