Underground cable locations
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Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
my experience has been that underground cables exist in continuous immersion
within conduits between vaults, without significant problems.
this was last winter, and while the vault was pumped out, and wires terminated,
this particular vault has about that much standing water in it right now, and
has had for most of this year... all of the 6,000' of conduit in this project is full
of water, and remains so. i have no doubt it'll stay like that for an indefinite period
of time. wire is southwire simpull.
i'm pretty sure that wire will be here energized long after i'm gone.
when i pulled this project, i meggered phase to phase and phase to ground on
each pull before i cut it free from the spools, so i'd know if i had any problems.
the readings went into the asbuilts. i'll probably down everything next spring or
summer, and see how the stuff is holding up.
- Location
- Massachusetts
In my area it is a rare thing not to have underground raceways full of water. As long as the insulation was not damaged while being installed everything works fine.
K8MHZ
Senior Member
- Occupation
- Electrician
Same here, even at power plants.
At the last plant we built, we had sump pumps to pump the water out of the trendways so the guys putting cable in them didn't have to swim, they could just wear waders. No kidding.
This is a power plant as well, 36kV @ 320A run on those cables. Though yesterday we pumped out the water as humidity might affect the installed svl (see red component). We have to fix the foundation drains for a long term solution.
K8MHZ
Senior Member
- Occupation
- Electrician
Where I was, there was no intent to even try to drain the trendways. The water table is too high. The engineers were content with that and to this day I'll bet the cables are all still underwater. That is why they have periodic checking of the conductors. Back then it was with a hi-pot. Now they use a device called, I think, a reflectometer that uses sound waves (IIRC) instead of high voltage to check for possible faults in the insulation.
Insulation checking is still hipot but in AC (VLF) since there was a study that DC have some effect on XLPE. VLF for integrity, TDR for location.
TDR works on principle as radar. Pulses of energy are transmitted, and will be based on impedance. There will be a discontinuity of impedance at cable fault.
The location of these turbines are up in the mountains so it is not normal to have the waters on the foundations.
- Location
- Illinois
- Occupation
- retired electrician
A plant that I often work at was having fairly routine failures of 5kv cables in their underground distribution system. The system consists of single conductor cables installed in duct banks with manholes. The manholes and duct banks were normally filled with water so the cables were always under water.
About 10 years ago, they installed sump pumps in the manholes and the number of failures went down drastically. Before they installed the sump pumps they would have a cable failure about every 15 to 18 months (not the same cable, there are about thirty 5kV feeders in the plant). In the 10 years after the pumps were installed they only had one failure.
About 10 years ago, they installed sump pumps in the manholes and the number of failures went down drastically. Before they installed the sump pumps they would have a cable failure about every 15 to 18 months (not the same cable, there are about thirty 5kV feeders in the plant). In the 10 years after the pumps were installed they only had one failure.
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