GROUND FAULT PROTECTIVE SYSTEM

• US 3,836,821 A
• Filed: 03/12/1973
• Issued: 09/17/1974
• Est. Priority Date: 05/07/1971
• Status: Expired due to Term

• Current Assignee: Provident Industries, Inc.
• Original Assignee: Ellwood S. Douglas, Wallace W. Wahlgren
• Inventors: Wahlgren, Wallace W., Douglas, Ellwood S.
• Primary Examiner(s): Miller, J. D.
• Assistant Examiner(s): Salce, Patrick R.
• Application Number:
  US05/340,169
• Time in Patent Office: 554 Days
• Field of Search: 317/18D,27R,33SC,141S,50 330/110,23
• US Class Current: 361/45
• CPC Class Codes: H01H 71/123 using a solid-state trip unit circuits H02H
• CPC Class Codes: H02H 3/331 responsive to earthing of the neutral conductor H02H3/338 takes precedence

Abstract

Highly sensitive ground fault protective system utilizing an operational amplifier, integrator and switching means for interrupting the flow of current in a distribution system when the amplified and integrated signal produced by a fault current reaches a sharply defined threshold value. The system can be built in highly miniaturized, modular form which can be incorporated in small circuit breakers and receptacle outlet boxes.

Claims

1. In a ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, the neutral conductor being connected to ground at the source, a differential transformer having a plurality of primary windings and a secondary winding, said primary windings being connected in a series with the conductors of the distribution system and arranged for producing a fault signal in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, means for amplifying the fault signal, integrator means for producing a trip signal having a magnitude corresponding to the magnitude and duration of the amplified fault signal, switching means connected to the output of said integrator means and adapted for actuation when the magnitude of said trip signal reaches a predetermined level, interrupter means having an operating coil and normally closed contacts for interrupting the flow of current in the distribution systEm conductors, said coil requiring a predetermined minimum current for opening said contacts, and power supply means for receiving input current from the distribution system conductors and delivering operating current to the remainder of the protective device, said operating coil being connected between one of the conductors and the power supply means whereby the input current passes through said coil, said input current normally being less than the predetermined minimum but increasing to said minimum upon actuation of the switching means.

2. In a ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, the neutral conductor being connected to ground at the source, a differential transformer having a plurality of primary windings and a secondary winding, said primary windins being connected in series with the conductors of the distribution system and arranged for producing a fault signal in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, highly sensitive amplifier means comprising an operational amplifier having its input connected for receiving the fault signal from said secondary winding, integrator means connected to the output of said amplifier means for producing a trip signal having a magnitude corresponding to the magnitude and duration of said fault signal, switching means connected to the output of said integrator means and adapted for closing when the magnitude of said trip signal reaches a predetermined level, and temperature compensation means comprising a first feedback resistor connected between the output and the inverting input of the operational amplifier, a temperature dependent conductive element having one terminal connected to the output of the operational amplifier, and a second feedback resistor connected between the other terminal of the temperature dependent element and the inverting input of the operational amplifier.

3. A ground fault protective device as in claim 2 together with a set of normally closed breaker contacts connected in series with at least one of said conductors and means controlled by said switching means for opening said contacts in response to the fault signal.

4. A ground fault protective device as in claim 2 wherein said amplifier means comprises an operational amplifier.

5. A ground fault protective device as in claim 2 wherein said integrator means includes a capacitor connected for being charged by the output of said amplifier means and a resistor connected for discharging said capacitor at a predetermined rate.

6. A ground fault protective device as in claim 2 wherein said switching means includes a solid state switching device and a zener diode connected between the output of the integrator means and the control element of said switching device.

7. A ground fault protective device as in claim 2 together with power supply means connected for receiving an a.c. input from the distribution system conductors and delivering d.c. operating power to the remainder of said device, said power supply means including means for suppressing surges from the distribution system voltage to prevent false tripping of the device in response to such surges.

8. In a ground fault protective device for use in connection with an electrical distribution system having a plurality of conductors carrying electrical energy between a source and a load, one of said conductors being connected to ground at source, means for monitoring the flow of current in the conductors and producing a fault signal when the currents flowing to and from the load through said conductors are not equal, interrupter means including an operating coil and contacts for interrupting the flow of current in the distribution system conductors, said coil requiring a predetermined minimum level of current for opening said contacts, power supply means connected for receiving input current from the distribution system conductors and delivering operating current to the protective device, said operating coil being connected between one of said conductors and said power supply means in such manner that the input current passes through said operating coil, said input current normally being less than the predetermined minimum level, and switching means for increasing the input current to the predetermined minimum level in response to the fault signal.

