Mains electric shock from plastic light switch - no fault found by electrician - what next?

jimster99 · Jun 6, 2022

Hi all! This is my first post and I have registered to ask for some help with troubleshooting an electric shock I got from a plastic light switch outside a bathroom today.

I turned the switch off with my left hand and got what felt like a mains electric shock through the switch into my left arm. It had the distinctive "throbbing" feel of mains electricity and I felt it mainly through my left arm and left torso. I pulled my arm away and my rib cage still feels a little sore this evening. It was definitely not a static shock. The apartment has an RCD box which didn't trip.

The light switch itself is plastic but has metal screws (although I don't think I touched them). I was wearing slightly damp socks on a thin carpeted floor and my hand was possibly a bit damp from washing my hands in the toilet so my resistance was probably relatively low.

An electrician has now been to visit and said he couldn't find anything wrong and he seemed to suggest I couldn't have had a shock. He very happily touched the switch and it was fine. The wires in the switch appeared to be screwed in properly and the same in the light. The switch is mounted in a metal frame but he couldn't find any live voltage where there shouldn't be with his two-pronged multi meter. He did suggest that a more thorough check of the property and this is being scheduled.

This may be unrelated but in the morning before the shock my wife had noticed that the light in the bathroom controlled by the switch (which is a ceiling mounted multi-LED type) was pulsing on and off approximately once a second when switched off. When turned back on it was working fine but then still pulsing when switched off. The light then started working normally again so we thought it had cleared up.

Any ideas as to what can possibly have caused this? I didn't imagine it but with the electrician not finding anything wrong, I am flummoxed! Note I am handy with a multimeter but only a hobbyist and I normally steer well clear of working on mains electricity.

Pics of the light switch are below:

[ElectriciansForums.net] Mains electric shock from plastic light switch - no fault found by electrician - what next?

LastManOnline · Jun 12, 2022

brianmoooore said:
There's a remote possibility that it's the floor that is 'live' as a result of a nearby fault, and that the shock came from touching correctly earthed metal in the switch.

Remote.But certainly a possibility.If the installation earthing proves to be good than it could even become a probability.Especially with static now ruled out

marconi · Jun 13, 2022

jimster99. Good evening Sir.

We are a nosy crowd and really like to know about how problems which have been presented are resolved especially when we have 'skin in the game'.

I noticed that the light fitting has been replaced some time which may be significant. We will not ask by whom. What we really want to know is whether this problem has been first explained and second resolved so that it is safe for all. Unless one understands how you received the shock ie explained by careful investigation it is uncertain whether it could happen again.

jimster99 · Jun 14, 2022

marconi said:
jimster99. Good evening Sir.

We are a nosy crowd and really like to know about how problems which have been presented are resolved especially when we have 'skin in the game'.

I noticed that the light fitting has been replaced some time which may be significant. We will not ask by whom. What we really want to know is whether this problem has been first explained and second resolved so that it is safe for all. Unless one understands how you received the shock ie explained by careful investigation it is uncertain whether it could happen again.

Hello Marconi! Thanks for the reply (and for your earlier very helpful comment that damp socks = no static!) and don't worry, I have not forgotten about this - I am very keen to avoid future shocks!

The light itself had been replaced by somebody before we moved into the property. We don't know who!

The outstanding points appear to be:

(i) checking the RCBO: why didn't it trip? I suspect because I got less than the 30ma trip level. It's worth noting that we had a faulty metal electric kettle a few months ago that DID trip the RCBO a few times, although I suppose it could have been a short circuit tripping the fuse rather than a voltage leak. But this should be checked (which requires taking the front of the RCBO box off).

(ii) Is the internal wiring of the LED itself correct? You asked for a photo of the inside of the light and I will do this when I get a moment (hopefully today). I have the VED screwdrivers ready to go!

(iii) should the internal lugs of the switch be bent back to avoid any future risks of electric shocks? EDIT - yes - and I will do this myself (since the electrician didn't) after cutting the power to the entire property.

(iv) check earth continuity back to board.

(v) The earthing resistance needs to be checked which requires a fault loop impedance test. Is this the same thing as (iv)?

(vi) an insulation test needs to be carried out on the circuit (rather than just the switch) which requires taking the front cover off the RCBO.

Lucien Nunes said:
Odd. Carrying out an insulation test on the circuit (as distinct from the switch) requires taking the front off the board to disconnect the neutral. In which case an RCD test can be done at the same time, taking less than a minute. If he did not test the circuit insulation and did not open the board, then it is still possible to do the RCD test from the switch position albeit with a bit of legwork to visit the board a few times to reset the RCD.

Hmm, he only checked the switch insulation, not the circuit insulation. Shall i get him to come back and check the circuit as well? He didn't seem to think it was a necessary check but better safe than sorry I think!

brianmoooore said:
There's a remote possibility that it's the floor that is 'live' as a result of a nearby fault, and that the shock came from touching correctly earthed metal in the switch.

