If the relay loses control power (or, in some cases, fails its self-test) the contacts revert to shelf state and will trip the protected devices. Some relays have jumper select-able NO/NC output contacts. The protection relay tripping circuit refers to the critical electrical control loop that executes trip/close commands from protective relays to circuit breakers, ensuring rapid fault isolation in power systems. This system integrates protection logic with breaker control functions. The application varies from one manufacturer to the next, but many relays offer a "Fail-safe" mode, wherein a contact which must close to perform a trip function is held open by control power and absence of trip condition. Motor relays are especially notorious for having failsafe logic. DC power is used because it allows for a battery bank to supply close/trip power to the breaker control circuits in the event of a complete (AC) power failure.
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