James G. Biddle Company

Northeast Utilities Service Company

Carl R Scharle

Peter H Reynolds

Joseph F Barresi

Harry L Latham

Charles Saile

Donald S Ironside

Three-phase power equipment while still connected to the three-phase line has the insulation qualities of its various phases tested. All ground connections of said equipment except one are disconnected and that one is reconnected through ground current sensing means. Coupling units which may be capacitors are respectively connected between each of the three phases of the power line to which the power equipment is connected and ground. Each coupler unit is a reference phase current sensing means. Adjustable phase and magnitude adjustor means are used to couple the respective phase current sensing means to a summing means so that they are adding. The ground current sensing means is similarly coupled to the same summing means but opposed to phase current effects. A null detector coupled to the summing unit detects when balance occurs. Each of the phases is checked in sequence using fixed predetermined settings of the phase and magnitude adjusting means for the other two phases and separately and sequentially varying the settings of capacitance and dissipation factor for the phase under test until nulling occurs. Deviation in the settings of the adjustable phase and magnitude adjustor means from previous settings are considered in determining whether changes suggest deterioration of insulation and in which phase such deterioration may be occurring.

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Glenside, PA, United States of America

Carl R Scharle