DC ground fault detecting apparatus with an auto-null circuit and method

US5530363A · kind A · utility

Assignees

• John Gaughan · US
• CONSOLIDATED EDISON COMPANY OF NEW YORK, INC. · US

Inventors

• John Gaughan · Los Angeles, US
• Robert Ward · Colorado Springs, US

Key dates

• Filing date: Oct 31, 1994
• Grant date: Jun 25, 1996
• Expiry date: Oct 31, 2014

Classification

• Technology area (CPC G): Physics
• CPC primary: G01R31/52
• WIPO field: Measurement
• WIPO sector: Instruments

Abstract

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.