EARTH CURRENT MEASURING CIRCUIT FOR A 3 POLE AIR CIRCUIT BREAKER

An earth current measuring circuit of a 3-pole air circuit breaker is provided to prevent a measurement error generated when phase difference between neutral-pole measurement current and measurement current of other three phases is changed through an NCT(Neutral Current Transformer) and a rogowski coil. In an earth current measuring circuit of a 3-pole air circuit breaker(51), rogowski coils are installed at three-phase lines of the 3-pole air circuit breaker, respectively to provide voltage output proportional to current flowing through the three-phase lines, as a three-phase measurement current signal. An external NCT is installed at an external neutral line of the 3-pole air circuit breaker to provide the voltage output proportional to neutral-pole current flowing in the external neutral line, as a neutral-pole measurement current signal transformed. An internal NCT(51a-1) is installed in the 3-pole air circuit breaker to provide a neutral-pole measurement current signal transformed by transforming output voltage transmitted from the external NCT into lower proportional output voltage. A time delay circuit(51a-2) connected to an output stage delays the phase of the transformed neutral-pole measurement current signal output from the internal NCT correspondently to the predetermined time to return phase difference, among the neutral-pole measurement current signal and the measurement current signals of other three phases(R,S,T) after transforming for current detection, into phase difference output before transforming.