Electrified plaster!

[John Rumm]

This was an interesting one!

Got a call from my Mum, who is currently having a fair amount of building work done on her place (large Victorian semi, built late

1800s), repairing damage caused by tree roots (settlement cracks etc).

She wanted me to disconnect some unwanted surface mounted sockets in a couple of rooms before the builders get to them. So I said I would pop round and sort it out. Then in passing she mentioned that a plasterer was complaining of getting electric shocks from a wall!

I figured this was worthy or more urgent investigation!

Apparently, the plasterer was just finishing off a small patching operation on the interior wall above a second floor window. This had involved hacking off existing plaster, resin bonding / rebricking the wall, covering with expanded metal lath and finally replastering inside. While doing the final polish on the plaster. he was getting a tingling sensation though his trowel. As he applied more water to the finish coat, it got worse, to the point he was getting a reasonably significant belt off it!

I thought some basic tests and inspections would be a good start. She has a "whole house" 30mA RCD that feeds two CUs via a Henley block. Both CUs are Wylex rewireable types - a 8 way doing most of the house, and a two way that feeds an electric shower in a cloakroom extension. The earthing has been updated relatively recently to PME (probably when a new meter was installed recently). There is also good cross bonding to gas and water services. Over the years we had eliminated all the remaining PBJ coated wire, and what remains is all PVC. Some bits of stranded T&E remain, but I think most of the sections of twin with separate earths have been replaced.

My first thought was that if he was getting a shock like that, it was surprising the RCD did not trip. So I did an RCD test, but it looked like it was working fine. At 15mA it did not trip in the two second window my tester measures. At 30mA it tripped in 20ms. I did a earth loop impedance test and that also tripped the RCD (which it is not supposed to do!). I may go back and repeat that.

The bit of wall he was plastering is not noticably near anything electric (although to be fair I did not investigate the loft above). Although one of the wires that was to be removed did run along the base of the wall and into the adjacent room, passing close to some disused lead pipes that were boxed into the corner of the adjacent room. This I thought might be a possible source since the pipe would have been close to where he was working. However, having taken the cable out there was no sign of damage to it. I could not get a voltage reading between the newly plastered wall and a water pipe - even on a very high impedance DMM. Needless to say the wall had dried out a fair bit by the time I was looking at it.

The only other possible cause I think of, was if he threw enough water about the place, he could have got the carpet wet along with a socket, junction box, and FCU on the skirting board, perhaps enough to make his metal ladder "live" so that as he trowled the wall he was actually getting an earth reference from the damp wall.

Any thoughts?

[Christian McArdle]

Could you explain how drawing 20A from live to earth doesn't trip the RCD? (Or do you have one of those fancy ones that claims to work with an RCD in circuit?)

Christian.

[Stefek Zaba]

Remember that last word:

The RCD seems to work OK. The "significant belt" you say the plasterer got with a wet wall may've been below the tripping threshold. But a thought which runs through my fevered brain is that the source of the belt might be from next door - t'other side of the party wall - which need not have an RCD in place, but might have sthg in contact with (say) the old lead pipes you mention.

Just another confusing possibility to add to your mystery - Stefek

[Andrew Gabriel]

I suspect capacitive coupling to a buried cable would do it. With the plaster all wet and conductive, the capacitance to something like a light switch drop with both wires live would be higher than normal.

When working in the loft on some earthed metalwork, which meant laying on my side to access it, I noticed I was getting a strong

50Hz hum feeling when brushing against it. Checked with a neon screwdriver, and it lit dimly. Then I realised I was proping myself up on one arm, which was laying along a lighting cable. Lift arm off cable, and the 50Hz hum feeling and lit neon both stopped. Cable was in perfect condition -- it was just capacitive coupling, helped by me being rather moist in a hot loft.

However, you should check this out thoroughly, remembering the death of the MP's daughter just recently after people had noticed something similar with a metal cutlery rack, which was in fact live.

