CAPACITANCE REJECTING GROUND FAULT PROTECTING APPARATUS AND METHOD

• US 20020140432A1
• Filed: 04/02/2001
• Published: 10/03/2002
• Est. Priority Date: 04/02/2001
• Status: Active Grant

• Current Assignee: ETI, Inc.
• Original Assignee: MSX, Inc.
• Inventors: Jones, Thaddeus M.
• Granted Patent: US 6,459,269 B1
• Time in Patent Office: Days
• US Class Current: 324/509
• CPC Class Codes: G01R 27/18 Measuring resistance to earth , i.e. line to ground
• CPC Class Codes: G01R 31/52 Testing for short-circuits, leakage current or ground faults
• CPC Class Codes: H02H 3/337 avoiding disconnection due to reactive fault currents

Abstract

A method of controlling a load in response to a ground fault condition includes measuring a ground fault alternating current flowing from the load. A real part and an imaginary part of the ground fault alternating current is ascertained. Electrical power is removed from the load and/or the ground fault condition is indicated to a user if a magnitude of the real part exceeds a first predetermined threshold and/or a magnitude of the imaginary part exceeds a second predetermined threshold.

Claims

1. A method of controlling a load in response to a ground fault condition, said method comprising the steps of:

measuring a ground fault alternating current flowing from the load;

ascertaining a real part and an imaginary part of the ground fault alternating current; and

at least one of removing electrical power from the load and indicating the ground fault condition to a user if at least one of;

a magnitude of the real part exceeds a first predetermined threshold; and

a magnitude of the imaginary part exceeds a second predetermined threshold.

2. The method of claim 1, wherein the first predetermined threshold is such that both personnel and equipment are simultaneously protected.

3. The method of claim 1, wherein said ascertaining step comprises:

determining a phase angle of the ground fault alternating current relative to a branch current entering the load;

measuring a magnitude of the ground fault alternating current; and

at least one of;

multiplying the magnitude of the ground fault alternating current by a cosine of the phase angle; and

multiplying the magnitude of the ground fault alternating current by a sine of the phase angle.

4. The method of claim 3, wherein said determining step comprises:

ascertaining a period of at least one of the ground fault alternating current, the branch current and a branch voltage;

measuring a time period between a zero-crossing of the ground fault alternating current and a zero-crossing of the branch current; and

dividing the time period by the period of at least one of the ground fault alternating current and the branch current.

5. The method of claim 4, wherein said step of measuring a magnitude of the ground fault alternating current is performed at a time corresponding to a peak of the ground fault alternating current.

6. The method of claim 5, wherein the time corresponding to the peak is approximately one-quarter of the period of at least one of the ground fault alternating current, the branch current and the branch voltage after the zero-crossing of the ground fault alternating current.

7. The method of claim 6, wherein said first predetermined threshold is approximately 6 milliamperes and said second predetermined threshold is approximately 30 milliamperes.

8. The method of claim 1, wherein said cosine of the phase angle and said sine of the phase angle are stored in a common array.

9. The method of claim 1, wherein said measuring and ascertaining steps are performed under sinusoidal steady state conditions.

10. The method of claim 1, comprising the further step of at least one of calibrating and adjusting at least one of said first predetermined threshold and said second predetermined threshold.

11. The method of claim 10, wherein said step of at least one of calibrating and adjusting is performed using a single positive power supply.

13. The apparatus of claim 12, wherein said ground fault current detector includes a current transformer connected to the load.

14. The apparatus of claim 13, wherein said current transformer has an output, said ground fault current detector including a non-inverting amplifier connected to said output of said current transformer.

15. The apparatus of claim 12, wherein said at least one ground fault signal includes a pulse signal indicative of a phase angle of the ground fault current relative to a branch current entering the load.

16. The apparatus of claim 15, wherein said pulse signal includes a plurality of pulses, a duration of each said pulse corresponding to a time period between a respective zero-crossing of the ground fault current and a respective zero-crossing of one of the branch current and a branch voltage.

17. The apparatus of claim 12, wherein said automatic control circuit includes a switch having a first position and a second position, said control circuit being configured for removing electrical power from the load when said switch is in said first position, said control circuit being configured for indicating a ground fault condition to a user when said switch is in said second position.

18. The apparatus of claim 12, wherein said automatic control circuit includes a contactor configured for removing electrical power from the load.

19. The apparatus of claim 12, wherein the load comprises a shielded heater wire.

12. A ground fault circuit interrupter apparatus, said apparatus comprising:

a ground fault current detector configured for measuring a ground fault current from a load and producing at least one ground fault signal indicative thereof, and

an automatic control circuit connected to said ground fault current detector, said control circuit being configured for;

receiving said at least one ground fault signal;

ascertaining a real part and an imaginary part of the ground fault current dependent upon said at least one ground fault signal; and

at least one of removing electrical power from the load and indicating a ground fault condition if at least one of;

a magnitude of the real part exceeds a first predetermined threshold; and

a magnitude of the imaginary part exceeds a second predetermined threshold.

20. A method of detecting a ground fault condition, said method comprising the steps of:

making a plurality of ground fault current measurements, each said ground fault current measurement being made at a respective point in time;

calculating an average of said ground fault current measurements; and

comparing said average to a predetermined threshold value.