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.