Line isolation monitor and alarm

Details Line isolation monitor and alarm Line isolation monitor and alarm

1975-04-25

1976-07-13

Caldwell, John W.

Communications: electrical

Systems

Call station

317 18R, 317 18D, 340253R, G08B 2100

Patent

active

039697118

ABSTRACT:
The monitor includes a pair of circuits; each circuit includes a resistor and an SCR in series and each circuit is connected to a respective end of a center-tapped primary of a current transformer. Means are provided to alternately and cyclically operate the SCR's from an OFF to an ON state. A current path is provided between the center tap of said primary and ground. Current through the pair of circuits and the current path is monitored to determine leakage to ground for controlling alarm means.
The novel alarm structure accomplishes the foregoing with a means that derives a DC voltage representative of the condition and combines that voltage in subtractive relationship with the voltage of a reference source and applies the sum to the gate of a Silicon Controlled Rectifier. A second and third voltage source provide AC voltage, oppositely poled, one of which is rectified by a rectifier, the second of which is rectified by the SCR, when in its current-conducting conditions, and means combine the rectified voltages in an essentially subtractive relationship. A first indicator operates when the combined voltage is of one polarity and a second indicator operates when the combined voltage is of a second polarity.

REFERENCES:
patent: 3579219 (1971-05-01), Wolfframm
patent: 3666993 (1972-05-01), Legatti
patent: 3696366 (1972-10-01), Parsons
patent: 3699392 (1972-10-01), Lee et al.
patent: 3754221 (1973-08-01), Stelter
patent: 3757169 (1973-09-01), Beresnikow
patent: 3816816 (1974-06-01), Schweitzer, Jr.
patent: 3868665 (1975-02-01), Treglown

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