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]

Need more information as if it's a modern office then the floor boxes are usually plugged in to a power rail under the floor.

 

[polo1]

Need more information as if it's a modern office then the floor boxes are usually plugged in to a power rail under the floor.

Yep indeed, looked like Legrand powertrak.

 

[snowhead]

U.P.S, but it would have shown live if tests were done.

 

[westward10]

Did someone ascertain whether it was locked off or not.

 

[polo1]

Did someone ascertain whether it was locked off or not.

As mentioned in my op, the/a circuit was locked off, but I couldn’t say for certain it was the correct one.
Tbh I was more interested in the possibility of something else causing the shock. The lad seemed genuine that he had locked off & tested for dead - he seemed to be in too much of a state to be trying to cover his back!

 

[Squid]

Should have tested the socket or unplugged it from the track, looks like failure from the spark to follow procedure, I'd kick him off site.

 

[PEG]

Did the same fellow "ascertain" it was a radial circuit?

 

[Rpa07]

Didn’t recently touch a plastic light switch did he? He might be working the circuit, you know, getting his name out there!
I’m just kidding, this is serious - he should have proved dead!

 

[Toneyz]

We did some Legrand track recently it had two earth terminals I think one for the earth in the track and one for the enclosure (if my memory serves me right) we linked out both the earth terminals. If not was it a UPS or human error.

 

[Lucien Nunes]

If he did in fact correctly prove the circuit dead with a functioning tester, then no, it's unlikely that current-using equipment on the circuit could re-energise it sufficiently to deliver a serious shock. Capacitors in large PSUs can store enough energy to inflict serious injury, but there's a rectifier in the way of that charge getting back out to the plug and bleeder resistors to discharge the capacitors. You would have to create a very unlikely double or treble fault scenario to be shocked on a general power circuit more than a second or two after switch-off, and provided the tester is capable of indicating DC, it would have shown up.

I would be more inclined to believe that the wrong circuit was isolated and the proving was invalid. Had it been a single-phase board with the circuit off at an SP MCB, I'd have said check the polarity at the DB. But in an office surely it would be a TP+N board.

 

[Squid]

We did some Legrand track recently it had two earth terminals I think one for the earth in the track and one for the enclosure (if my memory serves me right) we linked out both the earth terminals. If not was it a UPS or human error.

UPS will power essential equipment like servers so they can be shut down without damage, rarely is a UPS used for the general office and Comms and Computer rooms have their own dedicated circuits.

As Lucien posted the sparks most probably isolated the wrong circuit and then didn't test that it was isolated, remember the documentation and labelling on site may not always be correct another thing is always isolate and test the circuit that you are going to work on, never trust anyone one else saying the circuit is dead or you could be.

 

[marconi]

Static electricity? I have had to investigate some but not all folk receiving electric shocks in a data centre when they touched the earthed server cabinets. It turned out the sufferers were wearing clothes and insulated sole shoes which created the conditions for them to become highly charged. The raised, insulated flooring provided further isolation from terra firma and the air con was keeping the air dry.

I wonder also if the supply to the tracks has an unearthed neutral and a single pole mcb. So, with the mcb open, in doing the proving dead the tester he used does not detect any minimum 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 and so will be the line if any loads remain connected - (L and N then being at the same potential).

 

[Squid]

Most comms rooms and data centres have anti static mats, and to be honest only peasants wear rubber soled shoes, leather is best.........

 

[ChrisElectrical88]

Surely he plugged in his martindale first?

Failure on his part.

 

[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.