GROUND FAULT DETECTOR CIRCUIT

• US 3,700,967 A
• Filed: 01/10/1972
• Issued: 10/24/1972
• Est. Priority Date: 01/10/1972
• Status: Expired due to Term

• Current Assignee: Commander Electrical Equipment, Inc., Liberty Lighting Company Incorporated
• Original Assignee: Robert J. Hoss
• Inventors: Hoss, Robert J.
• Primary Examiner(s): Trammell, James D.
• Application Number:
  US05/216,526
• Time in Patent Office: 288 Days
• Field of Search: 317/18D,53,27R,33SC,49 323/60,70
• US Class Current: 361/46
• CPC Class Codes: H02H 1/06 Arrangements for supplying operative power power supply arrangements in general G05F, H02M
• CPC Class Codes: H02H 3/33 using summation current transformers H02H3/347 takes precedence
• CPC Class Codes: H02H 7/008 for protective arrangements according to this subclass H02H9/042, H02H9/043 take precedence; protection of spark-gaps H02H7/24

Abstract

A ground fault detector providing protection from leakage current to ground by sensing unbalance in supply and neutral conductor currents to actuate a circuit breaker. A solid state circuit including amplifier and detector and having the unbalance signal of a current transformer as an input for energizing a trip coil of the breaker when the unbalance current exceeds a predetermined level. Improved circuitry providing temperature compensation, insensitivity to turn on and other transients and noise, and protection against overheating due to breaker hangup.

Claims

1. In a ground fault detector for operation with a circuit breaker, a current transformer, and an a.c. power source with at least one supply conductor and a neutral or return conductor, with the supply conductor connected through a breaker contact set to a transformer primary winding and with the neutral conductor connected to another transformer primary winding, and with the transformer secondary winding providing an unbalance signal when there is an unbalance in the currents in the supply conductor and the neutral conductor, the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to said amplifier input for amplifying said unbalance signal;

detector means having said amplifier output as an input for detecting when said amplifier output exceeds a predetermined value and producing a control signal;

trip means having said control signal for an input for actuating said circuit breaker and opening said contact set;

a d.c. power supply for said amplifier means and detector means;

said detector means including a first transistor switched into conduction when said amplifier output exceeds said predetermined value, and said amplifier means including a first capacitor charged from said d.c. supply through a first resistor; and

circuit means interconnecting said first capacitor and first transistor for discharging said first capacitor through the conducting first transistor, with the discharged capacitor reducing the amplifier output to cut off said first transistor and provide a control signal pulse, with the pulse duration being a function of the capacitor discharge time and with the interpulse interval being a function of the capacitor charge time.

2. A ground fault detector as defined in claim 1 wherein said circuit means for connecting said transformer includes a second capacitor connected across said secondary winding forming a resonant circuit, said second capacitor having a capacitance to resonate with said secondary winding at a frequency in the order of that of the a.c. source, with the change of inductance of the secondary winding with change in temperature shifting the resonant frequency of the resonant circuit maintaining the secondary winding output substantially constant as the initial permeability of the transformer core material changes with change in temperature.

3. A ground fault detector as defined in claim 2 wherein said amplifier means includes a multistage negative feedback amplifier having a second transistor in one stage and a third transistor in a succeeding stage, with the signal to be amplified connected through the collector and emitter of said second transistor to the next stage, and means defining an a.c. feedback path from the output of said third transistor to the base of said second transistor providing a signal out-of-phase with the signal to be amplified, and means defining a d.c. feedback path comprising a third capacitor connected between said second transistor base and circuit ground providing a substantially constant d.c. potential at said second transistor base.

4. A ground fault detector as defined in claim 3 including means for reverse biasing said first transistor during power build-up after said power supply is energized and having a resistive circuit in an output stage of said amplifier connected to the base of said first transistor, with build-up at said base preceding build-up at said first capacitor.

5. A ground fault detector as defined in claim 3 including a third capacitor connecting the input of said d.c. power supply to the a.c. power source conductor on the load side of the circuit breaker.

6. A ground fault detector as defined in claim 3 wherein the circuit breaker has a magnetic trip mechanism with a magnetic armature, and said trip means includes a coil of wire wound around said armature and a solid state switch connected to the a.c. power source in series with said coil, with said switch actuated by said control signal of said detector means.

7. In a ground fault detector for operation with a circuit breaker, a current transformer, and an a.c. power source with at least one supply conductor and a neutral or return conductor, with the supply conductor connected through a breaker contact set to a transformer primary winding and with the neutral conductor connected to another transformer primary winding, and with the transformer secondary winding providing an unbalance signal when there is an unbalance in the currents in the supply conductor and the neutral conductor, the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to said amplifier input for amplifying said unbalance signal, including a first capacitor connected across said secondary winding forming a resonant circuit, said capacitor having a capacitance to resonate with said secondary winding at a frequency in the order of that of the a.c. source, with the change of inductance of the secondary winding with change in temperature shifting the resonant frequency of the resonant circuit maintaining the secondary winding output substantially constant as the initial permeability of the transformer core material changes with change in temperature;

detector means having said amplifier output as an input for detecting when said amplifier output exceeds a predetermined value and producing a control signal; and

trip means having said control signal for an input for actuating said circuit breaker and opening said contact set.

8. A ground fault detector as defined in claim 7 wherein said amplifier means includes a multistage negative feedback amplifier having a first transistor in one stage and a second transistor in a succeeding stage, with the signal to be amplified connected through the collector and emitter of said first transistor to the next stage, and means defining an a.c. feedback path from the output of said second transistor to the base of said first transistor providing a signal out-of-phase with the signal to be amplified, and means defining a d.c. feedback path comprising a second capacitor connected between said first transistor base and circuit ground providing a substantially constant d.c. potential at said first transistor base.

