Megging and High-Potting

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mull982

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Hello

Here at a new construction project we have began installing cables anywhere from low voltage 600V rated cable up to 5kV rated cable. For the 5kV rated cable the contractors are "Megging" and "High-Potting" each length of cable. They are megging the cable before it is installed, and then once it is installed they are high-potting it using a DC High Pot up to 30kV.

These two tests are new to me and I was hoping that someone could explain their purposes and exactly how they are performed. If anyone has any further information on the web they they could point me to that would be great as well.

Thanks Alot for the help.

Mull982
 
Thanks for the info guys. When performing these test however how do you make the actual connection with the testing instruments. More specifically with the megger, where do you connect the leads of a megger and what results or values should you see when checking wire. I've seen a megger used for checking ground faults in a transformer or motor, and I was wondering how it is used in those applications. What is a typical acceptale Resistance value when using a megger to insure that what you are testing is acceptable.

I was curious about similar connections with dc hi-pot and what acceptale for it would be.

Thanks alot for the help

Mull982
 
Most meggers have three leads the two potential leads and one ground lead. We seldom utilize the ground lead except for discharging the cable. Testing new cable on the reel at 1000 or 5000 VDC one would hope to see maximum full scale reading. But maximum full scale reading varies from one meter to the next.

We have about 20 different meggers with a maximum ranging from 2000 megohms and one with a maximum 1 teraohm. A minimum reading on 600 VAC equipment is 50 megohms (but there are some exceptions to this number) temperate humidity play into this as well as the distribution equipment type, length of conductors (be it cable or busway).

When testing any equipment it is best (TO AVOID A SHOCK) to discharge the conductor when you have completed the testing.

Long conductors take a while to charge, it is best to pick a time frame, like one minute) for all test, this makes comparisons easier to access.

Meggers are available with maximum test voltages from 100 VDC-10,000VDC with 1000 VDC being the most common.

mega=1,000,000, 1 megohms=1-million ohms
giga=1,000,000,000 1gigohm = 1-billion ohms
tera=1,000,000,000,000 1 teraohm=1 trillion ohms
 
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A 600V cable should have a minimum test voltage of 1000V applied and be >100Megohms (Tempture corrected to 20 degrees C) per NETA 2007 ATS.

DC Hipotting should be considered a destructive test, however it is still allowed by some standards as an acceptance test. The test voltage for a DC acceptance test depends on the voltage rating and insulation type of the cable under test. The test is conducted one phase ata time with the other 2 phases grounded. Test voltage is applied in steps not o exceed 1000V increments, and the final test voltage is held for at least 15 minutes. Leakage current should gradually decrease, an increase in leakage current indicates insulation breakdown and the test should be stopped. Afetr the test the cables should be discharged for 45 minutes.

Think of a DC hipot test as a pass-fail test, a cable can pass a DC hipot test and still have flaws like water trees that can lead to early failures. For a more comprehensive test you should conduct a VLF and either a tan delta or partial discharge test to establish baseline data for future condition assesment.
 
mull982,

If you're trying to learn how to do these tests I strongly suggest that you learn from an experience person that is standing right next to you. Some of these tests can damage the cable if it's done incorrectly.
 
WE had a customer destroy some busway a few years back, used a hipot in lieu of a megger at wrong voltage....NOTHING LIKE EXPIERENCE, it does look simple but it does take some understanding.
 
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