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A Reminder about Megohmmeter Testing
A technical tip from Electrical Apparatus' Engineering Editor, Richard L. Nailen
Although no single test can fully define insulations condition in a motor or
generator winding, the most common indicator is insulation resistance (IR)
measurement.
Debates continue about the proper test voltage for various insulation ratings; whether or not IR testing is "destructive"; and the minimum acceptable value of polarization index.
Less well publicized is the effect on personnel safety (and on the measurements themselves) of the capacitive charge such testing leaves on the winding, and the importance of discharging that stored energy before performing any subsequent tests or other work.
Applying a d-c megohmmeter voltage (of whatever magnitude) for a one-minute (spot) reading or ten minute (PI) test will store energy in the capacitance inherent between winding and core or frame. The capacitance is small. Stored energy is low. But the associated voltage can be dangerously high until the charge has time to leak off.
Because that dissipation time cannot be accurately predicted, IEEE Standard 43 warns us to consider the winding unsafe for at least four times the duration of the test voltage application. That would mean 40 minutes for a normal PI test. To ensure safety, the standard recommends measuring the residual voltage between winding and core, considering the safe maximum to be 20 volts.
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Technical Articles
by Richard L. Nailen
Click the links below to find summaries of many technical articles written for Electrical Apparatus magazine by Richard L. Nailen, P.E.
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