Line isolation monitor and alarm

Claim 2

A line isolation monitor for monitoring the leakage condition of an isolated electrical system having leads carrying AC voltages of a frequency of F hertz comprising: a core of magnetic material; a first winding on said core; a first end and a second end and a center tap on said winding; first circuit means connected to one end of said winding and adapted for connection to one of said power lines, comprising resistance means and series connected semiconductor controlled rectifier means; second circuit means connected between the other end of said winding and adapted for connection to a second of said power lines, said second circuit means including resistance means and series connected semiconductor controlled rectifier means; periodic switch means coupled to each of said semiconductor controlled rectifier means for alternately and cyclically triggering said semiconductor controlled rectifier means into the current conducting condition; said switch means having a periodic rate of less than said line frequency; means connected between said center tap and electrical ground potential for providing a resistive path therebetween to ground and measuring means associated with said last named means for indicating the current level through said path.

Claim 3

The invention as defined in claim 1 further comprising capacitor means connected between the ends of said winding, said capacitor means being sized to form with said winding a circuit resonant at said line frequency, whereby each controlled switch means is placed in the current conducting condition for a predetermined number of AC half cycles to complete a unidirectional current conducting path.

Claim 4

The invention as defined in claim 1 further comprising: a second winding located on said iron core, said second winding being inductively coupled to said first winding whereby current through said first winding generates a corresponding voltage in said second winding; means connected to said second winding for monitoring the level of voltage present in said second winding; and alarm means coupled to said voltage monitoring means for providing an alarm signal responsive to said voltage monitoring means for detecting a voltage greater than said predetermined level.

Claim 5

The invention as defined in claim 1 wherein said current carrying path comprises a bridge rectifier having a pair of input terminals, one of which is connected to said center tap and the second of which is connected to ground, and a pair of output terminals; resistor means connected across said output terminals and voltage monitoring meter means connected across and resistor means.

Claim 6

The invention as defined in claim 1 wherein said periodic switch means comprises: multivibrator means having an output; first and second light emitting diode means oppositely poled and connected across said multivibrator output; first and second light sensitive transistor means having an optical input and an electrical output; means optically coupling the output of said first light emitting diode to the input of said first transistor means; means optically coupling the light output of said second transistor to the input of said second transistior means; and means for connecting said outputs of said first and second transistor means to the respective trigger inputs of said first and second semiconductor controlled rectifier means for providing a trigger voltage thereto.

Claim 7

Condition responsive alarm means comprising: condition responsive means for providing a representative DC voltage of a first polarity and of a voltage level functionally proportional to the level of the condition being monitored; reference voltage source means for providing a first predetermined DC reference voltage; first means for combining said first predetermined DC reference voltage and said representative DC voltage in voltage subtractive relationship to obtain a voltage essentially equal to the difference therebetween; a second voltage source for producing a second predetermined DC reference voltage; a third voltage source for providing an AC voltage; semiconductor controlled rectifier means, said controlled rectifier means having a control input and being connected in circuit with said third voltage source for rectifying AC voltage from said third source to produce a third DC reference voltage when said SCR is in the current conducting condition, said third DC reference voltage source being larger than said second DC reference voltage; means coupling said first voltage combining means to the input of said SCR for biasing said SCR into the current conducting condition when the voltage from said combining means is above a predetermined level; a voltage divider network; means for combining said second and third DC reference voltages in voltage subtractive relationship across said voltage divider network; tap means on said voltage divider network; indicator means responsive to the voltage at said tap means for providing a first indication when said voltage is of a first polarity and a second signal indication when said voltage is of a second polarity.

Claim 8

Condition responsive alarm means comprising: condition responsive means for providing a representative DC voltage of a first polarity and of a voltage level functionally proportional to the level of the condition being monitored; reference voltage source means for providing a first predetermined DC reference voltage; first means for combining said first predetermined DC reference voltage and said representative DC voltage in voltage subtractive relationship to obtain a voltage essentially equal to the difference therebetween; a second voltage source for producing a second predetermined DC reference voltage; a third voltage source for providing an AC voltage; semiconductor controlled rectifier means, said controlled rectifier means having a control input and being connected in circuit with said third voltage source for rectifying AC voltage from said third source to produce a third DC reference voltage when said SCR is in the current conducting condition, said third DC reference voltage source being larger than said second DC reference voltage; means coupling said first voltage combining means to the input of said SCR for biasing said SCR into the current conducting condition when the voltage from said combining means is above a predetermined level; a voltage divider network; means for combining said second and third DC reference voltages in voltage subtractive relationship across said voltage divider network; tap means on said voltage divider network; indicator means responsive to the voltage at said tap means for providing a first indication when said voltage is of a first polarity and a second signal indication when said voltage is of a second polarity.

Claim 9

The invention as defined in claim 6 further comprising: second tap means on said voltage divider network, second indicator means responsive to the voltage at said second tap means for providing a first signal indication when said polarity of the voltage at said second tap means is of a first polarity and for providing a second indication when said voltage is of a second polarity.

Claim 10

The invention as defined in claim 6 further comprising a small container, an electrical heater located in said container for producing heat, and wherein said semiconductor controlled rectifier means is located in said container exposed to said heat whereby the temperature of said controlled switch means is stabilized.

Claim 11

The invention as defined in claim 8 further comprising thermistor means, said thermistor having a positive temperature co-efficient of resistance and being connected in said first voltage combining means, and wherein said thermistor means is located in said container means, whereby said thermistor is maintained at the same ambient temperature level as said controlled rectifier means.

Claim 12

The invention as defined in claim 6 wherein said first indicator means includes: a source of AC; semiconductor controlled rectifier means, said controlled rectifier means having an input; first lamp means; electrically actuated audible signalling means, means connecting said lamp and audible signalling means in a parallel electrical circuit; means connecting said controlled rectifier means, said AC source and said parallel electrical circuit in series circuit; and means coupling said input of said controlled rectifier means in circuit with said tap means.

