APPARATUS FOR ENVELOPE DETECTION OF LOW CURRENT ARCS

Arcing faults in an ac current are detected by apparatus which is responsive to a predetermined randomness in step increases in the current to eliminate false tripping caused by regularly occurring discontinuities produced by loa ds such as dimmers. A signal conditioner generates a bandwidth limited di/dt signal having pulses produced by the step increases in current. In one embodiment, a first tracki ng circuit tracks the envelope of the di/dt signal with a first time constant. A second tracking circuit 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 which responds to a time attenuated integrated valve of the pulses in the di/dt signal. In anoth er embodiment, the di/dt envelope signal is subtracted from the magnitude of th e pulses before they are applied to the time attenuated integrator, but with the subtraction time delayed to preclude attenuation of the first pulse.