Single or poly-phase network protection cct. - has storage capacitor between two triggers to giue hysteresis after leuel drops to safe

摘要

The protection circuit, for single or polyphase networks, has a filter inductively coupled to the phase line and producing a given output voltage in response to a short circuit current a.c. components in the phase line. A first trigger operates when this voltage exceeds a given level and charges a storage capacitor. The capacitor discharges at a given rate when the first trigger is reset by the voltage dropping below the given level. A second trigger responds to the capacitor charge state and its output acts as a criterion for switching the protective relay. Because of the capacitor the second trigger has a certain amount of hysteresis and its safe criterion is only released a given time after the first trigger has been reset.