Ground fault circuit interrupter utilizing a single transformer
First Claim
1. A ground fault circuit interrupting device for monitoring an electrical distribution circuit including a line conductor and a neutral conductor grounded at the source and interrupting the circuit in the event of a ground fault on the line conductor resulting in a ground fault current exceeding a predetermined current value and a ground fault on the neutral conductor of a resistance below a predetermined resistance value, said device comprising, in combination:
- A. a differential current transformer having1. a first winding for connection in series with the line conductor,2. a second winding for connection in series with the neutral conductor,
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Accused Products
Abstract
A GFCI utilizes a single transformer in responding to ground faults on either of the line and neutral conductors of a distribution circuit. The transformer, a differential current transformer, has a secondary winding which is periodically strobed during a first sampling interval for the presence of a fault signal indicative of a line-ground fault. During a second sampling interval, a negative resistance network sets up a ringing oscillation in the transformer secondary circuit. If the neutral conductor is grounded through a low impedance ground fault, the oscillation is damped, and circuit interruption is initiated. Absent a low impedance ground fault on the neutral conductor, the oscillation amplitude increases to exceed a predetermined threshold level during the second sampling interval, and circuit interruption is not initiated.
68 Citations
21 Claims
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1. A ground fault circuit interrupting device for monitoring an electrical distribution circuit including a line conductor and a neutral conductor grounded at the source and interrupting the circuit in the event of a ground fault on the line conductor resulting in a ground fault current exceeding a predetermined current value and a ground fault on the neutral conductor of a resistance below a predetermined resistance value, said device comprising, in combination:
A. a differential current transformer having 1. a first winding for connection in series with the line conductor, 2. a second winding for connection in series with the neutral conductor, - View Dependent Claims (3, 5)
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2. a third winding in which is developed a fault signal proportional in amplitude to the ground fault current occasioned by a ground fault on the line conductor;
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B. first means monitoring the amplitude of said fault signal and initiating circuit interruption when said fault signal amplitude exceeds a first predetermined threshold level correlated to said predetermined current value; C. second means connected with said third winding to form therewith a tank circuit, the resistive impedance of a neutral ground fault being reflected into said tank circuit via said second winding; D. a negative resistance network connected to shock excite said tank circuit and develop therein a ringing oscillation, said negative resistance network operative to increase the rate of change of the Q of said tank circuit for variations in neutral ground fault resistance about said predetermined resistance value and to establish a negative Q in said tank circuit for a neutral ground fault resistance equalling said predetermined resistance value, whereby the operation of said tank circuit is regenerative for neutral ground fault resistances equal to and greater than said predetermined resistance value; and E. third means monitoring the amplitude of said ringing oscillation during a prescribed time period succeeding shock excitation of said tank circuit to detect a neutral ground fault having resistance below said predetermined resistance level, whereupon to initiate circuit interruption. - View Dependent Claims (4, 6, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 21)
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7. A ground fault circuit interrupting device for monitoring an electrical distribution circuit including a line conductor and a neutral conductor grounded at the source and selectively interrupting the circuit in the event of a ground fault on either conductor, said device comprising, in combination:
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A. a differential current transformer having 1. a first winding for connection in series with the line conductor, 2. a second winding for connection in series with the neutral conductor, and 3. a third winding; B. first means monitoring the signal condition in said third winding to detect the presence of a ground fault on the line conductor; C. second means monitoring the signal condition in said third winding to detect the presence of a ground fault on the neutral conductor; D. a programmer synchronized to the frequency of the distribution circuit for alternately conditioning said first monitoring means to sample the signal condition in said third winding during a first sampling period and the second monitoring means to sample the signal condition in said third winding during a second sampling period, said programmer further operating to disable said second monitoring means during said first sampling period and to disable said first monitoring means during said second sampling period; and E. means responsive to said first monitoring means for initiating circuit interruption in the event fault current flowing through a ground fault on the line conductor exceeds a predetermined current level and responsive to said second means to initiate circuit interruption in the event the neutral conductor is faulted to ground through an impedance below a predetermined impedance level.
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10. A ground fault circuit interrupting device for monitoring an electrical circuit including a line conductor and a neutral conductor grounded at the source and selectively interrupting the circuit in the event of ground faults on either conductor, said device comprising, in combination:
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A. a differential current transformer having 1. a first winding for connection in series with the line conductor, 2. a second winding for connection in series with the neutral conductor, and 3. a third winding in which is developed a fault signal proportional in amplitude to any differential in the currents flowing in said first and second windings occasioned by a ground fault on the line conductor; B. first means monitoring the amplitude of said fault signal and initiating circuit interruption when said fault signal amplitude exceeds a predetermined threshold level; C. second means connected with said third winning to form a tank circuit; D. a negative resistance network connected to shock excite said tank circuit and develop therein a ringing oscillation, said network including means for producing either regenerative or degenerative operation of said tank circuit depending on the resistance impedance of any ground fault on the neutral conductor as reflected into said tank circuit via said second transformer winding; and E. third means monitoring the amplitude of said ringing oscillation and initiating circuit interruption should the amplitude thereof fail to exceed a predetermined threshold level within a prescribed time period succeeding shock excitation of said tank circuit.
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19. A ground fault circuit interrupting device for monitoring an electrical circuit including a line conductor and a neutral conductor grounded at the source and selectively interrupting the circuit in the event of ground faults on either conductor, said device comprising, in combination:
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A. a differential current transformer having 1. a first winding for connection in series with the line conductor, 2. a second winding for connection in series with the neutral conductor, and 3. a third winding in which is developed a fault signal proportional in amplitude to any differential in the currents flowing in said first and second windings occasioned by a ground fault on the line conductor; B. first, second and third switches; C. a burden resistor connected across said first switch; D. a capacitor connected across said second switch; E. a resistor connected across said capacitor through said third switch; F. a negative resistance network connected across said third switch; G. a programmer synchronized to the frequency of the distributing circuit and operating to control the condition of said switches, said programmer 1. maintaining said switches closed to short-circuit said third winding through said first and second switches, 2. opening said first switch to initiate a first sampling period during which current flow in said third winding is diverted through said burden resistor and to develop thereacross a fault signal voltage.
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20. closing said first switch to conclude said first sampling period and opening said switch to connect said capacitor and resistor in parallel across said third winding to form a tank circuit effective to damp out any current in said third winding,opening said third switch to initiate a second sampling period, andclosing said second and third switches to conclude said second sampling period;
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H. excitation means operable upon opening of said third switch to shock excite said tank circuit and produce a ringing oscillation therein, said oscillation increasing in amplitude over said second sampling in regenerative fashion at a rate established by said negative resistance network in accordance with the resistive impedance of any ground fault on the neutral conductor as reflected into said tank circuit via said second transformer winding; I. first means monitoring the amplitude of said fault signal developed across said burden resistor during said first sampling period and initiating circuit interruption when said fault signal voltage exceeds a first predetermined threshold level; J. second means monitoring the amplitude of said ringing oscillation and initiating circuit interruption should the amplitude thereof fail to exceed a second predetermined threshold level during said second sampling period.
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Specification