Megger testing

[SparkyPete]

When performing a insulation resistance test at 2500vdc on a piece of equipment the black lead is placed on ground or earth and the red is placed on the live part that you are measuring. A question was asked does it really matter if the leads are reversed and the ground lead is on the live part and the red lead is on ground? Besides the manufacture stating how to place the leads does really matter how the dc is applied to the equipment if you were to use and old fashion hand crank megger?

 

[waross]

I often reverse the leads to check for non-symmetrical leakage to ground. I also do this when checking continuity. loose copper connections may form copper oxide rectifiers and as a result, the indicated resistance will be quite different as indicated by a multi-meter.
That said, some Meggers have a third,shield, connection. If the shield connection is used I would go back to the literature and refresh my memory on the theory behind shield connections, both before using the shield connection and before reversing polarity.
And, as a parting thought, if the Megger manufacturer issues strong instructions to observe polarity, it may have to do with the internal insulation to ground.
My 1000 Volt Megger had no such restriction, but it may be a possibility to consider.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[catserveng]

Have you read this?

https://www.testequipmentdepot.com/megger/pdf/electrical-insulation-testing_guide.pdf

I think you'll find it does a really good job detailing how to an insulation tester and how to interpret readings.

Bill, page 50 of the attached discusses the "guard" terminal and its use.

Hope that helps, MikeL

 

[waross]

Thanks Mike.
By the way, I couldn't find any instructions on polarity. Do you have any comments?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[catserveng]

So if you look at most of the low voltage diagrams in the "Stitch in Time" publication, it shows the RED lead as being connected to ground and the BLACK lead connecting to the test point, my manuals for the AEMC and Fluke(both up to 1000V) testers show the same thing. My only higher voltage tester is from Megger

On voltages above 1000V note that they don't usually call out polarity, at least in most the info I have. I asked this in a class given by AVO several years ago, the instructor had said that for low test voltage applications they did see differences in readings when polarity was changed, mostly on older equipment. And that their recommendation on any testing below 1000V use the RED lead connected to ground. But when we did it in class during the hands on portion, never saw a significant difference, but it was in a lab with nice clean equipment, good grounds and nice environment.

In most of the stuff I test, usually generators, motors and cables, I've never really seen a difference, maybe someone else in the forum has a better answer or different experience, but in practice most of the time my problems in field testing are finding a good, clean "ground".

MikeL

 

[waross]

I can only see a problem if the Megger internal insulation is graded. As far as I know, the insulation is not graded.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!