After analysis, it turned out that someone, while searching for a cable of suitable length to connect the Megger to the motor, found an L.T. three-core cable lying around and used two cores of this cable. Since the Megger was a 10kV HT, it was measuring the insulation resistance between two cores of a low-tension cable.
Even though the motor at the end of the cable had attained high insulation resistance after heating, the lead insulation itself was much lower, and, naturally, the lower value was being indicated by the Megger. The Megger was measuring the inter-lead insulation, not the motor insulation value.
This entry was submitted by Raghavendra Girimaji and edited by Rob Spiegel.
Raghavendra Girimaji has spent more than 30 years in the electrical industry. In recent years, he has been involved in renewable energy. He now teaches engineering.
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