P12x/EN T/Eb6 - Schneider - error

Application GuideP12x/EN AP/Eb6MiCOM P120/P121/P122/P123 Page 27/78Case n°2 → the relay supervising one of the feeders is faulty:In this case, the watchdog contact of this relay is closed.Thus, for a phase fault on the busbar, thresholds tI>> and tI>>> activate their associatedoutput contact. However, threshold tI>> will clear the fault as this threshold has been set to alower value than the threshold tI>>>.For a phase fault on one of the ‘healthy’ feeders, thresholds tI>> and tI>>> of the relaylocated on the incomer being selectively set to higher values than the ones set for the phasethresholds of the downstream relays, the fault shall be cleared selectively by the relay of thefaulty feeder (selectivity between the relay of the incomer and relays of the feeders isensured thanks to intervals of selectivity correctly chosen or thanks to a suitable blockingdiagram).For a phase fault on the feeder of the failed relay, the threshold tI>> of the relay located onthe incomer operates via the watchdog contact of the faulty relay on the trip coil of the circuitbreaker of the faulty feeder. This threshold being selectively set to a value lower than thethreshold tI>>> (which operates directly on the coil of the incomer circuit breaker ), the faultis therefore selectively cleared.