The electric transmission and distribution systems require equipment to protect against a wide variety of electrical conditions that can damage or destroy system components. Circuit protection schemes rely on various interrelated protective equipment components. The schemes are engineered plans for disconnecting either sections of a circuit or a specific component in the event of a fault (an inadvertent electrical connection between an energized component and something at a different voltage including short circuits). Circuit protection schemes rely on various interrelated protective equipment components.
The main lines of defense from the high-energy conditions created by faults are fast-acting circuit breakers and fuses. These devices react quickly to high currents and disconnect the system before major damage is done. Some switches can also be used to similarly protect the system. Breakers are operated based on signals from relays that actuate the breaker to open given a specific system condition. Overcurrent relays may be installed to monitor the current on the system and automatically take the line out of service should currents rise to a level that would damage the conductor or other equipment. Over and under frequency relays are used to monitor the frequency of the system and shut down if frequency levels exceed strict limits.
Sometimes faults are transient, meaning that the cause of the fault only lasts for a short period of time. For instance a branch may fall across a line during a wind storm but then be blown off so it is no longer in contact with the line. In such cases it is desirable to restore service automatically. For this reason reclosing breakers, commonly called reclosers, are often used on distribution circuits. A recloser opens when a fault is detected, but after a period of time it recloses automatically. If the fault is no longer detected the recloser stays closed and customers have only a brief interruption. But if the fault is still detected then the recloser closes again. This cycle may be repeated three or four times before the breaker locks open, thus requiring a manual reset to restore service.