A portable instrument for measuring insulation resistance at high voltage

THERE has been an increasing trend toward measuring insulation resistance of high-voltage machines and cables at or above their rated voltage by the use of direct current. The advantage of high voltage lies in its ability to detect leakage paths which do not occur at lower voltages. ¹ D-c tests are advantageous because of the elimination of capacitance-current effects. Since the resistances measured are usually quite high — of the order of megohms — if high-voltage alternating current is used to test insulation, even small load capacitances offer impedances low enough to mask the insulation-resistance effect.