electric walls

[drsabrown]

Hi, I'd like to hang a large mirror above our fireplace. I checked the wall with a draper stud/voltage detector and discovered that the device indicates 'voltage' present all over the wall. The indicator stops whenever I place my hand on the wall. I've switched off the mains electricity and the detector ceases to register anything. I've tested other walls and over lamp cables etc. and the detector appears to be working as expected (beeps above outlets and light switches etc, silent elsewhere). The wall is a first floor external wall containing a chimney breast (its about 15ft wide and 10ft tall). The flat is a part of a conversion in a mid ninteenth century house. Is this something I should be worried about? Is it simply damp? Is it safe to put screws into the wall? Any help or advice very much appreciated.

Regards,

Simon

[The3rd Earl Of Derby]

Erm! you havn't got it set on metal have you? ie it might be detecting the flu liner. ;-)

-- Sir Benjamin Middlethwaite

[Lobster]

Or - again, this is if it's set to detect metal - just bits of metallic crap mixed in with the original plaster, along with shards of glass, broken crockery, horsehair etc... that's what I discovered to be the problem in a house of similar vintage which I used to own. Made the detector totally useless!

David

[david lang]

If there are cables being detected - where are they going to and from above a chimney breast?

Dave

[drsabrown]

Well, its set to voltage/metal. Voltage is indicated with a steady beep and flashing led, metal with a continuous tone and constant light. Its definitely showing voltage. The only other thing I can mention is that the affected wall was painted about 7 days ago. In the centre of the wall is a mantelpiece which was gloss painted at the same time the wall was painted. Bizarrely the voltage detector sounds against this and shuts up when i place a hand on the paintwork nearby. Again, the detector is silent against other walls... still baffled, so any clues appreciated.

Cheers,

Simon

[drsabrown]

I cannot imagine there are cables there - as you question - there's really nowhere they might possibly go (the cabling up to the attic is in a completely different place.) Plus the voltage appears to be present throughout the entire wall...

[The3rd Earl Of Derby]

Just hang the darn mirror and hope for the best, wear rubber gloves if necessary. ;-)

-- Sir Benjamin Middlethwaite

[Andy Dingley]

An old spur from a lighting circuit to a clock socket ? Not uncommon in the middle of the century.

I've seen people fitting new fireplaces find these things, grab firmly hold of them to pull them out, then get a belt as they were still attached to live lighting. Don't assume it's definitely a false alarm!

[Roger R]

breast

An old photo of my grandmothers living room showed that at the time of gas lighting it was not uncommon to have lanterns on the chimney breast, one each side. Possibly at a later time it was fashionable to have electric lights in this position, linked by a cable across the breast.

Could it be that subsequently the lamps were removed and the wall made good but the live cabling left in place? If so then it's certainly due for disconnection /replacement.

Roger

[Bob Eager]

Our (1903) house still had the pipes in place in the walls! One each side, as you say...

[Kalico]

I've seen that too. I wouldn't assume that it is totally safe.

Have a good scout around above and below the chimney breast for any wires that disappear into the vicinity.

The fact that you get no indication with the power off or when you press your hand on the plaster/paint makes me think the detector could be picking up a tiny induced voltage that you then, in effect, earth when you touch it. This would be made worse if there was any metal particles in the plaster mix or if it is slightly damp.

If you are not familiar with the concept, its the same technique they use to charge up rechargeable toothbrushes etc without there appearing to be any physical connection. Current travelling along a wire somewhere will produce a magnetic field about it (how electro-magnets work). If that field then crosses another wire, a current can be produced in it. Of course, it doesn't have to be a wire, but could be your slightly damp (and therefore conductive) plaster.

A better quality detector might be able to differentiate.

Rob

[Dave]

One thing to do before you drill the wall, go out and buy yourself a 'volt stick'. This will tell you that there is live mains under the wall. Volt/metal detectors can be confused, but a volt stick will only respond to volts.

HTH

Dave

[meow2222]

My money's on it being a live wall. The wall is somewhat conductive, due to either damp or conductive materials, and cable insulation is not what it should be at some point.

This explains why the detection stops when you earth the wall through yourself. It also explains why it stops with the leccy off. And why youre detecting all over the wall. I cant think of anything else that would explain all those.

If youve got a DMM, or a FET input analogue meter, conncet one probe to mains earth and the other to a metal plate pressed against the wall, held there with something insulating so you dont short it out or get bitten. If my guess is so, you'll get voltage and current readings.

Yes, I can think of one other thing that coud explain it. If you were live, via a semi-live floorboard nail etc, the same thing would occur.

NT

[The Natural Philosopher]

Or simpy capcititaive pickup with the dampish plaster.

If you stick a sheet of metal onto e.g. a high impedance scope and hang it in the middle of an average room, you will see about 20-50Ac, sometimes more, on it due to straight pickup from the wires around the place.

A simple test would be to slap a flat meatl plate on te wal and use a megger to se what impdenace there is to ground, and to live and to neutral, with all the electrics off, obviously.

My guess is its very very high...and its just picfking up enough radiation to tweak your test gear.

OTOH if its low, you need to investigate it, but it can't be or you would get a shock off it, or trip your RCD's/.

[Bill]

I had the same thing although the walls were foil backed plasterboard. Came to the conclusion it was the cheap stud/voltage detector I was using; went out and bought a new more expensive one with the same results. I'd go with the inductance idea.

[Lobster]

Have you tried doing your test with all incoming power to the house switched off?

David

[drsabrown]

Thanks everyone for the suggestions and advice.

Some other things I noted - the voltage is registered over the entire wall, including the protuding wooden mantelpiece (!) - on an adjacent wall, voltage is registered over what appears to be an entire section of plasterboard, however once accross the vertical stud (which shows up as metal), the rest of the wall registers nothing. - the phenomenon only occurs when I am holding the detector in one hand and then either place/remove another hand from the wall. If I leave the detector on the mantelpiece untouched, it registers nothing. If I hold it on the mantel then it beeps. If I then place another hand there it stops... other bits of wood in the house (e.g. a window ledge) register nothing

My plan is to switch off the power to the house, install the 2 inch screws in the chimney breast, restore the power and finally check the screws with an electric screwdriver to ensure they are safe. Is this reasonable?

Cheers,

Simon

Bill wrote:

[The3rd Earl Of Derby]

Thats providing the house was in its original state of electrics, which I very much doubt?

-- Sir Benjamin Middlethwaite

[The3rd Earl Of Derby]

But that wouldn't account for the whole wall being active(alledgedly)?

-- Sir Benjamin Middlethwaite

[david lang]

Are electric walls covered by part P?

Dave