Safe isolation - are you sure you are safe?

Susannah's diagram looks different to the one in the article to me - the above is TN-C-S while the one in the article is (now) TN-S. I wonder if the story started out as a L-PEN reversal...?

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?

The article seems to say the c.p.c. to the fitting was open circuit, but due to the reverse polarity "N" was at 230V, L & PE floating. So shock between "N" and some adjacent earthed metalwork perhaps could be plausible.

  - Andy.

mapj1 said:

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?

I have had one from a lamp fitting. Return path was screwdriver (not sheathed  ), me, synthetic footwear (not operating theatre ones, which deliberately conduct), wooden ladder, floorboards, joists, bricks, slate damp-proof course (could have been breached), more bricks and footings.

I wouldn't say that it was a "bad" shock, just a little nip really, but it made me feel foolish.

mapj1 said:

given how little the 's' in TN-s means these days

The diagram does not look like TN-s to me.

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

Another reason for having RCBO in CU rather than MCBs

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:

• Turns the light fitting on. He observes the light fitting operating with the switch.
• Turns off the circuit-breaker (CB). He observes the light fitting go out.
• Locks off the CB and attaches his warning notice. The system is 100 A single-phase TN-S earthing arrangement, so he is happy with single-pole isolation.
• Checks his proving unit and voltage indicator are functioning. All ok.
• Carefully removes the light fitting’s cover and tests between line-earth, neutral-earth and line-neutral. Records 0 volts on every test.
• Rechecks his voltage indicator with the proving unit. All ok. 

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

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! 

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.

mapj1 said:

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?

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 

Pretty common back then. I still come across an odd one on older properties where they borrowed lives and neutrals on the hall and landing lights. RCDs don't like it if you connect across two of them

Gary