Ground fault interrupters

• US 3,879,639 A
• Filed: 09/10/1973
• Issued: 04/22/1975
• Est. Priority Date: 01/04/1973
• 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): Envall, Jr., R. N.
• Assistant Examiner(s): Salce, Patrick R.
• Application Number:
  US05/395,422
• Time in Patent Office: 589 Days
• Field of Search: 317/18D,53,27R,18R,33SC
• US Class Current: 361/45
• CPC Class Codes: H02H 3/17 by means of an auxiliary voltage injected into the installation to be protected using summation current transformers H02H3/33
• CPC Class Codes: H02H 3/331 responsive to earthing of the neutral conductor H02H3/338 takes precedence

Abstract

In a distribution circuit for connecting a load to an a-c source, it is known that a ground-to-neutral short tends to reduce the sensitivity of ground fault detectors that use a differential current transformer to sense line-to-ground leakage. A resonated coil on a core links the neutral conductor forming part of a protective device for interrupting the load circuit in the event of reduced Q of the resonated coil caused by a neutralto-ground fault. The core must also link the line conductor, for otherwise a loop including the line, the load and the neutral would wrongly have a like effect. The same core and coil are used with a common amplifier for the neutral-to-ground fault detector and for the line-to-ground leakage detector, so that the apparatus can be economical and compact. Furthermore, so long as there is no neutral-to-ground fault, the amplifier delivers a signal of normal strength that monitors the integrity of both detectors. The detector also responds to '"'"''"'"''"'"''"'"'run-away'"'"''"'"''"'"''"'"' of the monitoring signal generator.

Claims

1. A neutral-to-ground fault detector for a distribution circuit to supply alternating current at about 60 Hz to a load, where the circuit includes line and neutral conductors extending to load-connection terminals, and where the neutral conductor has a ground connection, said neutral-to-ground fault detector including a differential current transformer having a core, primary windings forming current paths in series with said line and neutral conductors respectively, between said load-connection terminals and said ground connection of said neutral, said primary winding being arranged to produce mutually cancelling magnetic effects in the core due to load current carried thereby, and (a) multi-turn winding means on the core, means including a capacitor connected to said (multi-turn) winding means for producing resonance therein at a frequency substantially higher than 60 Hz, said core and said resonated winding means being formed to have a Q greatly in excess of unity at the resonant frequency, means for exciting said resonated winding within its pass band, (and) means for detecting the reduction of Q of the resonated winding resulting from a neutral-to-ground fault and means for detecting an abnormal rise in excitation of said resonated winding.

2. A neutral-to-gRound fault detector in accordance with claim 1, wherein said exciting means is an oscillator which includes said resonated winding as a frequency - determining part thereof.

3. A neutral-to-ground fault detector in accordance with claim 2, further including means for detecting oscillator run-away.

4. A neutral-to-ground fault detector in accordance with claim 2, including a circuit breaker having normally closed contacts in series with said line conductor and having tripping means for opening the contacts, and a control circuit for maintaining said tripping means inactive in response to sustained output of the oscillator, said control circuit being effective to cause tripping of the circuit breaker both in the event of failure of said oscillator and in the event of reduction of the output of the oscillator as upon reduction of Q of the resonated winding resulting from a neutral-to-ground fault.

5. A neutral-to-ground fault detector in accordance with claim 4, further including line-to-ground leakage detecting means for activating said tripping means, said last named means including a second control circuit selectively responsive to 60 Hz signals and having input signal means from said differential current transformer, said second control circuit being coupled to said tripping means for activating the latter in response to a 60 Hz signal exceeding a prescribed level.

6. A neutral-to-ground fault detector for a distribution circuit to supply alternating current at about 60 Hz to a load, where the circuit includes line and neutral conductors extending to load-connection terminals, and where the neutral conductor has a ground connection, said neutral-to-ground fault detector including a differential current transformer having a core, primary windings forming current paths in series with said line and neutral conductors, respectively, between said load-connection terminals and said ground connection of said neutral, said primary windings being arranged to produce mutually cancelling magnetic effects in the core due to load current carried thereby, and multi-turn winding means on the core, means including a capacitor connected to said multi-turn winding means for producing resonance therein at a frequency substantially higher than 60 Hz, said core and said resonated winding means being formed to have a Q greatly in excess of unity at the resonant frequency, means for exciting said resonated winding within its pass band, and means for detecting the reduction of Q of the resonated winding resulting from a neutral-to-ground fault, and line-to-ground leakage detecting means also coupled to said differential current transformer, said means for detecting a reduction of Q and said line-to-ground leakage detecting means including a common operational amplifier having frequency discriminating feedback means proportioned to provide much greater gain at 60 Hz than at said excitation frequency.