9. A ground fault protective device as in claim 2 wherein the temperature dependent element is a diode.

10. A ground fault protective device as in claim 2 further including means for providing an additional current in the neutral conductor in the event that said conductor is grounded remotely of the source.

11. In a highly miniaturized ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, the neutral conductor being connected to ground at the source, a differential transformer having a toroidal core with a diameter on the order of 1 to 2 cm for load currents up to about 30 amperes, a plurality of single turn primary windings connected in series with the conductors of the distribution system and passing through the opening in said core, and a secondary winding on said core, said primary windings being arranged in such manner that a fault signal is produced in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, and operational amplifier having its input connected for receiving the fault signal from said secondary winding, a capacitor connected for being charged by the output of said operational amplifier, switching means connected to said capacitor and adapted for closing when the charge on said capacitor reaches a predetermined level, a second toroidal core of small size surrounding the neutral conductor, oscillator means connected to a winding on said second core for inducing a high frequency signal on the neutral conductor in the event that said conductor is grounded remotely of the source, and temperature compensation means comprising a first feedback resistor connected between the output and one input of the operational amplifier, a temperature dependent resistive element having one terminal connected to the output of the operational amplifier, and a second feedback resistor connected between the other terminal of the temperature dependent element and said one input of the operational amplifier.

12. A highly miniaturized ground fault protective device as in claim 11 wherein said switching means includes a silicon controlled rectifier and a zener diode connected between said capacitor and the gate of said silicon controlled rectifier.

13. In a ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, wherein the neutral conductor is connected to ground at the source, a differential transformer having a plurality of primary windings and a secondary winding, said primary windings being connected in series with the conductors of the distribution system and arranged for producing a fault signal in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, interrupter means including an operating coil and contacts for interrupting the flow of current in the distribution system conductors, said coil requiring a predetermined minimum level of current for opening said contacts, power supply means connected for receiving input current from the distribution system conductors and delivering operating current to the protective device, said operating coil being connected between one of said conductors and said power supply means in such manner that the input current flOws through said operating coil, and switching means responsive to said fault signal, said switching means being connected to said power supply means in such manner that when said switching means is closed the input current is at least as great as said predetermined minimum level.

14. In a ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, wherein the neutral conductor is connected to ground at the source, a differential transformer having a plurality of primary windings and a secondary winding, said primary windings being connected in series with the conductors of the distribution system and arranged for producing a fault signal in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, means responsive to said fault signal for interrupting the flow of current in the distribution system conductors, power supply means connected for receiving a.c. input current from the distribution system conductors and delivering d.c. operating current to the protective device, and oscillator means coupled to the neutral conductor in the distribution system for inducing a high frequency signal in said conductor, said oscillator means including a switching element which fires when the voltage across its terminals reaches a predetermined level, said switching element being connected to the output of said supply means in such manner that it regulates the output voltage of said supply means at a value corresponding to said predetermined level.

15. In a ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, the neutral conductor being connected to ground at the source, a differential transformer having a plurality of primary windings and a secondary winding, said primary windings being connected in series with the conductors of the distribution system and arranged for producing a fault signal in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, highly sensitive amplifier means having its input connected for receiving the fault signal from said secondary winding, integrator means connected to the output of said amplifier means for producing a trip signal having a magnitude corresponding to the magnitude and duration of said fault signal, switching means connected to the output of said integrator means and adapted for closing when the magnitude of said trip signal reaches a predetermined level, power supply means connected for receiving an input current from the distribution system conductors and delivering d.c. operating power to the remainder of said device, said power supply means including means for suppressing surges from the distribution system voltage to prevent false tripping of the device in response to such surges, and interrupter means connected for interrupting the flow of current in at least one of said conductors, said interrupter means including an operating coil responsive to currents of a predetermined minimum level, said operating coil being connected in series between one of said conductors and one input of said power supply means so that the input current to said power supply means passes through said coil, said switching means being connected to the output of said power supply means in such manner that when said switching means is closed the current passing through said operating coil reaches said predetermind minimum level.

