Capacitor voltage and trip coil impedance sensor with high voltage

Abstract

A sensing circuit is disclosed for simultaneously and continuously monitoring the impedance of a trip coil of the type used to actuate synchronously operated circuit breakers, and the charge on a capacitor used to power the trip coil. The sensor comprises a circuit including the trip coil, the capacitor, a large resistance and a photo-diode connected together in series. A d.c. voltage is continuously applied across the capacitor, maintaining it charged and causing a constant direct current to flow through the other three components. The magnitude of the current will fall below a predetermined threshold value if either the capacitor voltage, which is proportional to the charge stored on the capacitor, falls below a certain level or the resistance of the trip coil rises above a certain level. The photo-diode produces a light signal representative of the current therethrough. The light signal is conveyed by an electrically insulative but optically transmissive medium, preferably an optical fiber bundle, to a circuit at ground potential which produces an indication of the magnitude of the light signal. Preferably the latter circuit comprises a relay which is closed when the photo-diode current is above the threshold level and open whenever the photo-diode current falls below the threshold level, indicating a fault in either the capacitor or the trip coil.