Possible Electric Shock?

[briand2]

I'm currently looking into an apparent electric shock incident. The bloke involved walked across a room and (reportedly) turned off three groups of lights, one at a time, via a three-gang metal faced switch. On operating the third switch, he saw a flash and felt a shock. At the time, he was wearing regular footwear (rubber soles, presumably). As far as I'm aware, no fuses or mcb's operated.

Shortly afterwards, an electrician found "nothing wrong", and that the Earth Loop Impedance was less than 2 ohms (Note: I don't know if the switch faceplate itself was connected to the adaptable box earth terminal). There is some suggestion that the shock was just static, but it seems from the signs / symptoms that a decent belt was received, not the momentary discomfort that static produces at its worst.

I've occasionally seen arcing while turning off light switches (particularly with inductive loads), so depending on switch design the arc between contacts can leave the switch housing. Does anyone have a view on whether such arcing could have momentarily reached the metal switch / front plate enough to produce a shock, but not be sustained for long enough to trip anything.

Alternatively, has anyone ever come across a "self-clearing" fault in this situation; for example, where a single strand of the conductor happens to touch an earthed part but then burns itself out before the fuse / circuit breaker operates?

Or any other ideas as to how a real shock could have occurred and yet the electrical system when inspected / tested later appears healthy!!

Thanks

Brian

 

[RalphChristie]

Can you confirm if there are any earth leakage relay/s? (besides any fuses or breakers)

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[itsmoked]

YES! I can confirm that strands can vaporize before any breaker decides to trip. (having done it several times)

I have also seen that flash/arc exit the switch into the room several times and my other-in-law just complained of an external arc on her bathroom switch.

Perhaps your victim got briefly connected to this? Though I might expect a finger burn to go with it?

 

[IRstuff]

You can't have it both ways.

If it shocked the operator, then the strand was NOT touching a ground.

If it was vaporized, then it was grounded and the operator should not have gotten a shock.

TTFN

 

[briand2]

Thanks all for your input so far.

With this being a truly international forum, I should have mentioned that this was in the UK; single phase is 240 volts. Also, of course, ground / earth are the same thing.

There was on Earth Leakage Circuit Breaker / Residual Current Circuit Breaker / Ground Fault Circuit Interrupter; just fuses (or, possibly, Minature Circuit Breakers). I don't know all the details until I visit site, in about two weeks.

I'm not convinced that the switch metal front plate was earthed; I guess I'll find out when I visit site. However, even if it was earthed, does anyone have a view on whether or not an excessive arc on switching (due to worn contacts / inductive load / etc) could have come out through the slight gap between the switch and the metal switch plate?

Thanks,

Brian

 

[itsmoked]

The arc comes out of the switch body between the plastic lever and the plastic surrounding the lever that borders the switch cover plate hole. In the USA the switch lever doesn't just project out thru the cover plate but is surrounded by this plastic island. Otherwise the switch arm eventually gets cut thru due to misalignment.

 

[RalphChristie]

Well, I'd be very surprised if the earth leakage relay didn't operate. They are normally set on 30mA. If there was maybe a problem with the E.L.relay for some reason, then the guy could be shocked without a fuse blown/breaker tripped.

I agree with itsmoked, I think the flash was during switching the light off. You see it many times when the contacts are old or damaged.

To be shocked however, there must been a current path from the conductor, through the hand / body of the person towards ground. You'll have to physical inspect the switch box to determine the cause of the shock.


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[briand2]

There was a typographical error (clumsy fingers) in my second post. Just to confirm; there was NO device designed to trip on leakage current to earth / ground. The ONLY device was either a fuse or a miniature circuit breaker, either of which will trip only on excessive current. Also, being subject to UK regulations (BS 7671), the allowed time before the protective device (fuse or miniature circuit breaker) trips on excessive current is five seconds for a lighting circuit. Perhaps this is more than long enough for someone to get a noticeable shock, and for the offending fault to clear itself?

 

[itsmoked]

briand2; You will need to look at the site....

It could be a wire on the switch is touching the metal box and the conduit/box are in fact NOT properly grounded.

Could also be not grounded and the conduit is being capacitively energized. This could also occur only when a specific leaky device is on. Which would allow operation of the switches by others who may access them while the leaky device is not operating.

