Differential Relay: Compares currents at two points; operates when there is a difference (used in transformers and generators). Types of Protective Relays: Protective relays are categorized by their mechanism (electromagnetic, static, mechanical) and function. A relay is a four-terminal electrical switch, used to control any electrical circuit with an independent low-power signal and also to control various electrical circuits with a single signal. The terminals of the relay mainly include; common, coil, NO (normally open) & NC (normally closed). Combines protection, sensors, control power, and circuit breaker in a single package Typically added to a breaker close circuit to prevent accidental reclosure after a trip. Three fundamental components required for each circuit breaker. First, relays were used as signal repeaters within long-distance. Overcurrent protection devices are not necessary for DC circuits.