16. In a ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, the neutral conductor being connected to ground at the source, a differential transformer having a plurality of primary windings and a secondary winding, said primAry windings being connected in series with the conductors of the distribution system and arranged for producing a fault signal in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, highly sensitive amplifier means having its input connected for receiving the fault signal from said secondary winding, integrator means connected to the output of said amplifier means for producing a trip signal having a magnitude corresponding to the magnitude and duration of said fault signal, switching means connected to the output of said integrator means and adapted for closing when the magnitude of said trip signal reaches a predetermined level, power supply means connected fore receiving an a.c. input from the distribution system conductors and delivering d.c. operating power to the remainder of said device, said power supply means including means for suppressing surges from the distribution system voltage to prevent false tripping of the device in response to such surges, interrupter means connected for interrupting the flow of current in at least one of said conductors, said interrupter means including an operating coil responsive to currents of a predetermined minimum level, said operating coil being connected in series between one of said conductors and one input of said power supply means so that the input current to said power supply means passes through said coil, and a shunt capacitor connected across the input of said power supply means, said operating coil and said shunt capacitor together forming a low pass filter to protect the device from high frequency transient disturbances arising in the distribution system.

17. In a ground fault protective device for use in connection with a system having a plurality of conductors carrying current between a source and a load, one of said conductors being connected to ground at the source, means for monitoring the flow of current in the conductors and producing a fault signal when the currents flowing to and from the load through the conductors are not equal, means including an operational amplifier for amplifying the fault signal, and temperature compensation means comprising a first feedback resistor connected between the output and the inverting input of the operational amplifier, a diode having one terminal connected to the output of the operational amplifier, and a second feedback resistor connected between the other terminal of said diode and the inverting input of the operational amplifier.

18. In a ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying eletrical energy between a source and a load, the neutral conductor being connected to ground at the source, a differential transformer having a plurality of primary windings and a secondary winding, said primary windings being connected in series with the conductors of the distribution system and arranged for producing a fault signal in said secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, highly sensitive amplifier means having its input connected for receiving the fault signal from said secondary winding, integrator means connected to the output of said amplifier means for producing a trip signal having a magnitude corresponding to the magnitude and duration of said fault signal, switching means connected to the output of said integrator means and adapted for closing when the magnitude of said trip signal reaches a predetermined level, and means for producing an additional current in the neutral conductor in response to grounding of said conductor remotely of the source, said means for producing an additional current in the neutral conductor including a pulse generator having its output coupled to said conductor through a transformer, said generator including a switching element which fires when the voltage across its terminals reaches a predetermined level anD thereafter conducts until the current flowing through it decreases to a predetermined level, said generator also including a capacitor connected to said switching element and thereafter discharging through said switching element, said switching element being connected to the remainder of said protective device in such manner that it serves as a voltage regulator for the operating power supplied to said device.

19. In a highly miniaturized ground fault protective device for use in connection with an electrical distribution system having line and neutral conductors carrying electrical energy between a source and a load, the neutral conductor being connected to ground at the source, a differential transformer having a toroidal core with a diameter on the order of 1 to 2 cm for load currents up to about 30 amperes, a plurality of a single turn primary windings connected in series with the conductors of the distribution system and passing through the opening in said core, and a secondary winding when the currents flowing to and from the load through the line and neutral conductors are not equal, an operational amplifier having its input connected for receiving the fault signal from said secondary winding, a capacitor connected for being charged by the output of said operational amplifier, switching means connected to said capacitor and adapted for closing when the charge on said capcitor reaches a predetermined level, a second toroidal core of small size surrounding the neutral conductor, oscillator means connected to a winding on said second core for inducing a high frequency signal on the neutral conductor, and power supply means for receiving a.c. power from the distribution system conductors and supplying d.c. operating power to the protective device, said oscillator means being connected to the output of said power supply means and including a bilateral switching element which fires when the voltage across its terminals reaches a predetermined level, said switching element serving to regulate the output voltage of said power supply means at a level corresponding to said predetermined level.

20. A highly miniaturized ground fault protective device as in claim 1 wherein said bilateral switching element is a diac.

21. A highly miniaturized ground fault protective device as in claim 19 wherein said power supply means includes means for suppressing surges in the distribution system voltage to prevent false tripping in response to such surges, said means for suppressing surges comprising a zener diode connected across the output of said power supply means, said zener diode having a breakdown voltage higher than the normal peak voltage in the distribution system and higher than the normal output voltage of said supply means.