You can have people with better footwear touch the conduit and avoid any shock. Plus people with lousy shoes that just happen to only touch the lever as compared to the face of the box.

I would seriously worry about others touching/using that switch before its ground quality is assured. One person's shock is another's electrocution.

 

[lyledunn]

Similar incident in a polythene factory where the pest control guy complained that he received a shock at the metal cases fly zappers! Tried it myself, no small discomfort- it was a mighty dig!
As the operative walked through the factory charge built up on him only to be discharged at the earthed zapper.

Regards,

Lyledunn

 

[briand2]

lyledunn:

"Tried it myself, no small discomfort- it was a mighty dig!".

Your description is fairly typical of a good static jolt, but I don't suppose it left you with any "damage" lasting more than a couple of seconds. As far as I'm aware, there really were longer term health effects in the incidenct I'm looking at.

In the "worst" case, the maximum charge that can be accumulated on the human body is something like 60 mJ. Now although some people find charges as low as 10 mJ very uncomfortable, others can discharge tens of mJ without feeling too bad at all. However, the IEC view is that it takes perhaps as much as 5 J (i.e. something like a thousand times the maximum amount that can typically be accumlulated on the body) before there is a significant risk to health. As I haven't yet had an opportunity to visit site, I'm assuming that if the medical evidence confirms health effects then static just can't have been the cause. (NOTE: I'm not naive enough to just take the word of the "victim" with regard to health effects; I believe the medical reports (A&E and later reports) really do show health effects). When I visit site (next week, hopefully), I'm expecting to see these medical reports for myself.

Brian

 

[PowerfulStuff]

Brian,
What type of lighting load is it? I came across one where the 'standard' 10A lighting switch is actually rated for only 2A of inductive load (e.g. fluorescent or discharge lighting). The switch construction was only a small spring to open contacts and nothing to stop and arc from pushing out through the front of the switch. The arc is almost designed to come out of the front of the switch as it lengthens. It was a plastic switch and resulted in an electric shock in an office running at 8A fluoresecent loading.

The manufacturer also sells a properly rated switch with a stronger spring to open the contacts quicker and a shield to stop anything coming out of the front of the switch. Not a small job to change a whole site!

Regards,
Martin

 

[briand2]

Martin,

Thanks for your input.

The first thing I want to check when I get to sight is the rating of the switch (inductive / resistive)!!

Brian

 

[lyledunn]

Brian,
The effect of a shock derived fom static in the situation described may well not have direct detrimental affect on the human body. However, many accidents have commenced with a mild shock that resulted in an involuntary reaction by the victim the consequences of which resulted in death or injury.
I still maintain static is a possible candidate for blame. Switching indutive loads could of course result in a visible arc, perhaps, coincidentally simultaneous with the static discharge.
The guy you say was wearing rubber footwear, presumably standing on a surface that was reasonably isolated from earth. Given the impedance of skin, body resistance, footwear, floor, and earth, the magnitude of current derived from a fault situation on the switch must have been relatively low lest he was also making contact with an adjacent earthed item.
As a matter of interest, i investigated an incident where an operative said that he received a shock from a laptop computer which was not connected to the mains. He claimed that the jolt caused him to fall and break his ankle. An out of court settlement was made. The laptop was removed, thoroughly checked by an independent body and proved safe.
As a precaution the laptop was destroyed. I guess that sometimes technical debate is superfluous in a litigious society.

Regards,

Lyledunn

 

[itsmoked]

Cripes lyledunn, sounds like a rabid dog!

I could see the switch arc come out of the plastic housing right there with the person's finger working the switch and connect with the the finger while meanwhile other fingers are in contact with the metal grounded face plate. This would absolutely connect the unfortunate with the mains supply for the duration of the arc life. He could be standing on glass insulators wouldn't make any difference.

 

[briand2]

update...it would seem that an offer of settlement has been made (after I put in writing various questions about periodic inspection and test records, etc) so I may not get to visit site after all!!!

 

[itsmoked]

I hate settlements! How is a settlement going to prevent a recurrence?!?

 

[lyledunn]

itsmoked.
Just what sounds like a rabid dog?

Regards,

Lyledunn

 

[itsmoked]

Taking out the laptop and destroying it. No doubt a laptop tens of thousands of other people use daily with no such "shock" nonsense and now one is deliberately destroyed.

Same thing they do with vicious dogs.