Megger usage

[jonathn1]

Originally posted in another area but couldnt get answer. Hopefully others can help.
Doing insulation tests on delta 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 procedure for testing you use i would certainly appreciate.

 

[SG-1]

I would ask them for a copy of the study, so I could better educate myself. I do not perform Megger Testing, so take this for what it is worth.

I do know that when you are megger testing a vaccum bottle, the test is performed twice with the polarity switched between tests. This information is in the breakers instruction manual, thats how I know this. The AC Dielectric Test is done in a simular manner, with 2 shots. I used to do this on a regular basis, every day.

BIL Testing is done using both a positive & a negative impulse on the test object.

They could very well have a point.

So far as safety is concerned the most important thing is the grounding & bonding of the test object so if it fails there is a low impedance path back to the megger. In my opinion it does not matter if the positive or the negative is grounded for safety.

When taking a voltage reading the black lead should contact the ground first per the meter's instructions. What if the measurement is line to line ?

I do not believe you were taught incorrectly, it is just that the learning never stops.

 

[jonathn1]

Well I received very interesting information I thought I would pass on. Seems that several tests have been done on the subject. If you are testing a motor and there is water intrusion. Then the water can become polorized in the test and actually creat a higher test reading by swapping the connectors around. It has to do with polorizing effects similar to a transistor passing a charge one direction but not if charged another. Problem is that many manufacturers take this into account and changed the internal wiring of the tester.
The other thing is this was much more common in very old systems. The new insulation we use today are not as likely to develope these traits even if placed in water. The most important thing is to develope a plan and then stay with it. Seems the number one accepted way of testing is to do it first the way the tester suggestes. Then stick to a 1 minute test. The actual test looks like a capacitor charging curve. SO the longer you leave it on the more charged it gets.
The experience people have had is that if the insulation is failing or there are issues. Then during the test you will see problems over time. But if the insulation is actually bad you will know it before the minute is up. Web site eng-tips.com has a couple good threads on it.
Thanks for the look and the assist.