×

Ground fault detection and protection circuit

  • US 4,159,499 A
  • Filed: 06/20/1977
  • Issued: 06/26/1979
  • Est. Priority Date: 06/20/1977
  • Status: Expired due to Term
First Claim
Patent Images

1. A ground fault circuit interrupter, which is connectable with the line and neutral conductors between an AC power supply of a first frequency and a load, for interrupting the line conductor when either (a) the leakage current from the line conductor to ground exceeds a predetermined maximum limit associated with a line fault and/or (b) the resistance between the neutral conductor and ground decreases below a predetermined minimum limit associated with a neutral fault, said ground fault circuit interrupter comprising:

  • a transformer core, said line and neutral conductors being wound on said core to define line and neutral windings, said line and neutral windings being wound on said core in phase opposition to produce a substantially net mmf of zero when the line and neutral conductor currents are equal which occurs in the absence of a line and/or neutral fault,a constant current AC source of a second frequency different from that of said first frequency of said AC power supply, said constant current source being connected across said neutral winding to supply thereto in the absence of a neutral fault a predetermined current for producing a predetermined voltage component across said neutral winding, current from said constant current source being shunted to ground without passing through said neutral winding in the presence of a neutral fault to reduce said voltage component across said neutral winding attributable to current from said constant current source,a secondary winding wound on said core in which a voltage is induced when a current imbalance exists in said line and neutral conductors in the presence of a line fault,an impedance connected across said secondary winding having a variable dynamic resistance which decreases with increasing voltage thereacross, said variable, dynamic resistance being reflected back to said neutral winding by transformer action to decrease said neutral winding resistance, and in turn said voltage component thereacross attributable to current from said constant current source, when said induced voltage across said secondary winding increases in response to a line fault,a voltage sensor connected across said neutral winding for providing a fault signal when said voltage component across said neutral winding attributable to current from said constant current source decreases to a value associated with a line fault and/or a neutral fault,a selectively actuable circuit breaker connected in said line conductor for interrupting power to said line conductor when actuated, andmeans responsive to said fault signal for actuating said circuit breaker when a line and/or neutral fault occurs.

View all claims
  • 0 Assignments
Timeline View
Assignment View
    ×
    ×