On a motor, doing the Insulation test with a megger at 500 volts.

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jonathn1

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Location
Macau, China
Recently involved with testing insulation on transformers for a site in macau China. It was proposed that the test be completed by installing the black lead of the megger to the three phase wires and the red lead to the ground wire. Since we are testing insulation only and not the voltages i understand it doesnt really matter which location you place the leads. But as a matter of consistency and safety I was always trained to place the black lead on the ground wire and move the red lead to each phase. This would have me practice safe skills for when I was trying to get voltage readings later.
Well they came back to me and said that they had a study done which showed placing the black lead on the power wire and the red on the ground created some type of boundary in the wire which gave a better result on the test.
Have I been mis trained? Is this true? I figured the first place to check is with the people here I have learned to trust and see what i can find out.
Any information on the correct procedure for testing i would certainly appreciate.
 
zog

zog

Senior Member
Location
Charlotte, NC
It inly makes a difference if water is present, when you have moisture in windings or in insulating oil. Water has a polarity and will effect your readings. There is a reference to this in IEEE-43, called electrodosmosis or something like that. Red on ground, black on copper is the correct set up, this is also shown in "A stitch in time" by Megger.

Reversing polarity if you suspect moisture in insulation is a good way to verify it, then you know if baking will be effective.
 
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