Ground fault detection system with sequential line testing

Claim 3

A system as claimed in claim 1, in which said detector circuit comprises a first active semiconductor unit connected to function as a constant current source, and a second active semiconductor unit, coupled to the first active semicondUctor so that the preset current level is established by the active current source, and the alarm unit is energized when the current flowing through the second active semiconductor unit exceeds the preset level.

Claim 4

A system as claimed in claim 1, and further comprising a resistive bridge circuit coupled between said lines, and a fault condition detection network, coupled to the resistive bridge circuit, for providing a current signal exceeding said preset level when an unbalanced signal is received from the resistive bridge circuit due to a single fault occurring on either line, or due to occurrence of a substantially unbalanced double fault on both lines.

Claim 5

A detector system for signalling the presence of a ground fault on one or both of a pair of lines connected to transfer passive a-c energy of given frequency and given amplitude between a power isolation transformer and a load, comprising: means for providing an apparent source of active energy of a frequency substantially equal to said given frequency and of an amplitude substantially equal to said given amplitude, but which active energy is either in phase with, or is phase-shifted by 180* relative to, the passive a-c energy present on said lines; a ground fault detection network, including a detector circuit connected to provide an alarm signal responsive to sensing fault current flow exceeding a preset level; a switching network, coupled both to said lines and to the apparent source of active a-c energy, operable in different states to sequentially test the condition of said lines, and to complete a path for passing a current signal exceeding said preset level to the detector circuit when a fault condition occurs; means for sequentially driving said switching network through its different states to continually test for ground faults; and means for energizing said system.

Claim 6

A detector system for signalling the presence of a ground fault on one or both of a pair of lines connected to transfer passive a-c energy of given frequency and given amplitude between a power isolation transformer and a load, comprising: means for providing an apparent source of active energy of a frequency substantially equal to said given frequency and of an amplitude substantially equal to said given amplitude, but which active energy is either in phase with, or is phase-shifted by 180* relative to, the passive a-c energy present on said lines; a ground fault detection network, including a detector circuit connected to provide an alarm signal responsive to sensing fault current flow exceeding a preset level; a switching network, coupled both to said lines and to the apparent source of active a-c energy, operable in different states to sequentially test the condition of said lines, and to complete a path for passing a current signal exceeding said preset level to the detector circuit when a fault condition occurs; means for sequentially driving said switching network through its different states to continually test for ground faults; and means for energizing said system.

Claim 7

A system as claimed in claim 4, in which the means for providing the apparent source of active energy includes a transformer having a control secondary winding, which system further includes a pair of resistor-capacitor networks, each network being coupled to one end of said control secondary winding, for providing substantially unity power factor for the source of apparent power seen by said switching network.

Claim 8

A system as claimed in claim 4 and in which said switching network comprises a series-coupled resistance of given value, and said ground fault detection network includes a variable resistor of a value at least an order of magnitude less than said given value, such that the alarm setting of the detector circuit can be adjusted while maintaining a high detector impedance to minimize leakage current from the detector system to the isolated lines.

Claim 9

A system as claimed in claim 4, and in which said ground fault detection network comprises an active semiconductor unit coupled in the detector current path for sensing a fault current exceeding a preset level, and further comprises means for indicating the amplitude of fault current flowing through the detector circuit.

Claim 10

A detector system as claimed in claim 4, and further comprising a test switch, connected for actuation to complete a path for detector current flow while still maintaining system isolation, thus testing the detector system without imposing a hazard on the a-c lines.

Claim 11

A detector system as claimed in claim 4, in which the means for providing an apparent source of active energy comprises a transformer, and an additional secondary winding on said transformer, a rectifier circuit coupled to said additional secondary winding to provide d-c energy, and an alarm circuit having at least one condition-indicating unit connected for energization by the d-c energy, thus minimizing capacitive fault leakage current which would otherwise be imposed by the detector system on the a-c lines.

