Apparatus for detecting a ground connection on load side of neutral conductor

Abstract

An improper or undesired second ground connection on the load side of the neutral conductor of an electrical supply system is detected by coupling an oscillator to the neutral conductor and to least one other conductor of the supply system in a balanced mode. In the absence of said improper or undesired second ground, oscillator signals cancel on the two conductors. When said second ground is present, oscillator current flows therethrough and the impedance change in the grounded neutral conductor is reflected back to the oscillator tank circuit to change the output signal of the oscillator. Change in output signal is detected and actuates circuit breaker to disconnect the supply system.

What is claimed is: 
     
       1. A fault protection circuit for use in an electrical supply system at a location therein between a current source and an electrical load, wherein said supply system has a plurality of conductors including a neutral conductor that is electrically grounded on the source side of said protection circuit, said protection circuit being operable to detect an undesired second electrical ground that is present on one of the conductors on the load side of said protection circuit, said circuit comprising an oscillator circuit for providing an oscillator signal at a frequency different from the frequency of the supply system current,   said oscillator circuit having a portion thereof whose impedance is dominant in determining a characteristic of the output signal of said oscillator,   means coupling said oscillator circuit to conductors of said supply system for coupling said oscillator signal to the coupled conductors in a balanced mode so that substantially no oscillator current flows in the conductors in the absence of an undesired second ground connection on the load side of one of the conductors and for reflecting back to the oscillator circuit the impedance to ground of at least one of the conductors when said undesired second ground connection is present thereon,   the normal impedance of said dominant portion of the oscillator circuit in the absence of the second ground connection being proportioned relative to said impedance reflected back to the oscillator circuit when said second ground connection is present to cause an appreciable difference in the equivalent impedance of said dominant portion of the oscillator circuit, thereby to cause a detectable difference in said characteristic of the output signal of the oscillator,   said means for coupling the oscillator circuit to the conductors being substantially non-responsive to supply system current, and   means coupled to the oscillator circuit for detecting the difference in said characteristic of the oscillator output signal and for providing a fault signal in response thereto,   said last named means being coupled to the oscillator circuit by means other than the supply system conductors or the means that couples the oscillator circuit to the conductors.   
     
     
       2. The combination claimed in claim 1 wherein the means coupling said oscillator circuit to said supply system conductors includes   a transformer core made of a material that exhibits substantial magnetic properties to said oscillator signal but is substantially nonmagnetic to said supply system current.   
     
     
       3. The fault protection circuit claimed in claim 1 and further including, current interrupter means operable in response to said fault signal for interrupting the flow of supply system current on the supply system conductors.   
     
     
       4. The faul protection circuit claimed in claim 3 and further including ground fault sensing means coupled to said neutral conductor and to at least one other conductor of the electrical supply system for detecting an undesired unbalance of supply system current flow in said conductors and operating in response thereto to produce a ground fault signal, and   means operable in response to said ground fault signal for operating said supply current interrupter means.   
     
     
       5. The combination claimed in claim 4 wherein said ground fault sensing means includes a second transformer core made of a material that exhibits substantial magnetic properties to the supply system current but exhibits substantially nonmagnetic properties to the oscillator signal. 
     
     
       6. An electrical fault protection circuit for use in an electrical supply system at a location therein between a system current source and an electrical load, wherein said supply system has a neutral conductor that is electrically grounded on the source side of said protection circuit, said protection circuit being operable to detect an undesired second electrical ground connection that is present on one of the conductors on the load side of said protection circuit, said circuit comprising an oscillator for providing an output signal at a frequency different from the frequency of the current from said source,   said oscillator including a tank circuit and having its circuit elements proportioned and arranged so that the impedance of the tank circuit is dominant in determining the magnitude of the oscillator output signal,   a transformer core effective to couple said oscillator output signal but ineffective to couple signals at the frequency of the supply system,   a primary winding on said core for coupling the tank circuit to the core,   said neutral conductor and at least said one conductor of the supply system being magnetically coupled to said core as secondary windings in a manner to be excited in a balanced mode by the oscillator signal on said primary winding and in a manner to reflect back to the tank circuit the impedance to ground of one of said conductors when said undesired second ground connection is present thereon, thereby to cause the effective impedance of the tank circuit to have first and second values and the oscillator signal to have first and second magnitudes, respectively, when the second ground connection is absent and present,   fault signal producing means coupled to said oscillator by means other than said core or the conductors of said supply system for producing a fault signal when the oscillator signal is at said second magnitude.   
     
