Case of the Plunging Insulation Resistance Values

I was overseeing the construction and commissioning of electrics at the Bhilai Steel Plant in central India. We were in the process of installing a high-tension slip-ring induction motor of 250KW for driving a cone crusher. Our standard practice was to check the insulation levels and only start the motor when the IR (Insulation Resistance) value was more than several megaohms.

On initial checking, we found that the IR value was much less than 1Mohm. We didn't consider this unusual, since the motor was shipped to us a while back, and during several weeks of transportation and storage, moisture could have seeped in. As a standard remedial measure, we heated the motor, first by using external means and later by passing a current in the rotor of a value higher than its rated load current.

We placed the motor in its intended location, which was about 25m below ground level, in a pit. Since it was very cumbersome to get down into the pit, every time we needed to measure the IR value, we ran a set of cables down from the Megger and into the pit to connect to the motor.

The heating was started, and, at one-hour intervals, we took the IR value readings of the motor. We were surprised to see that the IR value continued to remain low. After several rounds of heating, the result was the same, much to the annoyance of the commissioning crew. The engineers were perplexed.

The situation was becoming very critical. The plant commissioning date was fixed. In three days, we had to start the plant to coincide with a visit from the Minister of Steel and Mines.

After many hours of lost sleep and anxiety all around, it occurred to me to go down to the pit with the Megger and check the IR value by connecting the Megger leads directly to the motor terminals. The value indicated was more than 100Mohm, which was perfect! We took the readings many times to make sure the values were consistent.