Faulted circuit detector having isolated indicator

• US 5,475,371 A
• Filed: 12/18/1992
• Issued: 12/12/1995
• Est. Priority Date: 06/25/1990
• Status: Expired due to Term

• Current Assignee: Cooper Industries, Inc. (Eaton Corporation PLC.)
• Original Assignee: Cooper Industries, Inc. (Eaton Corporation PLC.)
• Inventors: Rector, Gary D., Dunk, Michael P., Banting, John F.
• Primary Examiner(s): Peng, John K.
• Assistant Examiner(s): Lefkowitz, Edward
• Application Number:
  US07/958,323
• Time in Patent Office: 1,089 Days
• Field of Search: 340/664, 340/654, 340/646, 340/641, 340/660, 340/663, 340/662, 324/133, 324/127, 324/508, 324/522, 324/555, 324/547, 361/92, 361/93, 361/88, 361/35
• US Class Current: 340/660
• CPC Class Codes: G01R 15/186 using current transformers with a core consisting of two or more parts, e.g. clamp-on type G01R15/142 - G01R15/16 take precedence; tong testers G01R1/22
• CPC Class Codes: G01R 31/52 Testing for short-circuits, leakage current or ground faults
• CPC Class Codes: G01R 31/58 Testing of lines, cables or conductors testing of electric windings G01R31/72
• CPC Class Codes: H02H 3/042 combined with means for locating the fault locating faults in cables G01R31/08
• CPC Class Codes: H02H 3/044 Checking correct functioning of protective arrangements, e.g. by simulating a fault for differential current circuit breakers H02H3/335

Abstract

A faulted circuit detector having an electrically isolated and remotely positioned indicator detects fault currents in a monitored conductor. When a fault of a predetermined magnitude is detected, a light pulse is transmitted from the detector to the indicator via a fiber optic cable where, upon receipt, the pulse is converted to an electrical pulse causing the indicator to indicate a "fault" state. A reset circuit within the indicator resets the indicator at regular intervals to the "normal" state as long as a predetermined minimum voltage is present on the monitored conductor. The detector includes a temperature compensation circuit and low pass filter circuit so as to prevent misoperation due to varying ambient temperatures or high frequency transients. A test circuit is also provided to allow service personnel to field-test the device with the monitored conductor in an energized state.

Claims

1. An apparatus for detecting the presence of faults on an electrical conductor;

comprising;

mean for sensing an overcurrent condition in the electrical conductor, said sensing means comprising a temperature compensation circuit for negating the effect of temperature change on said sensing means;

means remote from said sensing means for indicating the presence of an overcurrent condition in the electrical conductor;

means for communicating the presence of an overcurrent condition sensed by said sensing means to said indicating means;

means for automatically resetting said indicating means;

means for disabling the automatic operation of said resetting means when a predetermined minimum voltage is not present on the electrical conductor; and

said temperature compensation circuit comprising

a current transformer having a wire coil, said wire coil having a positive temperature coefficient;

a thermistor having a negative temperature coefficient; and

a resistor connected in parallel with said thermistor, said resistor having a negligible resistance change at temperatures within the range of approximately -40 degrees C. to +85 degrees C.

2. The apparatus of claim 1 wherein said parallel combination of said resistor and said thermistor is connected electrically in series with said wire coil, and wherein the resistance of the parallel combination decreases in proportion to the increase in resistance of said wire coil when the temperature of said wire coil and said thermistor is increased.

3. Apparatus for detecting a fault on a monitored electrical conductor, comprising:

means for sensing a fault in the conductor, said sensing means including a temperature compensation circuit for negating the effect on said sensing means of temperature changes within the range of approximately -40 degrees C. to +85 degree C., and including a low pass filter circuit for filtering out undesirable high frequency transients induced in said sensing means;

means for remote from said sensing means for indicating the presence of a fault in the electrical conductor;

means for communicating the presence of a fault sensed by said sensing means to said indicating means;

means for automatically resetting said indicating means; and

means for disabling automatic operation of said resetting means unless a predetermined minimum voltage is present on the conductor;

said communication means electrically isolating said sensing means from said indicating means;

said resetting means continually resetting said indicating means at regular predetermined intervals unless disabled by said disabling means;

said disabling means comprising a hold-off circuit for deactivating said resetting means when said predetermined voltage is not present on the conductor; and

said temperature compensation circuit comprising;

a current transformer having a wire coil, said wire coil having a positive temperature coefficient;

a thermistor having a negative temperature coefficient; and

a resistor connected in parallel with said thermistor, said resistor having a negligible resistance change at temperatures within the range of approximately -40 degrees C. to +85 degrees C.

