Coordinating Overcurrent Protective Devices with Automatic Transfer Switches in Commercial and Institutional Power Systems

Emergency and standby power is fast becoming a major consideration in commercial and institutional buildings and complexes. In fact, emergency power is a National Electric Code mandatory requirement for health care facilities and places of assembly. Transfer switches are a vital link in the portion of a distribution system that must provide power and illumination essential to life and property. Overcurrent protection is of equal importance. It is the purpose of this paper to objectively define and analyze important transfer switch functions and application criteria. Of these, the coordination of overcurrent protective devices to match the transfer switch ratings, under fault conditions, is perhaps the most important application aspect in providing reliable operation of a standby or emergency power system.