Strange Electric field

[dal5band]

In a 30s house we are getting a strange indication of " AC Live" on two of the internal walls using one of these metal & AC detectors. A whole area a meter wide of wall is bleeping as live. A neon test screwdriver indicates there is no live voltage as such, even on an RSJ. It is related to the ring main and dissapears if I dis off the ring main at the fuse box. The only thing common is that below the two places there is an RSJ running accross the under the floor in each room. (The house has only 6inch joists upstairs, supported by an RSJ half way accross each room).

Do RSJs give this strange reading? There is an RCD on the ring main which is quite happy and does not trip.

 

[securespark]

Is it a dry-lined wall with foil-backed p/board?

 

[Softus]

It is related to the ring main and dissapears if I dis off the ring main at the fuse box.

You might have a short - you need to carry out an insulation resistance test.

Is your earthing up to current standards?

 

[EddieCurrent]

Hello,

Out of curiosity I was wondering if inductance could be a possible factor ???

 

[dal5band]

It's a 30s house, traditional breeze& plaster internal wall, all dry - no damp.(My son has just bought the place so no other history known)

PVC wiring, multi-strand ring main - old but in good condition, recent fuse box with RCD on ring main. The earth is connected to the sheath of the incoming cable and copper water main.
I was wandering about some sort of inductance in the RSJs from the ring main which runs over the RSJ in a couple of places ? Either that or floating earth - don't know what that causes.

Like I said a neon screwdriver indicates no live - it's just the tester thing buzzes as if it was detecting an AC field.

 

[davy_owen_88]

Neon screw drivers are not a reliable way to test for a live situation. It's also known that those AC testers are not perfect either. Have you tried turning off all power at the consumer unit and testing again? Could it be a combination of 2 cables at right angles to each other on either side of the wall?

Davy

 

[dal5band]

EddieCurrent,

Do you think it could be eddy currents ?

 

[THRIPSTER]

What is needed here is an appreciation of how the two different voltage indication devices work. The neon screwdriver will detect minute currents flowing through the operator to earth. In your case, it has not made an indication which suggests that the (a) the wall is not damp and (b) that there is no conductive path through any resistance of the wall meaterial from a live conductor to the surface of the wall. The 'volt - stick' voltage detector works by detecting the magnetic field that surrounds a live conductor and therefore the fact that it has made an indication suggests that there is a live conductor behind the wall at the points you have written about.

Whilst on the subject of neon screwdrivers - a neon screwdriver will give you reliable results so long as you know the limitations andhave tested it on a known live supply before use. A neon screwdriver wiil give an indication of the voltage likely to be experinced by the body were you to contact the suspected live conductor/terminal/wall. Contrast this with an approved voltage tester or multimeter in the situation where both the live conductor and earth conductor are at a 230V - ie: a fault condition. The operator might well assume that it is safe to carry on and suffer the consequences. Use of a neon screwdriver would indicate the danger.


Regards

 

[EddieCurrent]

Dal5band,
What's an 'eddy current' ?

 

[Softus]

The operator might well assume that it is safe to carry on and suffer the consequences. Use of a neon screwdriver would indicate the danger.

Well done THRIPSTER - the first sensible information about neon screwdrivers that I've ever read on this forum. A marvellous post in general, IMHO.

 

[THRIPSTER]

Appreciate the support Softus.

Use of the spell checker might have further enhanced it.



Regards

 

[dal5band]

Eddie,
An edy current is a localised current within a lump of metal. I think it occurs when the metal is in a strong E/M field. Don't know what causes it - maybe imperfections in the metal. We need a physiscs prof to tell us exactly how this phenomenum occurs.

 

[THRIPSTER]

Dal,


Think you`ll find he was kidding.



Regards

 

[TicklyT]

Eddie,
An edy current is a localised current within a lump of metal. I think it occurs when the metal is in a strong E/M field. Don't know what causes it - maybe imperfections in the metal. We need a physiscs prof to tell us exactly how this phenomenum occurs.

An eddy current occurs in, say, a piece of iron subjected to a changing magnetic field. The magnetic field induces a current to flow at right angles to the field, just like in a transformer winding. If there is just a 'lump' of iron, this looks like a short circuited transformer winding to the induced current. Transformer cores are laminated out of thin pieces of material, or use materials with good magnetic permittivity and poor electrical conductivity to reduce these currents as far as possible.

 

[Softus]

JHC. Did you just vomit up an indigestible lump of encyclopedia?

While I'm here, my grandmother has an egg that she doesn't know how to suck - anyone have any tips?