4. Apparatus for indicating that a fault of a predetermined current magnitude has occurred in a monitored electrical conductor on the high voltage side of an electrical distribution transformer, comprising:

a sensor for generating a signal proportional to the current in the monitored conductor;

a low pass filter for filtering high frequency transients in the signal generated by said sensor to prevent the transients from causing misoperation of the apparatus;

an optic transmitter for transmitting a light pulse when a fault condition has been sensed by said sensor;

an optic receiver for receiving said light pulse and transforming said pulse into an electrical pulse;

an indicator for indicating the presence of a fault condition in the monitored conductor upon receipt of said electrical pulse, said indicator having first and second indication states for, respectively, displaying indications of "normal" or "fault" conditions;

a reset circuit for resetting said indicator from the second of said indication states to the first of said indication states; and

a hold-off circuit for delaying the reset circuit from resetting said indicator unless a predetermined minimum voltage is present on the low voltage side of the electrical distribution transformer.

5. The apparatus of claim 4 further comprising a temperature compensation circuit for compensating for temperature-induced variations in the signal that is generated by said sensor.

6. The apparatus of claim 5 wherein said optic transmitter comprises:

a light emitting diode adapted for transmitting a pulse of light through a fiber optic cable when current flows through said diode; and

a silicon unilateral switch which closes and allows current to flow to said light emitting diode when a predetermined voltage is present across said switch.

7. The apparatus of claim 6 wherein said optic transmitter further comprises a resistor for causing said predetermined voltage to appear across said silicon unilateral switch when a fault of the predetermined current magnitude has been sensed by said sensor.

8. The apparatus of claim 5 further comprising a test circuit for testing said apparatus and causing said optic transmitter to transmit a light pulse to said receiver to indicate the presence of an overcurrent condition in the monitored electrical conductor, said test circuit comprising a manually operable switch in said optic transmitter.

9. The apparatus of claim 4 wherein said low pass filter has a roll-off point within the range of approximately 1,000-2,000 hz.

10. The apparatus of claim 5 wherein said low pass filter has a roll-off point within the range of approximately 1,000-2,000 hz.

11. The apparatus of claim 10 wherein said sensor comprises a wire coil having a positive temperature coefficient and wherein said temperature compensating circuit comprises a thermistor having a negative temperature coefficient.

12. The apparatus of claim 11 wherein:

said temperature compensating circuit further comprises a resistor having a negligible resistance change at temperatures within the range of approximately -40 degrees C. to +85 degrees C. connected in parallel with said thermistor;

wherein said parallel combination of said resistor and said thermistor is connected electrically in series with said wire coil; and

wherein the resistance of the parallel combination decreases in proportion to the increase in resistance of said wire coil when the temperature of said wire coil and said thermistor is increased.

13. The apparatus of claim 5 further comprising a timer adapted for receiving the electrical pulse from said optic receiver wherein said timer, upon receipt of the electrical pulse from said optic receiver, transmits an output pulse causing said indicator to change from the first of said indication states to the second of said indication states and causing said reset circuit to delay a predetermined period of time before resetting said indicator.

14. An apparatus for detecting the presence of faults on an electrical conductor, comprising:

a sensor for sensing an overcurrent condition in the electrical conductor, said sensor comprising a temperature compensation circuit for negating the effect of temperature change on said sensing means, said temperature compensation circuit comprising;

a current transformer having a wire coil, said wire coil having a positive temperature coefficient;

a thermistor having a negative temperature coefficient; and

a resistor connected in parallel with said thermistor, said resistor having a negligible resistance change at temperatures within the range of approximately -40 degrees C. to +85 degrees C.;

an indicator for indicating the presence of an overcurrent condition in the electrical conductor; and

a signal transmitter for communicating the presence of an overcurrent condition sensed by said sensor to said indicator.