Hole in water pipe and electric cable mystery.

Posted by Les Desser on January 12, 2008, 8:51 pm

I started getting water coming up from my solid floor - concrete with a
2 inch screed on top.
We opened it up and found a 1/2 inch copper mains pressure pipe running
across a live flat pvc electric cable (240V - UK)
The copper pipe was wrapped in some special tape to stop it corroding.
The cable was laid bare in the screed.
There was a clean hole - no corrosion - about 1mm (1/24 inch) through
the bottom of the pipe directly over the electric cable which also has a
matching hole in it.
The circuit breaker did not rip though my plumber said he could hear
some electric buzzing when the cable was wet before we isolated it.
How come this hole? The installation is about 10 years old. The damage
could not have been mechanical and certainly was not there at time of
installation. It looked a clean hole, nearly round - as if done with a
drill bit.
Could there have been some interaction between the cable and the pipe.
Seems far fetched but what else could it be?
--
Les Desser
(The Reply-to address IS valid)

Posted by PeterD on January 13, 2008, 9:28 am

On Sun, 13 Jan 2008 01:51:09 +0000, Les Desser
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How much seperation between the wire and the pipe?
What kind of wire?
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Current flow was less than the breaker's capacity that's all. But it
could have beena problem eventually.
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Yes, it could have... Especially if they were touching (which would
not have been good...)
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Not far fetched at all! But not exactly as you expected it. I don't
know (you didn't say) what kind of cable it was, but clearly it had a
microscopic flaw in the insulation. THis is not uncommon or unheard
of. I'd guess the cable was not rated for direct burrial (I'm unclear
of whether this was in the cement or below it, but in either case...
the result is the same).
To prevent this from happening again:
1. All electrical cable shoudl be direct burrial rated if going in the
ground, or into cement.
2. Preferrable it shoudl be in conduit... Lot easier to pull the cable
out and run another one when/if it fails!
3. There are minimum seperation between power cables and other items
such as water lines, and non-power wires...
Regardless I'd strongly believe there was simply a micro-pore in the
wires' insulation which allowed it to fail, and the close proxminity
to the water pipe gave a good path to ground. THe hole in the pipe was
elctrolsis which wuold be expected.
If you can't put the entire run in conduit (plastic is best, IMH))
then at least put conduit where it crosses other metal such as rebar,
pipes and other wiring...
I'll agree, this is one odd problem! (I've seen it before but not
exactly the same circumstances, hence the diagnosis of a micro-pore in
the insulation).

Posted by Les Desser on January 14, 2008, 3:54 am

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Zero
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Standard 2.5 mm.sq twin & earth leading to and outside 1 KW light which
is hardly ever used.
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Apparently standard here - looks a bit like felt impregnated with some
very sticky oily substance. All pipes buried in concrete are wrapped in
it to stop corrosion.
[..]
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They were under at least 1.5 inches of sand/cement screed.
[..]
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I assume that must be the conclusion, but still a long shot - each
conductor has its own insulation and then there is an outer PVC bonded
sheath. And the fault should be directly under the piper, unless the
fault is more general - in which case I hope there are no more
cross-overs buried under my feet.
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Correct
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Buried in the cement
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A bit late now. I'm digging it all up :(
[...]
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Electrolysis sounds right.
[..]
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If there is ever a next time, I will watch the contractors!
Thanks for the response.
--
Les Desser
(The Reply-to address IS valid)

Posted by CWatters on January 13, 2008, 11:35 am

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Nearby lightening strike?

Posted by Edward Hennessey on January 13, 2008, 4:45 pm

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CW:
Very interesting conjecture which will be supported if characteristic signs
of melting are
present. When lightning hits certain sands, "fulgurites" (from "fulgur" for
lightning) form and
the flow marks are quite evident. But if we have holes that are clean,
careful, especially magnified, observation should yield a good conclusion as
to what made them.
Regards,
Edward Hennessey

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