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drying out cables

13K views 42 replies 18 participants last post by  Big John 
#1 ·
What is the proper way to eliminate moisture from cables before megging them? These are 750mcm three conductor armored cables carrying 4160.
 
Discussion starter · #7 ·
I am spectating this at my current job. One of the cables isn't megging out and they want to rule out moisture. There was a fault that trashed the line side of a transformer and they are wanting to rule out moisture invasion in the cable. I don't see how this would solve anything though. Once a cable causes a fault it is trash IMHO.
 
Discussion starter · #10 ·
Is it like a lead sheathed mv cable? Water inside a cable can be responsible for expediting the breakdown of already weak spots in the insulation. Drying a cable out doesn't always make things better though due to the impurities in the water that are left behind afterwards. I wouldn't think that water in a cable could produce a full blown fault but it's hard to say without knowing the type of cable and insulation in question.
Its pretty much teck cable. Plastic coated MC on a large scale. Armored tray cable by any other name.
 
Discussion starter · #33 ·
Not where I'm from. I will test, adjust, test again, adjust, and test yet again before pulling new MV cable. Big player or not, the cost of new MV armored cable and the labor required to install it (and remove the old) can be prohibitive unless tackled as a last resort. And regardless of the company name, they are probably not made of money in today's economy (oil companies not withstanding).

The fact that the cables "meggered" good when the stress relief was removed would indicate a possible stress cone failure, caused by moisture or some other contaminate or condition. It is not unusual for a stress cone to break down months or even years later due to improper cleaning or installation. Stress cones do just what the name implies; they relieve the effects of corona, or stress. Building a stress cone correctly takes time and patience, even with today's cold shrink 3M kits. Cutting corners is a recipe for disaster.

I would NEVER rely on a 5000 volt "megger" insulation tester simply because the cable is '4160'. You are in hi-pot land with shielded MV cable. A look at the Southwire DC hipot testing table will wake you up to MV cable testing.

http://www.southwire.com/support/DirectCurrentFieldTest.htm

There are strict guidelines and times involved to prove the stability of MV cables; rules that should not be taken lightly or omitted. Catastrophic cable failure is expensive in many respects beyond the actual cable loss. Loss of production can hurt a company very quickly.

Below are a couple more sites to help anyone understand what is needed to insure a safe, trouble free medium voltage installation. Anything less is simply a crapshoot.

http://www.generalcable.com/NR/rdonlyres/0129A307-6297-4D6A-B4A9-A21E527DE9A5/0/Spec_F025.pdf

http://www.chromausa.com/pdf/app-notes/AN-A Practical Guide to Dielectric Testing-092007.pdf

http://www.asresearch.com/events-training/pdfs/HotHipot.pdf

Again, if the the cable cleared after removing the stress relief, I would re-install another set of stress cones and hi-pot the cables again. It is by far the most economical method of repair. If your people are competent/ qualified to do hi-pot testing, then go for it. But if not, have professionals come in to do it. If it fails, you are only a grand or two into it. If it passes you are a hero.

And if everyone takes their time and does things right, no one will fool with those cables again until your retirement age. Then it won't be you anyway.

Mark
That's what is happening here. I overheard that the cable alone would be ~$50,000 before labor and termination material.
 
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