John J Gaughan, 7278 Gay Ridge Rd, Yorktown Hgts, NY 10598

5530363, Jun 25, 1996

Oct 31, 1994

8/331672

John Gaughan - New Rochelle NY
Robert G. Ward - Lake Grove NY

Consolidated Edison Company of New York, Inc. - New York NY

G01R 3102

324509

To locate ground faults in a normally ungrounded DC circuit a centertapped voltage divider is connected across the positive and negative buses. An oscillator controlled switch is connected between the centertap and ground which when operating modulates the ground fault currents with a square wave having a repetition rate no greater than 3 hertz. A Hall probe including a field nulling coil is clamped successively along the conductors under test. A capacitive storage circuit maintains the energization of the nulling coil at the level that provided a null indication immediately prior to actuation of a test button. The Hall probe is connected to a read-out oscilloscope which indicates the presence or absence of the square wave modulation. Fault currents as small as 1 ma can be detected.