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.
Click to see the full signature.