Claim 13

The invention as defined in claim 10 further comprising second lamp means, switch means, said switch means having a make contact, a break contact and a transfer contact, said transfer contact being connected in circuit with said AC source, said break contact being connected in circuit with said aubible signalling means and wherein said make contact is connected in circuit with said second lamp means.

Claim 14

Condition responsive alarm means comprising: first means for generating a DC voltage of a first polarity and of a level dependent upon the condition being monitored; reference voltage source means for providing a predetermined DC voltage of a second polarity, said voltage source means including a potentiometer having a movable tap, diode means, and resistor means; an output juncture; means electrically connecting said diode means and said resistor means in series circuit between said tap and said output juncture; thermistor means, said thermistor means being connected in series circuit with said first voltage generating means and said output terminal, whereby said reference voltage and said condition dependent voltage are connected in subtractive relationship to provide a voltage at said output terminal equal to the difference therebetween; first and second AC voltage sources for providing first and second AC voltages, the level of said second AC voltage source being greater than the level of said first AC source and approximately 180° out of phase therewith; rectifier means adapted to conduct current only during alternate half cycles of AC and first resistor means connected electrically in series with said rectifier means across said first AC source; semiconductor controlled rectifier means having an input and adapted to conduct current only during alternate half cycles of AC when said input has a predetermined voltage applied thereto; second resistor means; said second resistor means connected in series with said controlled rectifier means across said second source; each of said diode and semiconductor controlled rectifier means being poled in circuit to conduct current only during the same AC half cycles and means for combining the voltages as appear across said first and second resistors in a voltage subtractive relationship; means coupling said output juncture to said controlled rectifier input; and means responsive to said combined voltage across said first and second resistors for providing a first indication when said voltage is of a first polarity and a second indication when said voltage is of the opposite polarity.

Claim 15

Condition responsive alarm means comprising: first means for generating a DC voltage of a first polarity and of a level dependent upon the condition being monitored; reference voltage source means for providing a predetermined DC voltage of a second polarity, said voltage source means including a potentiometer having a movable tap, diode means, and resistor means; an output juncture; means electrically connecting said diode means and said resistor means in series circuit between said tap and said output juncture; thermistor means, said thermistor means being connected in series circuit with said first voltage generating means and said output terminal, whereby said reference voltage and said condition dependent voltage are connected in subtractive relationship to provide a voltage at said output terminal equal to the difference therebetween; first and second AC voltage sources for providing first and second AC voltages, the level of said second AC voltage source being greater than the level of said first AC source and approximately 180° out of phase therewith; rectifier means adapted to conduct current only during alternate half cycles of AC and first resistor means connected electrically in series with said rectifier means across said first AC source; semiconductor controlled rectifier means having an input and adapted to conduct current only during alternate half cycles of AC when said input has a predetermined voltage applied thereto; second resistor means; said second resistor means connected in series with said controlled rectifier means across said second source; each of said diode and semiconductor controlled rectifier means being poled in circuit to conduct current only during the same AC half cycles and means for combining the voltages as appear across said first and second resistors in a voltage subtractive relationship; means coupling said output juncture to said controlled rectifier input; and means responsive to said combined voltage across said first and second resistors for providing a first indication when said voltage is of a first polarity and a second indication when said voltage is of the opposite polarity.

Claim 16

Condition responsive alarm means comprising: condition respsonsive means for providing a DC voltage of a first polarity and a voltage level functionally proportional to the level of the condition being monitored; reference voltage means for providing a first predetermined DC reference voltage; first means for combining said first predetermined DC reference voltage and said representative DC voltage in voltage subtractive relationship to obtain a voltage essentially equal to the difference therebetween; a second voltage source for producing a second predetermined DC reference voltage; a third voltage source for providing a predetermined AC voltage; semiconductor controlled rectifier means, said controlled rectifier means having a control input and being connected in circuit with said third voltage source for rectifying AC voltage therefrom to produce a third DC reference voltage; means for combining said second and third DC reference voltages in voltage subtractive relationship; means coupling said first voltage combining means to the input of said semiconductor controlled rectifier for biasing saiid controlled rectifier into the current conducting condition when the voltage thereof is above a predetermined level and into a noncurrent conducting condition when said voltage thereof is less than said predetermined voltage level; and alarm means responsive to the voltage of said second combining means for producing an alarm indication in response to said voltage being of a first polarity and for producing a second alarm indication when said voltage is of the opposite polarity.

Claim 17

Condition responsive alarm means comprising: condition respsonsive means for providing a DC voltage of a first polarity and a voltage level functionally proportional to the level of the condition being monitored; reference voltage means for providing a first predetermined DC reference voltage; first means for combining said first predetermined DC reference voltage and said representative DC voltage in voltage subtractive relationship to obtain a voltage essentially equal to the difference therebetween; a second voltage source for producing a second predetermined DC reference voltage; a third voltage source for providing a predetermined AC voltage; semiconductor controlled rectifier means, said controlled rectifier means having a control input and being connected in circuit with said third voltage source for rectifying AC voltage therefrom to produce a third DC reference voltage; means for combining said second and third DC reference voltages in voltage subtractive relationship; means coupling said first voltage combining means to the input of said semiconductor controlled rectifier for biasing saiid controlled rectifier into the current conducting condition when the voltage thereof is above a predetermined level and into a noncurrent conducting condition when said voltage thereof is less than said predetermined voltage level; and alarm means responsive to the voltage of said second combining means for producing an alarm indication in response to said voltage being of a first polarity and for producing a second alarm indication when said voltage is of the opposite polarity.