Protection is the branch of electric power engineering concerned with the principles of design and operation of equipment (called "relays" or "protective relays") which detect abnormal power system conditions and initiate corrective action as quickly as possible in order to return the power system to its normal state. The quickness of response is an essential element of protective relaying systems—response times of the order of a few milliseconds are often required. Consequently, human intervention in the protection of system operation is not possible. The response must be automatic, quick, and should cause a minimum amount of disruption to the power system.

The Nature of Protection

In general, relays do not prevent damage to equipment; they operate after some detectable damage has already occurred. Their purpose is to limit, to the extent possible, further...