The concept of relay protection did not exist until the early 1900s. The rectangular devices are test connection blocks, used for testing and isolation of instrument transformer circuits. : 4 The first protective relays were electromagnetic. 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. CT's transform line current down to a signal level that is. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. While reliable, these relays.
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