This application guide assists the protection engineer in applying relays for the protection of generating plant equipment from damage caused by operation at abnormal frequencies including overexcitation. Consideration is given to the effect of abnormal frequency operation on those associated station auxiliaries whose response can affect plant output. The guide also presents background information regarding the hazards caused by operating generation equipment at abnormal frequencies. It documents typical equipment capabilities and describes acceptable protective schemes. Underfrequency protection can be provided by load shedding and/or a discrete underfrequency protective function. If both load shedding and a discrete protective function are used, then they must be coordinated. Guidance is provided to help meet requirements from regional entities and regulatory bodies. The recommendations made pertain to typical synchronous generator installations but does not displace manufacturer guidance. The protective functions discussed in this guide may be implemented with a multifunction microprocessor based protection system.