This guide has been prepared to assist the protection engineer in applying relays for the protection of generating plant equipment from damage caused by operation at abnormal frequencies including overexcitation. Emphasis is placed on the protection of the major generating station components at steam generating stations, nuclear stations, and at combustion-turbine installations. Consideration is also 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 generating equipment at abnormal frequencies. It documents typical equipment capabilities and describes acceptable protective schemes. Recommended methods for coordinating the underfrequency protective scheme with system-load shielding schemes are also included. Sufficient information is provided to apply suitable coordinated protection for given specific situations.