In the September issue of Wiring Matters e-newsletter, one of the articles looks at the case of 'Colin', a recently qualified electrician carrying out a safe isolation procedure. Unfortunately for Colin, he makes an oversight which leads to him receiving an electric shock.
Read the article here and let us know what would have been your safe method of working for the job Colin had.
Nice one - I think that I would have got a belt.
A quick Zs before removing the old fitting is a very easy precaution, but if Colin had been intending to issue a MEIWC, he would have done that anyway. A polarity check (also on the MEIWC) would have revealed the problem too.
Problem is that an electrician might not issue a MEIWC for very minor work - it would reduce the profit margin.
I am puzzled by the diagram above. If the little grey rectangle represents the meter cabinet, there appears to be a N-PE connexion between it and the DB.
Well I was told long ago, to switch things off but to still work as if it is live.
And it does help.
Back in the previous century I replaced a hall light fitting that was not working for someone, and I took out the downstairs lights fuse and had it in my pocket. (I know, but the isolation techniques of the past were different.) Anyway I duly removed the hideous chandelier thing and it's frayed flex and fitted a modern ceiling rose and holder, and then at the end, put the bulb in. And flinched a bit when it came on in my hand !!
The 'so and so' s who had done the hall and landing switches had taken the live feed for the hall light from upstairs, and because the original fitting was defective it had not come on !.
A volt stick or even a neon driver (boo hiss) would have revealed that, and the same applies to 'Colin' which I hope is not his real name.
Moral, Expect the unexpected. - actually the wrong kind of line reversal can also leave you with live plumbing !.
Mike.
This is the problem with 2 pole testers you need a potential difference, there has been times I have thought something isolated the cpc terminal a bit too deep for GS38 probes, I always follow up with my non contact tester or my single pole tester.
If your there to replace something think is the wiring a problem before you replace or dismantle
The 'so and so' s who had done the hall and landing switches had taken the live feed for the hall light from upstairs, and because the original fitting was defective it had not come on !
I learned to tread very carefully when re-wiring my daughter's house. It was originally a pair of Victorian farm-workers' cottages and some of the circuits were arranged vertically rather than horizontally. So the light switch in the dining room, which supplied a lamp above a door to the garden was fed from upstairs. The kitchen sockets were supplied by three different circuits!
Of course the safest solution, which the article mentions, is to isolate the whole DB, but that is problematic when the householder is present in the building.
Any thoughts on the diagram above?
Having had a few near misses under a variety of circumstances my final check after proving dead is to wave a voltage detector pen around just in case and to work as if live.
House has one lighting circuit and fuse was in my pocket. Removing wall lights and test one gang of a two gang switch, dead so remove wiring and wall lights. Wondered why there were two supplies to the switch instead of a link between switches. I found out why when fitting a single gang switch, the second switch was still live and wired from the ring main.
Along with shared neutrals, incorrectly wired rings with legs on two fuses/mcbs, lack of any labels to indicate a solar installation, no earth connection on PME and TT supplies, voltages on supply neutrals, earth from next doors PME supply via copper water pipe bond, reverse polarity 13A socket supplied by two core 0.5 mm flex under carpet, there are just too many possibilities for errors
A friend told me his ABC method, taught to him by RAF fighter and test pilots when he was learning to fly. Assume nothing, Believe no one, Check everything. Whenever I have had a near miss I realise I have not done one of the ABC's properly. Fortunately it is many years since I have managed to give myself a shock or tested the effectiveness of RCDs despite recent near misses.
In this case, if Colin was determined to do things strictly by the book he'd have notified the DNO or meter provider that the meter connections gave rise to danger (a so-called cat.3 situation). He would of course have decided that he couldn't break the all the seals, so he would hve isolated the entire house until the supplier arrived to put it right. At least that might have given Colin time to repair the broken earth connection in the lighting sub-circuit. But even so, not a way to endear him to a householder who'd asked him to do a minor job on a fitting, & who was probably baffled that an installation that seemed to be working perfectly was causing so much agro & expense!
Colin is here to change a light fitting. He has recently qualified as an electrician. He remembers his safe isolation procedure training and carries out the following sequence of operations and observations:
Now ask yourself whether he is safe to work?
There is no mention if he asked for permission to turn off circuit from the correct person
There is no mention that after LockOut TagOut that he put the key in his pocket
Another reason for having RCBO in CU rather than MCBs
would not help if it was a solid neutral type RCBO of course for this case of P-N reversal of supply. its not clear if the LE test read zero as the CPC was broken or because it was live, like the neutral - given how little the 's' in TN-s means these days. Its also not very clear how he got a bad shock - if all exposed cores are live how was the shock path completed - one foot in an external flower bed perhaps?
Mike
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