Designing an emergency power supply system

According to Section 3.3.4 of NFPA 110, an EPSS is "a complete and functioning EPS system coupled to a system of conductors, disconnecting means and overcurrent protective devices, transfer switches and all control, supervisory and support devices up to and included the load terminals of the transfer equipment needed for the system to operate as a safe and reliable source of electric power." If there is a need for a centralized emergency system to provide power to life safety loads, such as emergency egress lighting, a third type of EPSS would be required to comply with Level 1 requirements of NFPA 110, NFPA 101 and Article 700 "Emergency Systems" of the NEC. For this situation, a Class 1.5, Type U, UL 924 rated uninterruptible power supply system located in a two-hour rated space would be an acceptable solution. Because the UPS provides continuous power to the emergency loads during loss of normal power, it meets the NEC Section 700.12 requirement, which states that on loss of normal power, power shall be available within the time required for the application but not to exceed 10 seconds. NFPA 110 identifies time delay devices within the automatic transfer scheme of the EPS to provide reliable power to the essential loads through properly transitioning the loads from one power source to the other.