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Old 10-18-2006, 12:54 PM   #1
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Electric roof


We recently had our house re roofed. As its open plan with a concrete sub-floor all the wiring has to run in the walls or over the roof. It was felt and gravel before but we replaced it with insulation and plywood and composite tiles.
Unfortunately the ply is held down with 6 inch nails into the main tongue and groove roof and it appears that the roofers hammered one or more nails through the wiring.
The main wiring areas were covered with a metal sheet but thats not robust enough to stop a 6 inch nail.
We only discovered it when my wife touched the metal trim in the atrium and found it live. The roofers denied they could have done this but I discovered that the insulation is covered with a thin sheet of foil that conducts. Thus any nail touching a live wire is automatically ( where the insulation sheets touch) connecting to the roof.
We isolated one circuit that gave the 110v and eliminated this line but now I have a mystery. I now read 14v with a DVM against earth at one corner of the atrium on the exposed foil and it diminishes across the roof ( at the points I can reach) to almost nothing at the edges.
How can this be ?
I cannot measure any voltage on any neutral or earth in the house although we have few earth points as the house is old and only has twin core wiring. Turning off the kitchen / bathroom circuit almost eliminated the voltage ( but not quite) so that implies this is the problem but I'm not sure how I can get 14v ( no current - it won't drive a 12v light bulb) .
Turning on the kitchen light actually raises the voltage measured on the roof to 15v which implies a neutral connection problem but the neutral by the switch is clean.
I'd be grateful for any ideas.

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Old 10-18-2006, 05:15 PM   #2
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I think you are correct they have driven a nail into a cable. That cable is now damaged and will need to be replaced when you find it. The problem is going to be finding it. One solution would be to try and determine which cable it is from within the house and just run a new cable.
It will be a matter of trial and error. Disconnecting various cable at various junction boxes and see when the roof power goes off.

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Old 10-18-2006, 07:03 PM   #3
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Originally Posted by Mike_pelham View Post
I'd be grateful for any ideas.
Hey... here's an idea... quit troubleshooting this yourself.

This is a very obvious insurance claim againt the roofer's GL policy. Get them on the horn and let the insurance people deal with it. This might be your house, but the problem is not yours. The roofer's created it, and it's up to them and their insurance people to fix it. Hopefully you got a copy of their insurance certificate before they started your work.
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Old 10-18-2006, 08:41 PM   #4
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nice thought MD, but good luck... the roofer will blame the electrical install... who ever climbs around in the attic looking for wires before re-roofing?? no one. The wires should have been spaced into the center of the rafter and/or protected with thick metal like the plates that code specifies for example when a wire passes through a notched 2x6 and not through its center.
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Old 10-18-2006, 10:59 PM   #5
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nice thought MD, but good luck... the roofer will blame the electrical install... who ever climbs around in the attic looking for wires before re-roofing?? no one. The wires should have been spaced into the center of the rafter and/or protected with thick metal like the plates that code specifies for example when a wire passes through a notched 2x6 and not through its center.
Nope, you're wrong. Plain wrong. There is ZERO code that ever existed in the history of mankind to back up anything you just wrote.

If you're using 6" nails to attach roofing material, you darn well better know what you're about to nail into and potentially damage.
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Old 10-19-2006, 10:14 AM   #6
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Thanks. We had a electrician out and he couldn't explain it. As I said the first fault was definately a nail but now I'm leaning towards an induced voltage caused by a large area of metal foil almost touching a mains wire. Theres no current in the metal foil so its tough to explain 14 volts - it should be plain 110v if its connected. It decreases across the roof and even wwith this circuit off theres 3-4 volts elsewhere . Either lots of high resistance connections or inductance I believe.
Also connecting the foil to ground removes the voltage in the area - unsurprisingly.
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Old 10-21-2006, 08:03 PM   #7
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MD I was indicating that if a steel roof was going to be used with 6" nails that I would not have run wires up there or protected them.

As far as "plain wrong"... I was referring to running wires through holes in joists or notches in joists... from memory I believe the NEC text reads something like this.

"Affix a nail plate to any stud or joist with a wire closer than 1-1-4" from the face of the stud or joist."

rant off
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Old 10-21-2006, 09:57 PM   #8
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"Affix a nail plate to any stud or joist with a wire closer than 1-1-4" from the face of the stud or joist."
Nope, says nothing like that. You're close, but the devil is in the detail of the text.

Are you aware of how wiring is run on roofs where there are exposed beams with tongue and groove roof decking exposed inside? Permit me to explain. The roof is framed up. The tongue and groove boards are laid on top of the roof framing. The wiring is affixed on top of the roof decking. One to several inches of insulation board are laid on top, with grooves cut as necessary to eliminate pressure on the wiring. The roof gets the BUR or EPDM system installed. In this case, the OP had a BIR, and added a layer of plywood on top with some other roofing system installed. The long nails necessary to affix this plywood obviously pierced some of the wiring.
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Old 10-22-2006, 11:15 AM   #9
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Is it possible thats its not a nail ? If the wiring is old and covered by a thin metal sheet and maybe the contractor did not cut back the insulation over the metal cover then the pressure of nailing down the plywood would compress the insulation onto the metal cover onto the wiring and any weak spot might couple through to the cover and thus to the foil on the insulation.
My feeling is that a nail would always conduct to 110 volts and I only measure a weak 14v.
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Old 10-22-2006, 11:34 AM   #10
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Originally Posted by Mike_pelham View Post
My feeling is that a nail would always conduct to 110 volts and I only measure a weak 14v.
I must admit that I'm still puzzled why you're still troubleshooting this. This is a very clear cut insurance claim.
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Old 10-22-2006, 11:17 PM   #11
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Hey for everybody who doesn't know MD is a electrician for a living. He is also up on everything very insightfull to read
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Old 10-22-2006, 11:43 PM   #12
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Many thanks for all your advice.
My problem is that now we have 'fixed' the 110v connection, the electrician that the contractor bought in claims that its likely that this is not a result of the new roof. Of course it could not be detected without a conducting layer but I have to admit that no-one can tell me whether its possible to induce this sort of voltage locally with old mains wiring. ( no earth)
I perserviered this afternoon, finding 3 cables on this circuit that run over the roof at this point. Each one individually induces between 5 and 8 volts when connected and 14v when all connected. I cannot see how a partial short can produce this effect. A nail cannot be just touching all 3 !
Even if I have this circuit turned off I have 1-2 volts present so its definately having effects of wiring proximity all over.

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