Shock from a consumer unit even after Safe Isolation!

[vwes59]

Not a big one, but it bloody hurt and scared the pants of me! If anyone reading the following knows why this happened I'm all ears.

I safely isolated a garage consumer unit, but didn't turn off the associated MCB in the house, so the tails to the unit were still live, ready to put in a new external socket. Did all the wiring, and was ready to do my r1+r2 tests etc.

Disconnected the line from the 10A MCB and then began to remove the earth. I moved the line so that I could get a better grip with my test cables and that is when I got a shock. It was extremely brief, but hot and stung like hell. I have absolutely no idea what it was I inadvertently touched, because the whole thing except the tails was dead, and I was nowhere near the tails. I redid the safe isolation and according to that the unit was dead and should have been safe as usual to poke around as I've done countless times in countless consumer boards.

Is it possible even though I thought I was nowhere near the tails, that there was a break in the cable behind the MCB's and main switch that I came into contact with ? I really ought to find out what occurred in case there is a possible dangerous fault with the unit.

Any suggestions welcome.

 

[snowhead]

If there was something bare then you should or rather already should have investigated and found it.
It's going to be obvious if you know where the shock affected you.
Was it just across the hand or across body or??

Personally I almost never touch the bare end of a cable whether connected or not.
I do have a procedure before I do.
But I'm old,,school I was brought up with no stated safe isolation procedure and very few testers available.

 

[vwes59]

It was just a short sharp pain to my finger as if I'd touched something hot, but you could feel the buzz. To be honest it happened so fast (and I moved away pretty damn quick) I'm not sure if I had pain anywhere else (btw I was wearing proper electricians insulated boots). The tip of my finger still tingles and is numb even now, 3 days after it happened. Nothing tripped out on the board or in the house. I did do a visual check of the board after I had turned off the MCB feeding it in the house and couldn't see anything obvious, so I'm at a loss to be honest. I even did a check with the power back on with my probes, just to make sure I hadn't done something daft, like to test the wrong part of the main switch, but I was right, live with the power on, dead with the main switch off in the garage, on at the MCB in the house, so I hadn't done the safe isolation check incorrectly, so wasn't a case of being complacent.

 

[Lucien Nunes]

and then began to remove the earth.

One possibility is that there was a potential between the CPC you were removing and the CPC in the submain. If the main CU earth is disconnected, the external socket might have been providing the only earth reference and any leakage from the entire installation passing through it, and hence through you as you disconnected it. Did you get a Zdb reading with that CPC disconnected, and/or do you think it could be in effective contact with true earth outside the equipotential zone?

 

[pc1966]

Is the source from the house on RCD protection?

I would agree with Lucian that it is probably some sort of current between the two CPCs, but the fact you seem to have continuing sensation suggests it was a lot more than the ten or so mA that typically would be seen on a good installation.

I would be checking the house's main earth connection and bonding, also might be worth checking the feed cable, etc.

You could get that on PME if the garage has a good path to earth and there is a fault, but I would expect you would be seeing other bizarre problems due to the house supply being crazy under those conditions.

 

[littlespark]

Faulty mainswitch in the garage CU? so busbar was still live and you’ve been lucky not to touch it sooner.

 

[stevethesparks]

The tip of my finger still tingles and is numb even now, 3 days after it happened.

I think you should get yourself checked out.

 

[vwes59]

Thanks for the replies. I'm back on site on Sunday morning, as I'm adding a couple of MCB's so all the power will be switched off, so I'll check everything - I'll need to anyway for the remainder of the paperwork (Ze, RCD tests and so on), so if anything is amiss it'll show up. Yes, a faulty main switch did cross my mind, and yes it is an RCD protected circuit. Have to admit, for a house that is only 20 or so years old, the wiring is shocking.

 

[littlespark]

Have to admit, for a house that is only 20 or so years old, the wiring is shocking.

Bad choice of words there.... “shocking”!

 

[Marvo]

I'd suggest when you go back that you assume nothing. Don't assume CPS's are earthed, don't assume bonding wires are earthed and don't assume that neutral wires are actually neutral.

First thing I'd do is repeat exactly your initial isolation procedure and see if you can find the elevated voltage that gave you a whack. Do the usual tests at the incoming supply at the main CU then run a wander lead from the main house earth bar (MET) as a reference for your tests on the garage subDB.