9. In a ground fault detector for operation with a circuit breaker, a current transformer, and an a.c. power source with at least one supply conductor and a neutral or return conductor, with the supply conductor connected through a breaker contact set to a transformer primary winding and with the neutral conductor connected to another transformer primary winding, and with the transformer secondary winding providing an unbalance signal when there is an unbalance in the currents in the supply conductor and the neutral conductor, the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to said amplifier input for amplifying said unbalance signal;

detector means having said amplifier output as an input for detecting when said amplifier output exceeds a predetermined value and producing a control signal; and

trip means having said control signal for an input for actuating said circuit breaker and opening said contact set;

said amplifier means including a multistage negative feedback amplifier having a first transistor in one stage and a second transistor in a succeeding stage, with the signal to be amplified connected through the collector and emitter of said first transistor to the next stage, and means defining an a.c. feedback path from the output of said second transistor to the base of said first transistor providing a signal out-of-phase with the signal to be amplified, and means defining a d.c. feedback path comprising a first capacitor connected between said first transistor base and cIrcuit ground providing a substantially constant d.c. potential at said first transistor base.

10. A ground fault detector as defined in claim 9 wherein said detector means includes a third transistor switched into conduction when said amplifier output exceeds said predetermined value, and said amplifier means includes a second capacitor charged from said d.c. supply through a first resistor;

and circuit means interconnecting said second capacitor and third transistor for discharging said second capacitor through the conducting third transistor, with the discharged capacitor reducing the amplifier output to cut off said third transistor and provide a control signal pulse, with the pulse duration being a function of the capacitor discharge time and with the interpulse interval being a function of the capacitor charge time.

11. In a ground fault detector circuit for operation with a circuit breaker, a current transformer, and an a.c. power source with at least one supply conductor and a neutral or return conductor, with the supply conductor connected through a breaker contact set to a transformer primary winding and with the neutral conductor connected to another transformer primary winding, and with the transformer secondary winding providing an unbalance signal when there is an unbalance in the currents in the supply conductor and the neutral conductor, the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to said amplifier input for amplifying said unbalance signal;

detector means having said amplifier output as an input for detecting when said amplifier output exceeds a predetermined value and producing a control signal;

trip means having said control signal for an input for actuating said circuit breaker and opening said contact set;

means for desensitizing the circuit to unwanted transients which may be detected as an unbalance signal;

means for pulsing the trip means to limit heat generation in the breaker coil and associated electronics; and

means for suppressing electronics turn-on transients which may be detected as an unbalance signal.

12. A ground fault detector circuit as defined in claim 11 wherein said means for suppressing includes a resistor-capacitor network for tuning said amplifier means to have a passband at the frequency of the a.c. power source.

13. A ground fault detector circuit as defined in claim 11 wherein:

said trip means includes a solid state switch having a control element;

said detector means includes a DC level adjustment and a regenerative feedback circuit for driving said switch control element;

with said DC level adjustment located at the output of said amplifier means so that the AC signal at the amplifier output is impressed on a variable DC potential; and

with said regenerative feedback circuit in a cut-off state until the sum of said AC signal and said DC potential upon which the AC signal is impressed is greater than said predetermined value, at which time said regenerative feedback circuit will conduct providing current to said control element.

14. A ground fault detector circuit as defined in claim 13 including a d.c. power source, and wherein said regenerative feedback circuit includes a capacitor charged from said d.c. source and discharged into said detector means, with said capacitor providing the driving current for said feedback circuit;

and with said capacitor comprising said means for suppressing transients by blocking said detector means operation during initial charging of said capacitor.

15. A ground fault detector circuit as defined in claim 11 wherein:

said detector means includes a phase emitter, a diode rectifier, a DC level adjustment, and a threshold sensing circuit;

with said phase splitter coupled to said diode rectifier such that the AC signal at the input of the phase splitteR is full wave rectified at the output of the diode rectifier;

with said DC level adjustment connected to the output of the diode rectifier such that the full wave rectified AC signal is impressed on a variable DC voltage;

with said threshold sensing device comprising a Schmitt trigger or unijunction or other bistable or mono-stable, circuit which is activated when the sum of the DC potential and rectified AC potential reaches said predetermined value.

16. A ground fault detector circuit as defined in claim 11 including means for adjusting the phase shift of said amplifier means such that the peaks of the control signal occur a few degrees before the peaks of the AC power cycle supplying the trip means power for causing the circuit breaker contact set to open just prior to the zero crossing point of the AC power cycle minimizing contact arcing time.

17. A ground fault detector as defined in claim 11 wherein said means for desensitizing comprises a filter network of first and second capacitors and a resistor, with said first capacitor connected across the transformer secondary winding, and with one terminal of said second capacitor connected to one terminal of the secondary winding and the other terminal of said second capacitor connected to said resistor which is connected to the other terminal of the secondary winding, whereby said second capacitor and resistor and the transformer inductance determine the response frequency and bandwidth of the filter network, with the output of said filter network appearing at the terminals of said second capacitor.

18. A ground fault detector as defined in claim 1 wherein said circuit means interconnecting said first capacitor and first transistor includes a rectifier with its output connected to said first transistor, and a phase splitter driven by the amplifier output and connected to said rectifier as an input.

19. A ground fault detector as defined in claim 18 including an additional capacitor connected between the input of said detector means and circuit ground and connected to said d.c. power supply through an additional resistor for charging when the detector is energized for holding said detector means in an off-state for a few seconds while transients subside.