     
       7. A ground fault protection circuit for use in an electrical supply system at a location therein between a system current source and an electrical load, wherein said supply system has a neutral conductor that is electrically grounded on the source side of said protection circuit, said protection circuit being operable to detect an improper or undesired second electrical ground connection that is present on one of the conductors on the load side of said protection circuit, said circuit comprising an oscillator for providing an oscillator signal at a frequency different from the frequency of the current from said source,   said oscillator including a tank circuit and having its circuit elements proportioned and arranged so that the impedance of the tank circuit is dominant in determining the magnitude of the oscillator signal,   a transformer core effective to couple said oscillator signal but ineffective to couple signals at the frequency of the supply system,   a primary winding on said core for coupling the tank circuit thereto,   said neutral conductor and at least said one conductor of the supply system being magnetically coupled to said core as secondary windings in a manner to be excited in a balanced mode by the oscillator signal on said primary winding,   the impedance of the tank circuit in the absence of said second ground connection being greater in magnitude than the reflected impedance to ground of a conductor having a second ground connection thereon which is to be detected, whereby the magnitude of the oscillator signal changes by a detectable amount when said second ground connection is absent and when it is present, and   means coupled to said oscillator circuit by means other than said core or supply system conductors for detecting a change of given magnitude of said oscillator signal.   
     
     
       8. A fault protection circuit for use in an electrical supply system at a location therein between a current source and an electrical load, wherein said supply system has a plurality of conductors including a neutral conductor that is electrically grounded on the source side of said protection circuit, said protection circuit being operable to detect an undesired second electrical ground that is present on one of the conductors on the load side of said protection circuit, said circuit comprising an oscillator circuit for providing an oscillator signal that is different in frequency from said supply system current,   said oscillator circuit having a portion thereof whose impedance is dominant in determining a characteristic of the output signal of said oscillator,   means coupling said oscillator circuit to conductors of said supply system for coupling said oscillator signal to the coupled conductors in a balanced mode so that substantially no oscillator current flows in the conductors in the absence of an undesired second ground connection on the load side of one of the conductors and for reflecting back to said dominant portion of the oscillator circuit the impedance to ground of said one conductor when the second ground connection is present thereon, thereby to cause a detectable difference in said characteristic of the output signal of the oscillator when the second ground connection is present,   first fault signal producing means coupled to said oscillator circuit by means excluding said conductors of the supply system and excluding the means for coupling the oscillator circuit to said conductors for producing a first fault signal when the oscillator signal has the characteristic associated with said second ground connection being present,   ground fault protection means for detecting an undesired unbalance of supply system current flow in said conductors of the supply system,   said ground fault detector means being substantially nonresponsive to an unbalance of oscillator signal currents on the conductors of the supply system, and the means coupling the oscillator signal to conductors of the supply system being substantially nonresponsive to an unbalance of supply system current in said conductors,   means operable in response to said first fault signal and operable in response to a detected unbalance of supply system current flow by the ground fault detector means for interrupting supply system current flow on the supply system conductors.   
     
     
       9. The combination claimed in claim 1 wherein said characteristic of the oscillator output signal which changes by a detectable difference is the magnitude of said output signal. 
     
     
       10. The combination claimed in claim 9 wherein said oscillator circuit is constructed and arranged so that the frequency of its output signal remains substantially fixed when the magnitude of the output signal changes. .Iadd. 11. The fault protection circuit claimed in claim 1 wherein the magnitude of the oscillator signal changes upon the occurrence of said undesired second ground connection and wherein said means for providing a fault signal includes   a semiconductor device having at least first and second electrodes and operable between conducting and nonconducting states,   means for establishing a voltage bias of given magnitude on said first electrode of said device,   peak detector means coupled to the second electrode of said device,   means including first diode means for coupling said oscillator circuit to said peak detector means,   said first diode means being connected to prevent discharge therethrough of said peak detector means,   said peak detector having a given discharge time constant in a path other than through said first diode means,   the characteristics and parameters of said bias means, said peak detector means and said semiconductor device being so proportioned that the voltage on said second electrode in the absence of said undesired second ground connection maintains said device in one of its states, but transfers the device to its other state in the presence of an undesired ground on said   
     
