When a fault occurs, current increases and voltage decreases. The increased current causes excessive heating, which depending on where the fault occurs, can result in a fire or an explosion. If the fault is not quickly isolated, it can cause damage that may result in loss of service. Various types of control systems are used to detect and isolate faults with minimum disturbance. A key component of all of these control systems is the protective relay. This course examines the functions and operation of some types of protective relays.