Meggering Test Voltage

[czping]

Hi, it's me again..

Just like to check on something that i dun really understand..

I am asked to use a test voltage of 500V to megger the 400V breakers but use 5kV to megger 6.6kV breakers. Are the test voltages correct respectively? If it is, that why is it that we use a higher than rated voltage for 400V but lower for 6.6kV breakers?

Please enlighten... Thank u!!

 

[dpc]

The exact voltage used for the megger test is not really critical. Generally the dc voltage used is higher than the ac operating voltage, at least for new equipment. It is common to use a 1000V megger on 460V motors. If you use a voltage that is significantly lower than the operating voltage, it may not find problems that will become apparent when you apply the ac voltage.

The megger test is looking for major insulation defects or problems, so small changes in the megger test voltage generally will not change the results significantly.

But if you are doing periodic testing, you should always use the same voltage for testing so you can compare results from year to year.

 

[gigahertz]

For medium/high voltage cables (>600V), you should contact the cable manufacturer for their recommended test voltage, which is usually done for acceptance testing after installation of the cable. This then gives a comparison to the factory testing results to look for damage.

Once installed and accepted, I have frequently been advised not to routinely test at elevated voltages unless you are prepared to replace the cable, as the tests can be destructive. Testing after installation would normally be to confirm a suspected problem. 5kV testing on a 6.6kV cable may not really tell much.