What actually happened.

[polo1]

So a bit of a story....
Was in checking out an office for its first EICR. Another company of electrical contractors was in adding some floor boxes.

Heard a yell and found a lad looking as if he’d seen a ghost, said he had got a massive belt when removing a radial circuit from an existing box. Adamant that he had locked off and proved dead. The boards were in a locked riser, the circuit he was working on was locked off, and five other tracks were in the off position but not locked off.

Now I guess there is a possibility he locked off the wrong track, but is there anything in a typical office that would have capacitance which would not be discharged from safe isolation tests and give someone a major belt?

 

[Squid]

Surely he plugged in his martindale first?

Failure on his part.

 

[johnduffell]

voltage/current flow L-N, L-E or N-E because L and N are isolated from terra firma - but the neutral is live nevertheless

Voltage is potential difference, so if you're saying there's no voltage between earth and anything, even if you touch a live cable it will be floating relative to earth so you can't get a shock.
That's a bit like isolating transformers and even workers being clipped to energised power lines while doing work.

 

[Strima]

Didn't test for dead correctly and there was reversed polarity?

 

[marconi]

Re: Johnduffell #17: Thank you. What you have written I do not disagree with. I started my theory with 'I wonder if...' to indicate I was not certain. My theory rests on the minimum current proving dead test equipment requires to indicate something as being live - a figure I do not know and could not find out - and a circuit path for it flow through.

Being pedantic, an isolation transformer provides galvanic isolation (ie: no conductive path for electrons) between primary and secondary but does not normally prevent 'displacement currents' caused by electrodynamic induction ie: capacitance between primary and secondary windings. And cabling has capacitance to things around it. Similarly, the body has an inherent capacitance and capacitance to objects around it. I was musing whether the belt the lad experienced was the result of a displacement current flow which was lower in magnitude than the minimum the tester required to say something is live to avoid indicating something is energised when it is not.

I was trying to come up with an explanation which tied in with:

a. all track mcbs being off including for the circuit he was working on.

b. that he did indeed prove dead correctly.

c. his tester was functioning correctly.

so he did what would be expected and yet he still got a shock.

We will never know because all involved we be on the defensive.

 

[marconi]

A simple 1:1 isolation transformer with an extra dielectric barrier and an electrostatic shield between primary and secondary. The grounded shield prevents capacitive coupling between primary and secondary windings.

 

[Squid]

Re: Johnduffell #17: Thank you. What you have written I do not disagree with. I started my theory with 'I wonder if...' to indicate I was not certain. My theory rests on the minimum current proving dead test equipment requires to indicate something as being live - a figure I do not know and could not find out - and a circuit path for it flow through.

Being pedantic, an isolation transformer provides galvanic isolation (ie: no conductive path for electrons) between primary and secondary but does not normally prevent 'displacement currents' caused by electrodynamic induction ie: capacitance between primary and secondary windings. And cabling has capacitance to things around it. Similarly, the body has an inherent capacitance and capacitance to objects around it. I was musing whether the belt the lad experienced was the result of a displacement current flow which was lower in magnitude than the minimum the tester required to say something is live to avoid indicating something is energised when it is not.

I was trying to come up with an explanation which tied in with:

a. all track mcbs being off including for the circuit he was working on.

b. that he did indeed prove dead correctly.

c. his tester was functioning correctly.

so he did what would be expected and yet he still got a shock.

We will never know because all involved we be on the defensive.

He screwed up.

Simples......

 

[Lucien Nunes]

This is said to be a socket-outlet circuit fed from a DB in a commercial building, so we can be pretty sure it is supposed to have an earthed neutral and is not fed from an isolating transformer or any other non-standard configuration. Regardless, a GS38-approved tester would have indicated something abnormal, even if not the correct voltage, had the source impedance of whatever was supplying the shock current been low enough to cause harm.

Occam's razor. He poked the probes into the socket contacts to prove dead and didn't make contact. We've all been there. That's one of the reasons I carry a voltstick as a cross-check.

 

[The Ghost]

Maybe he used a fluke voltage tester under recall and it had actually failed?

 

[polo1]

Thanks all for your thoughts. Probability suggests that he didn’t prove dead, but I was keen to get views on other possibilities.
Another salutary lesson hopefully learned.

 

[marconi]

What makes me uneasy about saying the lad failed in some way and moving on is that he may not have done or his test kit may be defective or there may be some other fault so far undetected to blame. This is a serious 'near miss' involving a young inexperienced person (I assume because the OP called him a lad). He could therefore do the same again and be seriously harmed or killed. Where is the supervision? Without an investigation without blame the actual reasons why he got a shock are never established: he is young and therefore cannot be expected to be perfect even in the small details eg; did you prove test equipment first on a live supply, did you touch copper of the circuit you were about to work on, did you test for dead at least three times? Should he have been issued with a voltstick to do the Lucien last check? Why did he make a mistake identifying the circuit or is the labelling wrong? I'd want my son/daughter better looked after at work than appears to be the case here. Of course, we older folk have fewer excuses for getting it wrong. Yes, he may well/most likely have been at fault but that is not known for certain.

 

[Toneyz]

It's a bit difficault for the OP as it is a different company to get involved and go over it with the lad.

 

[westward10]

There certainly should have been an investigation into the chain of events which led up to this incident because, somewhere along the line isolation procedures have not been followed.

 

[Moley]

I totally agree with Westward10, an investigation should take place. What was a near miss this time could be fatal next time. I had a similar experience many years ago. We work working on a 415V switchboard at a power station. The permit had been issued, all the points of isolation were listed. We tested for dead at all the incomers and happy that things were as the permit said, we started work. It was a long switchboard and we had to do checks/maintenance on various contactors. We all did our checks for dead on each contactor before stripping it to check/replace the contacts. All except one. The one that wasn't checked turned had been altered at some time, the drawings not updated and was fed from a currently live feed. Luckily for the person concerned he only had a belt from it. Obviously work was stopped immediately and a rapid trip to the permit office followed. The power station did an immediate investigation and the following day reps from the company I worked for came to site to do their investigation. The AP who did the isolation was blamed by both parties and very nearly lost his job. His excuse for not checking that all points were isolated was that he was too busy as he was the only guy on site authorised to check isolations on the permit. Our guy who had the shock was also bo**ocked for not checking. Always, ALWAYS check, check and check again. The next time could be your last .

 

[gazdkw82]

Something has gone wrong in his safe isolation procedure or the equipment used to complete that procedure.

 

[BruvDunk]

Near miss !!! Would require looking into to stop it happening to the next person.
Does seem like a failure to prove dead on top of an incorrect isolation.