Nitrogen Purging of Conductors

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drbond24 said:
What I'm saying is that with water inside the insulation, there is a path for the current that shouldn't be there. It doesn't have to go through the insulation anymore, it can go around.
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To do that, you'd have to be talking about at the ends, where the conductor is stripped for termination.

When you connect the leads of the meggar to the conductor and the insulation, the water is a short between them.
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How do you connect a lead to insulation???

You aren't testing the insulation at all, you're measuring the insulation resistance of the water which is going to be waaaaaaaaaaaay lower than the insulation. There is water in the copper and there is water on the inside wall of the insulation and it is the same water. You can connect your leads to the copper and the insulation but all you're getting is the water.
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All you're talking about here is making the 'contact' between the copper and the insulation more intimate. Who cares? Insulation is supposed to be insulative.

Isn't it?
 
Apparently, the pressure of the nitrogen simply displaces the water, eventually forcing it back out through whatever penetrations it came in.

If water entered a conductor's insulation during installation, there is no compromise of the insulation, and the water shouldn't matter.

If the water entered after installation, it can get in again through the same pores or other breach. That's why I feel it is a temporary solution.
 
LarryFine said:
To do that, you'd have to be talking about at the ends, where the conductor is stripped for termination.


How do you connect a lead to insulation???


All you're talking about here is making the 'contact' between the copper and the insulation more intimate. Who cares? Insulation is supposed to be insulative.

Isn't it?
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I'm obviously suffering from a lack of field experience on this subject. All I have is an equation that tells me what the insulation resistance should be and some pdf files about how to measure it. IMHO, going on with this debate will not accomplish anything constructive. iwire is going to do what is best for him and I would do the same thing in his place. No hard feelings. :D
 
drbond24 said:
All I have is an equation that tells me what the insulation resistance should be and some pdf files about how to measure it. IMHO, going on with this debate will not accomplish anything constructive.
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I would really like to know how the water inside of the insulation along with the conductor changes the resistance of the dielectric insulation. That appears to be what you are trying to tell us.
Someone must have some idea as to what actually happens as they have a procedure to drive the water out. I don't see how having the water inside really makes any difference as to the resistance of the inuslation.
 
I just went through the entire booklet that iwire added a link to in post #38. There are several references to moisture being a problem, as well as several references to humidity being a potential problem. It does not provide any technical information as to why it is a problem, but they are clear that the cable being tested must be clean and dry to get an accurate test.

Edited to correct spelling.
 
don_resqcapt19 said:
I would really like to know how the water inside of the insulation along with the conductor changes the resistance of the dielectric insulation. That appears to be what you are trying to tell us.
Someone must have some idea as to what actually happens as they have a procedure to drive the water out. I don't see how having the water inside really makes any difference as to the resistance of the inuslation.
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You guys are thinking short term, it is the pressence of water in the inslation of long periods of time causing the insulation resistance to decrease, the water works its way into the insulation over time, just like air leaks out of tires over time. The water trees that form cause the effective insulation values to decrease.
 
zog said:
You guys are thinking short term, it is the pressence of water in the inslation of long periods of time causing the insulation resistance to decrease, the water works its way into the insulation over time, just like air leaks out of tires over time. The water trees that form cause the effective insulation values to decrease.
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And you seem to have moved back to MV cable and not the 600 volt cable we have been talking about.

As far as I know XHHW is impervious to water when it is intact.
 
Yes, there are references to wetness and water, but not when the water is inside the insulation. As far as an accurate test, if the wire is listed for wet locations and installed in a wet location I would prefer to have it wet when testing. I can expose a section of bare conductor, pull it into the conduit and have it test perfect if there is no water or other contaminates and the bare copper is not touching the metallic conduit, but as soon as I put water in the raceway, I will know that I have a problem.
 
iwire said:
And you seem to have moved back to MV cable and not the 600 volt cable we have been talking about.

As far as I know XHHW is impervious to water when it is intact.
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Thats what they thought about XPLE for many years, utilities buries thousands of miles of the stuff before water trees started leading to cable failures.

XHHW is just a LV cable with XLPE sisulation and a PVC jacket to protect it from EXTERIOR water, the PVC jacket dosent protect the XPLE insulation from water INSIDE the cable.
 
zog said:
You guys are thinking short term, it is the pressence of water in the inslation of long periods of time causing the insulation resistance to decrease, the water works its way into the insulation over time, just like air leaks out of tires over time. The water trees that form cause the effective insulation values to decrease.
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That is fine, but first this is a new installation and the water should not have damaged the insulation at this point in time. Also this is a wet location conductor and I don't see how water on the inside is any worse than water on the outside. If the a new cable with water under the insulation tests bad with the water and good after the water is blown out, would you accept that cable? I wouldn't.
 
zog said:
You guys are thinking short term, it is the pressence of water in the inslation of long periods of time causing the insulation resistance to decrease, the water works its way into the insulation over time, just like air leaks out of tires over time. The water trees that form cause the effective insulation values to decrease.
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Well, then you're describing what I consider to be porous insulation, and defective.
 
zog said:
XHHW is just a LV cable with XLPE sisulation and a PVC jacket to protect it from EXTERIOR water, the PVC jacket dosent protect the XPLE insulation from water INSIDE the cable.
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I have never seen single conductor XHHW with an outer jacket.
 
The only time I was ever involved with nitrogen purging conductors it was as temporary remediation while a shutdown could be scheduled when a conductor or conductor set meggered scary-bad. This was at a military facility. The Nitrogen was added through a T-tap rubber fitting designed for that use while the conductors were still in service. This business of nitrogen purging XHHW to pass a megger test seems like total nonsense. It will get your numbers up (probably), until the water returns. There's still a fundamental insulation compromise someplace. PMDMS dielectric injection (much like is done on MV cables) seems like it would be a more permanent repair. Nitrogen purging is a temp repair (when the bottles are left in place) or a way to "fake" a good megger test. The problem is not "fixed".
 
KnobnTube said:
I was in industrial for 26 years. We had a similar situation with moisture in some 750mcm tray cable. We brought in a company who did the purging 0n 6 runs about 85ft each. They connected a shrink tube with fitting over the ends of each conductor and 1 over the overall sheath. Purged them overnight and all was well after.
That was in 1982 and everything was great and still operating well (I left in 2006. Never kNew what or why on the cable as it was delivered new to the site with sealed end caps. But, something??
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Nitrogen purging might be a more permanent corrective effort for a cable assembly that somehow leaked water in one of the cut ends and got in amongst the individual conductors that aren't waterproof themselves.
 
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