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Megging a motor through MCC has any restriction?

Megging a motor through MCC has any restriction?

(OP)
Usually maintenance execute IR test and megging directly from motor cable box, but as an effort to save time testing it and also to cover the entire system (motor+cables) my fellow engineers are sugesting changing this procedure, making this tests throught MCC terminals.

Does it have any restriction? The test results are going to be as accurate as direct in motor cable box?

RE: Megging a motor through MCC has any restriction?

As long as you isolate the connections path so it does not back feed to some electronic component inside the mcc such as a VFD, softstart, or solid stat overload. If you did not I think you would damage these type of electronics.

RE: Megging a motor through MCC has any restriction?

My utility uses this approach in medium-voltage metalclad switchgear by means of a ground-and-test device racked into the cubicle in place of the circuit breaker; provided the precautions controlsdude listed are undertaken, shouldn't be an issue, in my view.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Megging a motor through MCC has any restriction?

It's a cheap and quick and dirty test.
That said, it has value.
If your reading is good, then the motor, any part of the starter still in the circuit and the feeders are all good.
If the reading is bad, then you have to test further to determine whether the failing resistance is in the motor or external to the motor.
This test has the advantage that tests may be completed so quickly and conveniently that it lends itself to much more frequent testing than a test of the motor only with the motor disconnected.
I would support this testing method.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Megging a motor through MCC has any restriction?

That is how we test all of our installations. Then if there is an issue, you can work back to the motor as waross said. If you have been trending, a new procedure would change your comparison data. I wouldn't expect much difference but there will be some from testing directly at the motor. If your just passing or failing, then there will be no difference.

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