[Bob Mannix]

I have experienced shocking plaster once. It was caused by a connector block buried in the wall where a shaving light had been replaced. It was OK until the wall was retiled. The wet tile cement caused the area being tiled to become live, the voltage dropping as you moved away from the source - quite interesting really. I discovered it while tiling behind the bath taps as my hand was touching the bath tap as my finger touched the wet tile cement. Nothing serious, just a tingle, but somewhat unexpected.

[John Rumm]

I was using one of those fancy ones that is supposed not to ;-)

(Megger LT5 - seems to do what it says on the tin when I try it here (although I am on a TT setup so hence a lower max fault current))

[John Rumm]

Ah, yes see where you are going...

Not being there at the time it is idfficult to quantify the level here... he still managed to get a decent finish on the plaster - so it could not have been that bad ;-)

Good thinking... alas one potential flaw in this theory is that the wall in question is a far from the party wall as it is possible to get - oposite side of the house in fact. Also the adjoining house was split into three flats ten years ago or so, and in the process I think they upgraded the electrics to include RCDs on each flat.

[John Rumm]

That was one of my thoughts... although I am not aware of any cables there (above a window, opposite side of the room from the door). Also that area of plaster had been hacked back to brickwork and lath without revealing anything. There is a possibility that there is something routed above it in the loft - but again that seems less likely since it is right over to the side of the house away from any lights etc.

I ran a metal detector over the wall and could not find evidence of any buried metalwork (some parts of the house still have buried pipes from the old gas lighting!

There is a wall mounted electric radiant heater about 7 foot away that gets power from a cable drop from the loft though. So that may warrant further investigation.

Yes point noted! I want to get to the bottom of what was going on anyway, if even just out of curiosity.

[Tony Williams]

In article , [snip]

Turn the logic upside down?

Any chance he was standing or touching something vaguely live so the wet plaster was actually acting as the Earth?

[The Natural Philosopher]

Trn the logic sideways.

Does the plasterer have a friend who is an electrician?

[nightjar]

I wonder whether you would have got a different result if you had dug down to the expanded metal mesh and tested that?

Colin Bignell

[Mark S.]

Knowing little about electrics I didn't want to say but I thought some mention of the metal mesh would have been made before now?

If it ended up near or touching some old cabling/pipework etc. Then when the wet plaster was applied over it and now it's dry? Or am I still not understanding electrics... ;-)

Mark S.

[John Rumm]

What you mean like what I said in the last paragraph of my original post? ;-)

I expect that is one of the most likely possibilities....

[John Rumm]

You mean like a thick ear from one's mother for messing up her nice new plaster? ;-)

I figured that since that was inserted by hand, plugged and screwed into place, and then plastered over with wet bonding plaster; had it have been live he would have got at least a tingle sooner. It seems it was only when he was throwing lots of water about when polishing the top coat that it got interesting.

[John Rumm]

That would be plausable, although I would expect him to have got a shock sooner if that was the case. The other problem is that it seems an unlikely place to have any wires - other than the metal lath I could find nothing with my metal scanner and there are no electrical fittings in the area.

The only pipes around are some disconnected ones that run floor to ceiling in the corner of the adjacent room in a boxed in section. These used to be the rising main and feed to the hot water cylinder to/from the cistern in the loft which is made out of slate and still there!. (I went through all the plumbing and eliminated all the remaining lead pipe that was in use when I was about sixteen IIRC - but never shifted the slate tank since it must weigh a ton all by itself!).

Perhaps I ought to go back and megger the upstairs light and power circuits...

[sPONiX]

Can you try a cable detector to see if there are any live cables in the vicinity?

sPoNiX

[sPONiX]

Maybe his metal stepladder was standing on an extension lead and damaged the insulation?

sPoNiX

[tony sayer]

In article , John Rumm writes

Nothing coming UP through the floor perchance, and the wall is "earthing" him?....

[John Rumm]

Did that, but could not find anything. I need to look in the loft to be sure though.

[Capitol]

Old unearthed gas pipes? I've seen this effect in a Victorian house back in the past.

Regards Capitol