I hope this can be ruled out because of the fact that the light itself was intermittlently pulsing when switched off shortly before the shock - too much of a coincidence otherwise.

littlespark · Jun 14, 2022

OP…. My suggestion of bending the top and bottom lugs off aren’t a regulation or anything.
Some makes of switches have terminals very close… and are only a problem if you’re taking a switch off the wall without isolating first.. very easy to brush against, then bang. Or click… if it’s on an rcd.

Does your rcd work at all by using the test button?
If it’s been going off with a big enough fault, it maybe not working properly.

Your electrician should have checked this as a matter of course with the right equipment.
It would tell him how fast it trips… has to be under 40ms, and how sensitive it is.

jimster99 · Jun 14, 2022

marconi said:
If you feel confident and only if you do please turn off the main switches on your consumer unit and all the circuit breakers. Then take the cover off the lamp and then take the lid off the white terminal box I have pointed to with green arrows. Take a photo of the terminal block inside and post it.

I've checked inside the chocolate box, I think it looks like the wires are correctly connected (especially the earth!) Photos attached as requested. Thanks!!

littlespark · Jun 14, 2022

Oh well… the light fitting has been connected properly. It did look like there was a swapped over conductor from the earlier pictures when it still had the lid on the connector.

Make sure you put that lid back on

brianmoooore · Jun 14, 2022

There's something a bit odd about that set up. Live and neutral come from one direction, cable sheath barely entering the fitting, and the cpc comes from a different direction.

DPG · Jun 14, 2022

brianmoooore said:
There's something a bit odd about that set up. Live and neutral come from one direction, cable sheath barely entering the fitting, and the cpc comes from a different direction.

I think L, N and E all come in through the grommet at the bottom side of the connector block don't they?

brianmoooore · Jun 14, 2022

Yes, the brown and blue are going to the driver, and the incoming cable is the white wires. HR sleeving?
Cable sheath still doesn't come into the fitting.

marconi · Jun 15, 2022

Now I would take the fitting down and examine the wiring underneath.

Jimster99 - if you feel confident first turn off both red main switches and all the circuit breakers and then remove the fitting securing screws while taking the weight of the fitting. Then take a photo of what you see of the wiring above the fitting especially of any connectors. Then carefully put everything back in place, turn on mains switches and the circuit breakers.

jimster99 · Jun 22, 2022

marconi said:
Now I would take the fitting down and examine the wiring underneath.

Jimster99 - if you feel confident first turn off both red main switches and all the circuit breakers and then remove the fitting securing screws while taking the weight of the fitting. Then take a photo of what you see of the wiring above the fitting especially of any connectors. Then carefully put everything back in place, turn on mains switches and the circuit breakers.

Thanks again Marconi, will do this during the course of the current week!

Lucien Nunes · Jun 22, 2022

Returning to the possibility that the shock was received from the switch mounting screws, which indicates that the CPC (Earth) conductor of the circuit is not earthed, I was thinking about the severity of the shock received. The most likely cause in that case would seem to me to be loss of connectivity at the consumer unit, rather than a local problem to the bathroom light.

If the CPC is disconnected locally it will float, but absent another fault would not collect enough leakage current from the nearby cables to give a heavy shock. To do that it would likely have been connected to the earth bar in the CU, while the CU was disconnected from main earth and any bonding due to an intermittent connection. Just possibly, loss of connectivity between all or most of the lighting circuit CPC and CU earth bar (which would leave the entire circuit's capacitive leakage accessible at all points) would be enough to deliver a heavy shock.

The above scenarios of intermittent earth connections should be fairly easy to locate by inspection and testing, unless one is very unlucky and there is a concealed intermittent e.g. where a cable is partially cut through, that is at the moment making very good contact and not detectable by unusually high resistance.

Alla Musica · Jan 21, 2024

LukeD said:
LAP and BG are now well and truly on my avoid list

What went wrong with BG switches? Would love to know - I have one at home and I'm slightly worried

timhoward · Jan 21, 2024

Alla Musica said:
What went wrong with BG switches? Would love to know - I have one at home and I'm slightly worried

Don’t over think this or over worry! They just don’t always last as long as other brands, especially if switching heavier loads.

Alla Musica · Jan 21, 2024

timhoward said:
Don’t over think this or over worry! They just don’t always last as long as other brands, especially if switching heavier loads.

Aha, no wonder the 30-year warranty

(just kidding)

Mains electric shock from plastic light switch - no fault found by electrician - what next?

jimster99

LastManOnline

marconi

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jimster99

littlespark

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jimster99

littlespark

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brianmoooore

-

DPG

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brianmoooore

-

marconi

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jimster99

Lucien Nunes

-

Alla Musica

timhoward

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Alla Musica

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