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

[jimster99]

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:

 

[jimster99]

Just curious about the led light. Is it in a loft ceiling or is there another floor above this?
If it is in a loft do you have tanks up there?
Did the electrician look inside the light?
Any chance of a picture with the bulbs OP?

The bathroom is in a low-rise block of apartments, on the first of four floors. So there are two more floors above it. I don't think there's a leaking boiler above it (the boilers are in a cupboard next to the bathroom). The electrician did look inside the lamp, picture below! I've also had the switch changed, so hopefully that resolves it (picture of the new switch below for completeness ).

The lamp:

The new switch:

 

[littlespark]

Good getting a new switch, but did he bend back the unused screw lugs?

Was there any further testing done? There is still the issue that you got a shock, and the RCD didn’t trip.
This is far from resolved.

 

[marconi]

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. You should see the two white wires connected to the blue and brown ones and the green yellow going in connected to the green yellow coming out to the screw terminal. One cannot be sure but it it may be from studying the order of the wires that one white ( the neutral) has been connected to the green yellow coming out and the green yellow going in has been connected to the blue coming out. Such a fault could cause a voltage on the green yellow wires ( the earths/ circuit protection conductors) at the switch and its screws which is high enough to cause an electric shock especially when the light is turned off as you reported. ( The L to E current may be too low to cause the rcd to trip). If you do find the crossed wires I mentioned then do not use the light or its switch. Others will advise you what to do next.

After taking photo replace lid, cover and finally turn main switches and then circuit breakers on again.

 

[littlespark]

Well spotted, @marconi

Certainly looks like they don’t follow the same order unless they’re crossing inside the white box.
White box missing a couple screws too.

 

[LastManOnline]

This leaves me baffled as well.But there will be a logical explanation (When it's eventually found).So if I may ,I,d like to add my twupence worth.

Neither you nor the investigating electrician found any evidence of moisture?That's peculiar because without it ,it's extremely difficult for you to make any kind of contact with switch terminals.The physical gap between a standard light switch terminal and the part of the switch you touch is a minimum of one centimetre.Its impossible to get any kind of shock unless there is a clear moisture pathway.Secondly moisture does,nt appear and disappear in the space of a few hours.Thirdly If any kind of arcing had taken place it would leave visable marks.

I am leaning towards 2 possibilities.
My first step would be to have your earthing checked.A high earth resistance will cause shocks to be felt in any earthed metalwork when certain appliances or plugged in.A fault loop impedance test will quickly establish this.

The second possibility is static from the carpet you stood on.I know you have ruled it out but static can be very unpleasant.Experiencing it while wearing damp socks and having damp hands can can be extremely unpleasant

As regards your strobing lights....I would 't even begin to speculate

 

[Sapper]

I may go against the grain here but I just don't like the LAP brand - personal experience includes short life spans caused by burnt out drivers and poor terminals. Even if there is not a wiring problem I'd personally get it replaced with something better.
The earthing problem still takes priority though.

 

[LukeD]

LAP and BG are now well and truly on my avoid list

 

[littlespark]

If you search through the forums, you’ll find stories of not so good LAP or BG products, but usually followed by a positive example of another product by the same.

It’s like a lot of other things… sometimes you get a good design, sometimes not.

I really don’t think this threads issues are down to the choice of fitting, but either installation mistake, or breakdown of much older equipment..

 

[SJD]

I may go against the grain here but I just don't like the LAP brand ...

While I agree about not liking LAP, I thought the new switch looked like MK Logic ... ?

 

[jimster99]

T

Good getting a new switch, but did he bend back the unused screw lugs?

Was there any further testing done? There is still the issue that you got a shock, and the RCD didn’t trip.
This is far from resolved.

Thanks for the follow up!! He did not bend the unused screw lugs back, should I do this myself? I feel confident that I can safely disconnect the circuit via the RCD/fuse box prior to doing this. He did do a 500v insulation test (as you suggested!) and found no problems. It was about a 1 hour visit in total. He didn't check the RCD (said it would take too long and require dismantling the box...).

I see there are some additional comments including to dismantle the light (which I feel confident enough to do). I really appreciate the feedback on this very friendly and helpful forum. I will take a look at those later today and post back!

 

[jimster99]

Neither you nor the investigating electrician found any evidence of moisture?

Re: moisture, there was no sign of moisture inside the switch found by me or the electrician. There was also no sign of arcing damage.

Although now I think about it I do remember seeing a few droplets on the wall high above the switch when the shock happened, which I remember thinking was odd because I don't know where they could have came from - possibly after I washed my hands I might have shaken them before touching the switch leaving drops on the wall and the moisture on my hands itself was sufficient to conduct? Although it's outside the bathroom there's no possible way water could splash off the shower or sink and into the switch because of the angles. The switch is in a cavity wall insulation and we did also have a heavy thunderstorm over the weekend in London so I wondered whether perhaps some moisture might possibly have come in through the roof (although seems unlikely given we are on the first of four floors). I have never seen any other signs of damp or water leaks.

The second possibility is static from the carpet you stood on.I know you have ruled it out but static can be very unpleasant.Experiencing it while wearing damp socks and having damp hands can can be extremely unpleasant

Re: the possiblity of it being static, I am certain it was an AC mains electric shock. I have had a mains shock once years ago from (ironically) the screw on a faulty light switch on a bathroom in a hotel in the middle east, and it has a distinctive throbbing feel and causes throbbing muscle contractions down your arm into your body and is very frightening/unpleasant/painful. This was the same. Static is (in my experience) a much sharper single shock with no throbbing and that only hurts at the contact point - I've played with e.g. a van de graff generator so I do know what a bad static shock feels like I would love for it to be a simple static shock explanation but I just don't think it is.

My first step would be to have your earthing checked.A high earth resistance will cause shocks to be felt in any earthed metalwork when certain appliances or plugged in.A fault loop impedance test will quickly establish this.

Thanks! I will check if the electrician did this test and if not will arrange to have it done (I assume I can't do it myself with a simple consumer multimeter unit)?

 

[Lucien Nunes]

. He did do a 500v insulation test (as you suggested!) and found no problems. It was about a 1 hour visit in total. He didn't check the RCD (said it would take too long and require dismantling the box...).

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.

 

[marconi]

What you experienced was not due to static electricity because you were wearing slightly damp socks which would have discharged you.

 

[LastManOnline]

What you experienced was not due to static electricity because you were wearing slightly damp socks which would have discharged you.

Good point.That effectively rules static out .I personally would be very confident that "dampness" is also not the issue so looking forward to hearing what results from testing the earthing.

 

[brianmoooore]

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.