在电气工程中，保护继电器是设计用来当检测到故障时，跳脱断路器达到保护特定设施的装置，特定设施可以是发电机、变压器、输电线路等等。

ANSI Device Numbers列出不同功能的电驿代号，举例来说，一个87L电驿代表这是一个输电线路差流电驿，87代表差流电驿，L代表保护标的是输电线路。

这种电驿的工作原理是基于克希荷夫电流定律，此定律是说，流进与流出某一个节点的电流总和为零。差流保护需要在输电线路的两端或每一个变压器端设置比流器，差流电驿透过比流器取得流经量端的电流并计算两边的差异。

In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, over-voltage, reverse power flow, over-frequency, and under-frequency. Microprocessor-based digital protection relays now emulate the original devices, as well as providing types of protection and supervision impractical with electromechanical relays. Electromechanical relays provide only rudimentary indications of involved phase and zone targets. In many cases a single microprocessor relay provides functions that would take two or more electromechanical devices. By combining several functions in one case, numerical relays also save capital cost and maintenance cost over electromechanical relays. However, due to their very long life span, tens of thousands of these "silent sentinels" are still protecting transmission lines and electrical apparatus all over the world. Important transmission lines and generators have cubicles dedicated to protection, with many individual electromechanical devices, or one or two microprocessor relays.