Protective system for electrical appliances

Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection

Patent

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Details

361 50, 361 91, 361 92, H02H 328

Patent

active

040682765

ABSTRACT:
An a-c power supply for a load such as a household appliance, including a line wire, a neutral wire and a ground wire, has a safety switch and an overload-sensing resistor inserted in its line wire and also comprises a ground-fault detector in the form of a differential transformer with a toroidal core traversed by the line and neutral wires. The safety switch, which may be a manually resettable circuit breaker or an armature of a self-locking relay, has an operating winding in series with an SCR connected between a positive and a negative terminal, one terminal being connected via respective diodes to the line and neutral wires, the other terminal being connected via respective diodes to the neutral and ground wires. The gate circuit of the SCR includes the secondary of the differential transformer, the secondary of another transformer connected across the overload-sensing resistor, a pair of voltage dividers -- one inserted between the positive terminal and the neutral wire, the other inserted between the same terminal and the ground wire -- for detecting ground loss, neutral loss, line
eutral reversals and excessive neutral-to-ground voltages, and Zener diodes for detecting overvoltages and undervoltages. In a three-phase system, the two terminals are both connected via respective diodes to all three phase wires in parallel. With the use of a pair of triggerable semiconductors (SCRs or triacs) in lieu of a single SCR, the operating winding can be tied directly to the line wire while being connected to the other two wires through the two semiconductors, respectively.

REFERENCES:
patent: 3766434 (1973-10-01), Sherman
patent: 3932790 (1976-01-01), Muchnick
patent: 3946282 (1976-03-01), Weiss et al.
patent: 3961319 (1976-06-01), Asberry
patent: 3987341 (1976-10-01), Clark
patent: 3987425 (1976-10-01), Lawton
patent: 4002968 (1977-01-01), Reid

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