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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.
 
#20 ·
We pulled in 15kv cables yesterday and the amount of water that came out of the conduit was enough to fill a small pool. The new cables tested out.

The old cables (in a parallel conduit) had been in there for years and were only replaced as it was a job spec for a site upgrade
 
#6 ·
Most conductors are rated for moisture. Problems arise when the ends aren't sealed properly and moisture enters between the insulation. How wet did it get, how old is it, what makes you think it's got a moisture issue? All relevant questions. 4160 is nothing to monkey with and is the reason why I asked why you were asking. If it's for a new installation I hope to hell there is someone around who knows this stuff.
 
#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.
 
#9 ·
butcher733 said:
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.
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.
 
#16 ·
Shock-Therapy said:
Im not grilling you, Im just interested in hearing more about the impurities left behind part....
Pure water is actually an insulator. But pure water (distilled) is not typically what we're talking about when it comes to water damage in a cable. Rain water is formed around dust and pollutants and ground water contains minerals and salts; all of this is still present after the water evaporates and however minimal the conductivity of the material it still can attribute to high resistance faults, especially in mv
 
#31 ·
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
 
#23 ·
whats the readout you are getting exactly?
If we are talking direct short like 0 ohms then that would be a first for me from moisture.
Sure ive had moisture give not ideal readings but ive never seen a direct short from it.

I'd check the connector first and make sure the connector "teeth" actually grabbed ahold of the armor and didnt grab the cable instead.

just curious, how are you testing it? at what voltage?
 
#24 ·
Typically there's no armor grounding teck connector involved on an MV cable. The cable enters through a gland like what would be used in flexible cable, complete with its jacket. The armor gets bonded inside the termination kit/boot.

The only real solution here if there was a fault and the cable does not meg out is to replace it. That's it.
 
#27 ·
99% of the medium and high voltage failures I see are in the terminations. If there's any doubt, and it's at all possible, cut off the terms and re-test.

It's why I always have to laugh when we're asked to do a cable hipot on commissioning prior to the installation of the stress cones: If it's gonna fail, it's gonna fail because some screws up the terminations.

That said, while I've successfully dried a lot of equipment, never MV cable, and it'd be interesting to know if that's even possible?
 
#28 ·
99% of the medium and high voltage failures I see are in the terminations. If there's any doubt, and it's at all possible, cut off the terms and re-test.

It's why I always have to laugh when we're asked to do a cable hipot on commissioning prior to the installation of the stress cones: If it's gonna fail, it's gonna fail because some screws up the terminations.

That said, while I've successfully dried a lot of equipment, never MV cable, and it'd be interesting to know if that's even possible?

We have had ends get wet cut off, splice on new piece, reterminate, test
 
#29 ·
We were installing 500 feet of 2/0 mc mv(outer plastic/rubber coating) cable in a vertical shaft ,when we cut the wax ends off with a bandsaw the wire pissed out water from the cable assemble,can't remeber if it came out of the center of each conductor or just around them.But i was surprised to see that happen the guy installing it said he had seen it before.The only thing we could figure is it had been stored outside in extreme cold and when we brought it in the shaft where it was considerably warmer it may have built up some internal condensation.The wire was new and we megged it out at 5000v afterwards we fired it up on 2400 volts ,no problem but it was strange.
 
#34 ·
Is this going to be a problem? Going on 3 weeks and I don't know how much longer. There are 6 open ends like this that are not sealed and one is laying in the grass with all of this rain and snow that we have had. It's 15kv cable with 7.2k going through it but a few feet will be cut off before terminating.

 
#37 ·
Is this going to be a problem? Going on 3 weeks and I don't know how much longer. There are 6 open ends like this that are not sealed and one is laying in the grass with all of this rain and snow that we have had. It's 15kv cable with 7.2k going through it but a few feet will be cut off before terminating.

View attachment 32133
Are the cut ends just stuffed down in the conduits? any tape at all on the cut ends?

Since the ends in the picture are pointing down I think the water intrusion should be minimal.

But as was said, it is the poco's problem. :thumbup:
 
#35 ·
8V71 said:
Is this going to be a problem? Going on 3 weeks and I don't know how much longer. There are 6 open ends like this that are not sealed and one is laying in the grass with all of this rain and snow that we have had. It's 15kv cable with 7.2k going through it but a few feet will be cut off before terminating.
I'd say that's their problem (poco). Right?
 
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