     
        neutral conductor. .Iaddend. .Iadd. 12.  The combination claimed in claim 11 and further including, a time constant circuit coupled to said first electrode of the semiconductor device. .Iaddend. .Iadd. 13. The combination claimed in claim 12 wherein said time constant circuit and said semiconductor device comprise a relaxation oscillator having a repetition frequency to assure repetitive transfer of states of the semiconductor device during the occurrence of a half cycle of the source current. .Iaddend. .Iadd. 14. The combination claimed in claim 13 wherein said peak detector means is a time constant circuit having a charging time constant that is longer than the time constant circuit coupled to said first electrode. .Iaddend. .Iadd. 15. The fault protection circuit claimed in claim 11 and including   ground fault protection means for detecting an undesired unbalance of supply system current flow in the conductors of the supply system,   a second diode having one terminal coupled to the output of said ground fault protection means and its other terminal connected to a junction between said oscillator circuit and said first diode,   like poles of said diodes being coupled to said junction,   said second diode operating in response to an output signal of said ground fault protection means to block conduction from said oscillator circuit to said peak detector means, whereby the magnitude of voltage in said detection circuit changes to transfer the semiconductor device to its other state. .Iaddend. .Iadd. 16. The combination claimed in claim 8 wherein said means operable in response to said first fault signal and in response to a detected unbalance of supply system current comprises,   a time constant detection circuit for providing a d.c. voltage, said detection circuit having a given discharge rate in the absence of an input signal thereto,   means including a first diode for coupling said oscillator circuit to said detection circuit for maintaining a d.c. voltage of a first magnitude in said detection circuit in the absence of a fault in said system,   said first diode being connected to prevent discharge therethrough of said time constant detection circuit,   a second diode coupled between said ground fault protection means and a junction between said first diode and oscillator circuit,   said two diode means having like terminals connected to said junction, whereby conduction of said second diode diverts current from said first diode and detection circuit and the voltage in said time constant detection circuit will change to a second d.c. magnitude,   said second diode conducting in response to the ground fault protection means sensing an undesired unbalance of supply system current flow,   a semiconductor device having at least two terminals and operable between conducting and nonconducting states,   means for coupling the first terminal of the semiconductor device to said detection circuit,   means for coupling a voltage source to the second terminal of the semiconductor device, the magnitude of the voltage provided by said voltage source being proportioned relative to the characteristics and parameters of said device to maintain the device in one of its conduction states when the voltage in said detection circuit is at said first magnitude and to transfer said device to its other state when the voltage magnitude in said detection circuit is at said second magnitude. .Iaddend.   
     
     
        .Iadd. 17.  The combination claimed in claim 16 wherein said time constant detection circuit is comprised of parallel connected resistor and capacitor having parameter values proportioned to smooth transient signals and prevent undesired triggering of the semiconductor device. .Iaddend. .Iadd. 18. The combination claimed in claim 17 wherein said ground fault protection means includes an operational amplifier, said second diode having one terminal connected to the output of said operational amplifier. .Iaddend. .Iadd. 19. The combination claimed in claim 18 and further including   means coupled to the second terminal of the semiconductor device for causing repetitive changes of state of the semiconductor device when the voltage of the detection circuit changes to said second magnitude. .Iaddend. .Iadd. 20. A fault protection circuit for use in an electrical supply system to disconnect one or more conductors of the system from the system source in the event of a fault in the system, said circuit comprising   fault sensing means coupled to the conductors between the system source and load for producing an electrical signal in response to a fault condition in the system,   a time constant detection circuit for providing a d.c. voltage, said detection circuit having a given discharge rate in the absence of an input signal thereto,   voltage means for maintaining a d.c. voltage of a first magnitude in said detection circuit in the absence of a fault in said system,   first diode means coupled between said voltage means and said detection circuit and connected to prevent discharge of the time constant detector circuit therethrough,   second diode means having one terminal coupled to said fault sensing means and its other terminal coupled between said voltage means and said first diode means,   said second diode means being rendered conductive by said electrical signal from the fault sensing means,   said second diode means being connected with a polarity to block current flow through said first diode means upon occurrence of said electrical signal from the fault sensing means, whereby the voltage magnitude in said time constant detection circuit will change to a second magnitude,   a semiconductor device having at least two terminals and operable between conducting and nonconducting states,   means for coupling the first terminal of the semiconductor device to said detection circuit,   means for coupling a voltage source to the second terminal of the semiconductor device, the magnitude of the voltage of said voltage source being proportioned with respect to the characteristics and parameters of the semiconductor device to maintain said device in one of its states when the voltage in said detection circuit is at said first magnitude and to transfer said device to its other state when the voltage magnitude in said detection circuit is at said second magnitude, and   means coupled to said semiconductor device for interrupting supply system current on said conductors in response to said semiconductor device   
     
     
        transferring to said other state. .Iaddend. .Iadd. 21.  The combination claimed in claim 20 wherein said fault sensing means includes an operational amplifier,   said second diode being coupled to the output of said operational amplifier and being rendered conductive by the operational amplifier upon the sensing of a fault. .Iaddend. .Iadd. 22. The combination claimed in claim 21 wherein said first and second diodes are connected to conduct current to said detection circuit and to said operational amplifier, respectively. .Iaddend. .Iadd. 23. The combination claimed in claim 20 and further including,   a second time constant circuit coupled to said second terminal of the semiconductor device,   said second time constant circuit and said semiconductor device comprising a relaxation oscillator to repetitively change the conduction state of the device upon occurrence of a fault. .Iaddend.