Claim 12

A system as claimed in claim 4, in which the output signal from the detector circuit is a series of pulses, and further comprising an incanDescent lamp having a filament winding connected to receive the pulse signals from the detector circuit such that the output signal from the lamp is an average value signal for indicating the presence of a fault condition.

Claim 13

A system as claimed in claim 4, in which the means for providing an apparent source of active energy includes a center-tapped secondary winding, and the ground fault detection network is coupled between the secondary winding connections and the pair of a-c lines under test, such that the means for driving the switching network in effect provides a phase modulation of the active energy by polarity reversal of the voltage provided over the secondary winding in synchronization with the sampling of the possible fault conditions by the ground fault detection network.

Claim 14

A system for detecting ground faults as claimed in claim 5, and in which said switching network comprises a plurality of photocells connected for sequential illumination by the means for sequentially driving the switching network, said photocells being disposed to complete current flow paths between each end of the control secondary winding and one of the two a-c lines over which passive energy is passed to the electrical load.

Claim 15

A detector system for signalling the presence of a ground fault on one or both of a pair of lines connected to transfer passive a-c energy of given frequency and given amplitude between a power isolation transformer and a load, comprising: means for providing an apparent source of active energy of a frequency substantially equal to said given frequency and of an amplitude substantially equal to said given amplitude, but which active energy is either in phase with, or is phase-shifted by 180* relative to, the passive a-c energy present on said lines; a ground fault detection network, including a detector circuit connected to provide an alarm signal responsive to sensing fault current flow exceeding a preset level; a switching network, coupled both to said lines and to the apparent source of active a-c energy, operable in different states to sequentially test the condition of said lines, and to complete a path for passing a current signal exceeding said preset level to the detector circuit when a fault condition occurs; a resistive bridge circuit coupled between said lines, for providing an unbalance signal when a single fault occurs on either line, and when a substantially unbalanced double fault occurs on both lines; a fault condition detection network, connected to pass a signal to the switching circuit responsive to receipt of the unbalance signal from the resistive bridge circuit when the switching network is in certain ones of its different states; means for sequentially driving said switching network through its different states to continually test for ground faults; and means for energizing said system.

Claim 16

A detector system for signalling the presence of a ground fault on one or both of a pair of lines connected to transfer passive a-c energy of given frequency and given amplitude between a power isolation transformer and a load, comprising: means for providing an apparent source of active energy of a frequency substantially equal to said given frequency and of an amplitude substantially equal to said given amplitude, but which active energy is either in phase with, or is phase-shifted by 180* relative to, the passive a-c energy present on said lines; a ground fault detection network, including a detector circuit connected to provide an alarm signal responsive to sensing fault current flow exceeding a preset level; a switching network, coupled both to said lines and to the apparent source of active a-c energy, operable in different states to sequentially test the condition of said lines, and to complete a path for passing a current signal exceeding said preset level to the detector circuit when a fault condition occurs; a resistive bridge circuit coupled between said lines, for providing an unbalance signal when a single fault occurs on either line, and when a substantially unbalanced double fault occurs on both lines; a fault condition detection network, connected to pass a signal to the switching circuit responsive to receipt of the unbalance signal from the resistive bridge circuit when the switching network is in certain ones of its different states; means for sequentially driving said switching network through its different states to continually test for ground faults; and means for energizing said system.

Claim 17

A detector system as claimed in claim 13, and further comprising circuit means, coupled between the ground fault detection network and the resistive bridge circuit, for providing parallel current paths from the ground fault detection network to each of the pair of lines, such that under a balanced fault condition, the indicating means of the detector system can be calibrated for the same level of current as for operation with single line fault conditions.

Claim 18

A detector system as claimed in claim 13, in which the means for sequentially driving the switching network operates first to test for a ground fault on one line of the pair, second to test for a ground fault on the other line of said pair, third to test for simultaneous faults on both lines when the fault is unbalanced in a given direction, fourth to test for simultaneous faults on both lines when the fault is unbalanced in a direction opposite said given direction, and, during both the third and fourth steps, also tests for a balanced fault on both lines.