GROUND FAULT INTERRUPTER WITH PULSED NEUTRAL-TO-GROUND FAULT DETECTOR

• US 3,794,884 A
• Filed: 05/29/1973
• Issued: 02/26/1974
• Est. Priority Date: 05/29/1973
• Status: Expired due to Term

• Current Assignee: Challenger Acquisition Corporation, Challenger DPT Corporation, Challenger Fuse Corporation, Challenger Lic Corporation, Federal Pioneer Ltd. (Schneider Electric SE)
• Original Assignee: Richard C. Sircom
• Inventors: Sircom, Richard C.
• Primary Examiner(s): Miller, J. D.
• Assistant Examiner(s): Salce, Pat
• Application Number:
  US05/364,890
• Time in Patent Office: 273 Days
• Field of Search: 317/18D,337,53,38,27R,52
• US Class Current: 361/45
• CPC Class Codes: H02H 3/331 responsive to earthing of the neutral conductor H02H3/338 takes precedence

Abstract

Apparatus for monitoring an alternating-current circuit having a differential current transformer to sense line-to-ground faults includes a pulsed ringing circuit for evidencing freedom of the alternating-current circuit from neutral-to-ground faults that could reduce the sensitivity of the line-to-ground fault detector.

Claims

1. Apparatus for monitoring an alternating current supply circuit having at least one line conductor, a neutral conductor, and means forming a ground-return, wherein said conductors extend from supply terminals to load terminals and wherein the neutral conductor has a ground connection to the ground-return, the monitoring apparatus including neutral-to-ground fault monitoring means having a differential current transformer having a core and primary windings forming series segments of said line and neutral conductors so arranged that load current carried thereby tends to produce mutually cancelling flux in said core, the differential current transformer being disposed between said ground connection and said load terminals, resonant means resonant at a higher frequency than that of the alternating current supply circuit including a further winding linking said core and a capacitor connected to said further winding, means for periodically pulsing said resonant means, and fault-evidencing means coupled to said transformer for responding thereto in one manner to indicate normal operation when the resonant means rings at least at a minimum amplitude for a prescribed period after each pulse of the pulsing means and for responding thereto differently to indicate abnormal operation in the event of a fault between the neutral conductor and the ground-return means at the load-terminal side of said transformer causing damping of the resonant means and decay of the ringing below said minimum amplitude during said period.

2. Monitoring apparatus in accordance with claim, 1, wwherein said neutral-to-ground fault evidencing means and said resonant means include said further winding in common.

3. Monitoring apparatus in accordance with claim 1, wherein said pulsing means is coupled to the alternating current supply circuit to be synchronized therewith.

4. Monitoring apparatus in accordance with claim 1, wherein said fault-evidencing means includes a transistor pulsed into saturation at the ringing frequency so long as the minimum ringing amplitude is sustained.

5. Monitoring apparatus in accordance with claim 4, further including time-constant circuit means restraining response to the transistor for intervals only slightly longer than the period of the ringing frequency.

6. Monitoring apparatus in accordance with claim 1, further including means coupled to the differential current transformer for evidencing the appearance of a line-to-ground fault which causes flux unbalance in said core at the frequency of the alternating current circuit.

7. Monitoring apparatus in accordance with claim 6, wherein said neutral-to-ground fault evidencing means and said line-to-ground fault evidencing means and said resonant means include said further winding in common.