An apparatus for detecting arcing fault in an ac electrical system

摘要

Arcing faults in an ac current are detected by apparatus (45, 61, 191) which is responsive to a predetermined randomness in step increases in the current to eliminate false tripping caused by regularly occurring discontinuities produced by loads such as dimmers. A signal conditioner (49, 69) generates a bandwidth limited di/dt signal having pulses produced by the step increases in current. In one embodiment, a first tracking circuit (53) tracks the envelope of the di/dt signal with a first time constant. A second tracking circuit (51) also tracks the di/dt envelope, but with a second, shorter time constant. An arcing fault is indicated if the second tracking signal falls to a predetermined fraction of the fist tracking signal. This circuit may be used alone to detect arcing faults or to increase the sensitivity to arcing faults over dimmers of a circuit (61) which responds to a time attenuated integrated valve of the pulses in the di/dt signal. In another embodiment (191), the di/dt envelope signal is subtracted from the magnitude of the pulses before they are applied to the time attenuated integrator (111), but with the subtraction time delayed to preclude attenuation of the first pulse.