The ability of engine-powered generators to start and supply standby power of acceptable frequency and voltage within 10 seconds of a utility outage has made these generators the standby power system of choice for a wide variety of facilities including mission critical. However, the rules defining these starting requirements are complex and often dependent on the specific standby power application. This has led to uncertainty in the industry regarding what is included in those crucial 10 seconds. Following are the details that system designers and consulting engineers need to know about the NFPA 110 (and the related NFPA 70) and how it applies to emergency standby generator applications.
NFPA 110 defines performance requirements for emergency and standby power systems that provide an alternate source of electrical power to facility loads in the event that the primary power source fails. As part of these requirements, standby power resources are also segmented by “type” (see Table 4.1b). The type refers to the maximum time, in seconds, that the Emergency Power Supply System (EPSS) will permit the load terminals of the transfer switch to be without acceptable electrical power. For example, an EPSS designated as Type 10 will not permit the load terminals of the transfer switch to be without acceptable power for more than 10 seconds.