Importance

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Abstract

A microprocessor based trip unit for a circuit breaker is disclosed. The instantaneous current magnitudes in each phase and in the neutral of the circuit being protected are simultaneously and sequentially sampled by respective sample and hold circuits. The measured values are then applied to a respective analog-to-digital converter and the digitized values are sequentially applied to a microprocessor. The microprocessor calculates true RMS current and sums the signals of all phases and neutral to determine presence and magnitude of a ground fault current. To permit use of an 8-bit microprocessor, while maintaining high resolution of measured current over a large range, novel automatic scale adjustment is provided. The memory of prior thermal history of the circuit breaker is provided electronically by a capacitor circuit which is discharged at a rate representative of the rate of cooling of the circuit breaker. The output of the capacitor circuit is loaded into the microprocessor when the circuit breaker comes back onto the line after a prior opening operation. Any desired number of trip units can be remotely located from a switchboard frame containing a respective plurality of circuit breakers without local trip units.

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First Claim