DC Hipots and Megohmmeters are fairly common test sets used by test technicians and engineers. The basic operation for both a DC Hipot and a Megohmmeter is the same; a known DC voltage is applied across insulation. The applied potential causes several small currents through and over surfaces of the insulation being tested. The total current is measured and the result is displayed as either an ohmic value or as a current value.
With modern insulating materials there is little, if any, difference in the reading obtained, regardless of which way the terminals of the test set are connected. However, did you know that it is industry standard to connect the positive (+) lead to ground? In fact, the output polarity of DC Hipots is designed to be negative polarity, positive ground.
The reason for this is that on older insulation, a little known phenomenon called electroendosmosis (i′lek•trō¦en•däs′mō•səs) causes a lower insulation reading to be obtained with the positive terminal connected to the grounded side of the insulation being tested. Since insulation testing is typically concerned with safety, maintenance and /or troubleshooting the worst case reading would be the one which yields the most relevant information.
Along with DC Hipots and Megohmmeters, cable fault locating equipment such as Surge Generators (thumpers) also use a negative output with reference to ground.
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Editor’s Note: This post was originally published March, 2012 and has been updated freshness, accuracy and comprehensiveness.