GROUND FAULT DETECTOR AND METHOD OF GROUND FAULT DETECTION

• US 3,754,221 A
• Filed: 04/19/1971
• Issued: 08/21/1973
• Est. Priority Date: 12/22/1970
• Status: Expired due to Term

• Current Assignee: Manfred Karl Georg Stelter
• Original Assignee: Manfred Karl Georg Stelter
• Inventors: Stelter, Manfred Karl Georg
• Primary Examiner(s): Caldwell, John W.
• Assistant Examiner(s): Myer, Daniel
• Application Number:
  US05/135,334
• Time in Patent Office: 855 Days
• Field of Search: 340/255 317/18D 307/94
• US Class Current: 340/651
• CPC Class Codes: H02H 3/16 responsive to fault current to earth, frame or mass with balanced or differential arrangement H02H3/26 ; monitoring earth connection H02H5/105

Abstract

A ground fault detector for detecting both balanced and unbalanced faults, eg. leakages to ground or other faults in a floating direct current system, comprises, first and second resistances connectible respectively to the positive and negative buses of the system, a two-way switching device operable to connect to the first and to the second resistances alternately, a fault condition being detected by voltage level detecting means connected between the switching device and ground, and a method of detecting ground faults in such a system by continuously alternately checking positive and negative sides of such system against predetermined known reference resistances.

Claims

1. A ground fault detector for detecting balanced and unbalanced faults in a floating electrical system having positive and negative sides and comprising:

first and second resistance means connectible respectively to positive and negative sides of said system;

two-way switching means connectible with each of said first and second resistance means;

fault signal detection means connected between said switching means and ground;

means for operating the two-way switching means to connect said first and second resistance means alternately in series with ground, and, gate means controlling said fault signal detection means and maintaining same normally insensitive and rendering same sensitive to fault signals after a predetermined delay following operation of said two-way switching means whereby to avoid sensing of transient fault signals by said fault signal detection means during said predetermined delay.

2. A ground fault indicator as claimed in claim 1, including means for independently varying the resistance values of said first and second resistance means whereby to vary the response characteristics of the ground fault detector.

3. A ground fault detector as claimed in claim 1 wherein said fault signal detection means includes third resistance means connected between said switching means and ground;

and a voltage level detector connected across the third resistance means, the voltage level detector being adapted to detect fault signals exceeding a predetermined voltage level.

4. A ground fault detector as claimed in claim 3 including means operable in synchronism with said switch operating means for rendering the voltage level detector unresponsive to fault signals for a predetermined duration following each change-over of the two-way switch;

and alarm means responsive to the condition of the voltage lever detector and operable in accordance with the detection of fault signals by the voltage level detector.

5. A ground fault detector for detecting balanced and unbalanced faults in a floating electrical system having positive and negative sides, comprising:

first and second resistance means connectible respectivey to said positive and negative sides of said system;

third resistance means connected to ground;

a two-way switch connectible between said third resistance means and each of said first and second resistance means;

means for operating the two-way switch at a predetermined frequency to connect said first and second resistance means alternately in series with the third resistance means whereby to derive a train of fault signals in accordance with a continuing fault condition;

a voltage level detector connected across the third resistance means, the voltage level detector including means for detecting the polarity exceeding a respective predetermined voltage level;

gate means operable in synchronism with said switch operating means for rendering the voltage level detector unresponsive to fault signals for a predetermined duration following each change-over of the two-way switch and rendering the same sensitive to fault signals after said predetermined delay; and

alarm means connected to the voltage level detector and operable in accordance with the occurrence of said trigger pulses.

6. A ground fault detector as claimed in claim 5 wherein the alarm means includes an alarm indicator and an energizing circuit therefor, said energizing circuit being connected to an electronic switch responsive to said trigger pulses and operable to actuate the energizing circuit in response to a fault condition.

7. A ground fault detector as claimed in claim 6, the electronic switch having a set condition and a reset condition, and including means for resetting the electronic switch from its set condition, and timing means connected to the resetting means for delaying resetting of the switch for an interval corresponding to tHe cycle of operation of said two-way switch.

8. A ground fault detector for detecting balanced and unbalanced faults in a floating electrical system having positive and negative sides, comprising:

first and second resistance means connectible respectively to said positive and negative sides of said system;

third resistance means connected to ground;

a two-way switch connectable between said third resistance means and each of said first and second resistance means;

means for operating the two-way switch at a predetermined frequency to connect said first and second resistance means alternately in series with the third resistance means whereby to derive a train of fault signals in accordance with a continuing fault condition;

a voltage level detector connected across the third resistance means, the voltage level detector including means for detecting the polarity exceeding a respective predetermined voltage level;

means operable in synchronism with said switch operating means for rendering the voltage level detector unresponsive to fault signals for a predetermined duration following each changeover of the two-way switch;

a first alarm indicator and an energizing circuit therefor, said energizing circuit being connected to an electronic switch responsive to said trigger pulses and operable to actuate the energizing circuit in response to a fault condition;

second and third alarm indicators and respective energizing circuits therefor, the ground fault detector further including means for deriving first and second trains of control pulses coincident with the respective alternate positions of the two-way switch, and first and second gating means for gating the trigger pulses with the first and second trains of control pulses respectively, the energizing circuits of the second and third alarm indicators being connected to the first and second gating means and being actuated in accordance with the gating of the triggering pulses whereby each of the second and third alarm indicators is responsive to a respective fault condition.

9. A ground fault indicator as claimed in claim 8 including first and second gating means for gating the trigger pulses with the first and second control pulses respectively, and second and third energizing circuits being connected to the first and second gating means and being actuated in accordance with the gating of the trigger pulses whereby each of the second and third alarm indicators is responsive to a respective fault condition.

10. A ground fault detector as claimed in claim 5 including means for independently varying the resistance values of the first and second resistance means, first and second reference voltage sources for determining said respective voltage levels, and means coupled to said resistance varying means for independently adjusting the reference voltage sources to vary the respective voltage levels in accordance with the respective values of said first and second resistance means.

11. The method of detecting ground faults in an electrical system having a floating battery and positive and negative conductors connected thereto, and comprising the steps of:

connecting resistances of predetermined value to each of said conductors;

alternately connecting each of said resistances to ground whereby to complete an electrical circuit between one said conductor and the resistance connected thereto and a groundd fault present in the other said conductor, thereby to establish a fault signal and temporary transient signals;

temporarily blocking said signals to permit collapse of said transient signals resulting from said connection, and, detecting said fault signal a predetermined time after connection of a said resistance to ground and after collapse of said transient signals.

12. The method as claimed in claim 11, including the step of generating an alarm signal in response to a fault signal greater than a predetermined value.