another megger question

[electricalperson]

what voltage should i test romex,thhn and other common insulation found in the construction industry. id like to use my megger to test MC,BX,romex, thhn, triplex. the voltages i work with is 120/240 120/208 277/480

 

[brian john]

600 VAC Cable - 1000 VDC is a standard test voltage on most meggers, though some meggers only go to 500 VDC.

 

[mdshunk]

I guess Brian has the textbook answer, but I've never felt especially compelled to test a conductor or cable at more than twice operating voltage. That is to say, it is unlikely that a cable with a 600 volt rating, that is being used on a 120/208 system today, will be suddenly be put to a use at 277/480 tomorrow. I would feel comfortable using a cable that "failed' a 1000 volt test at 120/208 if it would "pass" a 500 volt test. I put pass and fail in quotes because there doesn't seem to be clear agreed mark under which the results will necessarily condemn the cable.

 

[electricalperson]

thank you guys

 

[nakulak]

-make sure you read the literature that came with your megger
-never test a conductor at a voltage higher than its rating (this might seem like a no-brainer, but . . .)

 

[electricalperson]

this insulation testing process seems to be pretty complicated..i read so many online manuals and pdf pages and most of them are all different. im having a hard time understanding what proper voltage and ohm reading i should get

 

[nakulak]

ok, I should qualify my statement above - never test using ac at higher than rated voltage. this doesn't apply to hipot testing. you need to do some research, possibly training to be qualified to test high voltage. here's a few interesting links:



http://tdworld.com/business/power_highvoltage_underground_cable/

http://www.phenixtech.com/A4-Cable.asp

http://www.okonite.com/engineering/high-voltage-testing.html

http://www.cirris.com/testing/guidelines/hipot_testing.html#h2

 

[electricalperson]

we used an ac hi pot tester to test a 25kv cable. we used 60kv and it blew the load break up because of water seeped into the dielectric. so i take it the hi pot test can be pretty destructive i just dont want to ruin a good cable because the voltage was too high

 

[micromind]

The very few times I've had a cable or termination fail a hi-pot test, I was extremely thankful that the test didn't invlove a high energy system. Like closing it onto the grid.
Believe me; there's a huge difference between a little 'pop', and a loud 'BLAM'.