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?

--
Cheers,

John.

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