An underground lateral installed in ridged metal conduit to a riser up a utility pole where the POCO connects the type THHN/THWN at the top without any form of drip loop to prevent water from entering the wire.
So, we have the inside of the cable full of water from the point of connection at the top of the pole to the lugs in the meter, and the inside of the ridged conduit will have water in the low points underground.
Well, there seems to have been an insulation failure of one of the wires in the riser, causing a burn out on one of the hot legs to the ridged.
We just had 2 days of freezing temps (0 degrees F) and then 38 degrees on the morning of the fault. Could the freezing of the water inside the wire and subsequent thaw have caused the insulation to fail?
I would think it is more likely that there was a nick in the insulation from installation. If you're thinking that water would creep down between the copper and insulation, I would think that surface tension of the water would keep it from running, unless I'm not understanding surface tension correctly.
Another thing, I don't know if the expansion ratio of water to ice would be enough to split the insulation from the inside out. If water could get all the way down the insulation it would only be a small amount. Although the plastic would probably more brittle at these lower temperatures, I wouldn't expect it to split like a copper water pipe.
[This message has been edited by Jps1006 (edited 12-17-2005).]
Re: Water inside wire#59874 12/17/0504:00 PM12/17/0504:00 PM
jps, i too have seen water run inbetween the insulation and the conductor. if one side like on a pole is way above another side, like in a panel. water will bubble up in the panel. to prevent this a drip loop is needed.
Re: Water inside wire#59876 12/17/0508:08 PM12/17/0508:08 PM
The water in the conductor could very well have split the insulation when it froze. The cold temperature makes the insulation brittle and the 10% expansion caused when the water becomes ice could cause this type of problem. Remember that the manufactures say that this type of cable should not be handled below 15°F to prevent insulation cracking when you are installing it. It can be used, but not handled (moved) at lower temperatures. This connector is sometimes used to solve this problem. The shrouded part of the barrel goes over the insulation, and this prevents water from entering the conductor. I worked on a job where we were replacing 5kV conductors from the service drop to the switch gear. The joints at the switch gear bus were taped and when we took off the tape, we got about a quart of water out of each conductor. When we replaced the conductors we used the shrouded lugs. Don
Re: Water inside wire#59877 12/18/0502:06 AM12/18/0502:06 AM
Any sort of moisture getting into copper wires, will at the first stage cause them to darken. This is the first stage of oxidisation. The second stage is where the formation of "Verdigris" takes place and the copper starts to break down. If the 2nd stage is allowed to continue, it will eat right through the conductor. I've seen a lot of pre-wired A/C units where the ends of the cables are not sealed, strip the wire back and you can almost tell how long the wire has been exposed to the elements for. The water/moisture moves up the wire/cable by Capillary Effect.