Transient-free isolation monitor

Claim 2

In a line isolation monitor for detecting the existence of ground faults on power conductors normally isolated from ground: first and second phase-shift networks connected between the conductors for providing first and second signals to be monitored, signal monitoring means responsive to a current flowing between the input of the monitoring means and ground, a first photoisolator connected between the first phase-shift network and the input of the monitoring means, a second photoisolator connected between the second phase-shift network and the input of the monitoring means, means for providing first and second generally rectangular timing signals of predetermined frequency and opposite phase, means including a delay network responsive to the timing signals for delivering generally rectangular control signals which rise gradually from a first level to a second level a predetermined time after a transistion of predetermined polarity in the timing signals, and means for applying the generally rectangular control signals to the photoisolators to alternately condition the photoisolators to pass the first and second signals to the monitoring means, the photoisolators being conditioned to pass the first and second signals to the indicator means when the respective control signals are at the second level.

Claim 3

The line isolation monitor of claim 1 wherein each of the photoisolators comprises a light emitting element and a light sensitive element having a resistance dependent on the amount of light impinging thereon.

Claim 4

In a line isolation monitor for detecting the existence of ground faults on power conductors normally isolated from ground: first and second phase-shift networks connected between the conductors for providing first and second signals to be monitored, signal monitoring means responsive to a current flowing between the input of the monitoring means and ground, a first photoisolator connected between the first phase-shift network and the input of the monitoring means, a second photoisolator connected between the second phase-shift network and the input of the monitoring means, means for providing first and second generally rectangular timing signals of predetermined frequency and opposite phase, means responsive to the timing signals for delivering generally rectangular control signals which rise from a first level to a second level a predetermined time after a transistion of predetermined polarity in the timing signals, and means for applying the generally rectangular control signals to the photoisolators to alternately condition the photoisolators to pass the first and second signals to the monitoring means, the means for delivering generally rectangular control signals comprising for each of the photoisolators: a one-shot multivibrator having a period corresponding to the predetermined time, a NOR gate having one input connected to the output of the multivibrator, and means for applying one of the timing signals to a second input of the NOR gate and to the input of the multivibrator.

Claim 5

In a line isolation monitor for detecting the existence of ground faults on power conductors normally isolated from ground: first and second phase-shift networks connected between the conductors for providing first and second signals to be monitored, signal monitoring means responsive to a current flowing between the input of the monitoring means and ground, a first photoisolator connected between the first phase-shift network and the input of the monitoring means, a second photoisolator connected between the second phase-shift network and the input of the monitoring means, means for providing first and second generally rectangular timing signals of predetermined frequency and opposite phase, means responsive to the timing signals for delivering generally rectangular control signals which rise from a first level to a second level a predetermined time after a transistion of predetermined polarity in the timing signals, and means for applying the generally rectangular control signals to the photoisolators to alternately condition the photoisolators to pass the first and second signals to the monitoring means, the means for delivering generally rectangular control signals comprising for each of the photoisolators: a one-shot multivibrator having a period corresponding to the predetermined time, a NOR gate having one input connected to the output of the multivibrator, and means for applying one of the timing signals to a second input of the NOR gate and to the input of the multivibrator.

Claim 6

In a line isolation monitor for detecting the existence of a ground fault on power conductors normally isolated from ground: ground fault detector means, first and second photoisolators connected between the conductors and the ground fault detector means, each of said photoisolators having a conductivity determined by the level of a control signal applied thereto, and means for applying cyclical control signals of predetermined non-instantaneous rise time to the photoisolators for alternately increasing the conductivities thereof in a predetermined gradual manner.

Claim 7

In a line isolation monitor for detecting the existence of a ground fault on power conductors normally isolated from ground: ground fault detector means, first and second photoisolators connected between the conductors and the ground fault detector means, each of said photoisolators having a conductivity determined by the level of a control signal applied thereto, and means for applying cyclical control signals of predetermined non-instantaneous rise time to the photoisolators for alternately increasing the conductivities thereof in a predetermined gradual manner.

Claim 8

The line isolation monitor of claim 4 wherein the means for applying control signals includes means for delaying the rise of the control signal applied to the first photoisolator until a predetermined time after the control signal applied to the second photoisolator drops to a predetermined level.