Unsure how to ensure light switch is grounded

[CollC]

Hi
I have a dimmer light switch that would not turn on first time anymore, but if I gave the switch housing a thump it would come on. Assuming a loose connection in the switch, I turned off the circuit at the fuse box, and unscrewed the switch. A small screw dropped out of the switch on to the floor - it had come from the Earth on the inside of the switch.

This is a metal faceplate 2 gang switch, housed in a metal back box. There's two black leads, which each attach to an L1 on each switch. There's two red leads, which attach to each C, with a small red lead also linking the two Cs together. There's two bare copper wires cut back about as far as they go but they don't seem to be attached to the back box in any way. The earth screw on the inside of the faceplate (where the screw fell out when I took the faceplate off) has a small piece of copper sitting in the hole.

Not realising I had to earth this metal faceplate, I tightened all the L1 and C (the black wires and red wires), screwed the faceplate back into the wall and turned the power on, and the switch now works first time ever time. Then I got paranoid that it's not earthed, and it's metal faceplate and I'm going to die. So I turned the circuit off again at the fuse box and now I'm here. I've tried to contact my usual electrician but he's not answering. I have no RCD at the fuse box. Is my light switch safe or not?

Many thanks!

 

[securespark]

Pictures would be great so we can see exactly what you got.

The loose earth connection should not make the switch intermittent in operation, but a loose L1 or C terminal might have.

Post some pictures!

 

[CollC]

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• CollC
• 11 Jan 2016

Image

• CollC
• 11 Jan 2016

Hope this helps! Yes I was just working out how to add media!

 

[CollC]

I realise a loose earth would not make for the switch working intermittently. I'm confident I've fixed that now as some of the L1 or C screws were a little loose. I'm now just worried about whether the switch is earthed or not. Many thanks for your help!

 

[Taylortwocities]

The two bare wires are the earth wires. The front plate of that switch MUST be earthed. Thats what the terminal with the earth symbol is for.

You need to terminate the two bare wires on to the front plate, somehow. First of all you need to check that those conductors are connected to earth!
Alternatively, change the dimmer for one with a plastic front plate.

 

[CollC]

Ok, I thought the bare wires would be earth. How do I check they are actually connected to earth? I have a multimeter.

Does this suggest that this light switch was not actually earthed before I opened it, or did my opening it break the earth connection? The piece of copper wire I found in the earth terminal of the switch may have been long enough to make a connection with the back box, but certainly not long enough to meet the two bare wires - and the back box doesn't appear to be earthed so what good would earthing switch to back box be??

 

[Echo the husky]

Ok, I thought the bare wires would be earth. How do I check they are actually connected to earth? I have a multimeter.

The easiest way with a multimeter is probably the R1+R2 test. See here and read the parts in the section about test method 1. It is easier to do this test than to explain how to do it. The mains supply must be switched off during this test.

Does this suggest that this light switch was not actually earthed before I opened it, or did my opening it break the earth connection?

You probably didn't damage anything, it is most likely the person who installed the wiring couldn't be bothered to connect the cables. During the changeover from twin to twin and earth cables there was some resistance with some electricians not wanting to use the CPCs. If it is like this in this switch, it is possible it could be like it in every switch, light fitting and junction box on the circuit.

The piece of copper wire I found in the earth terminal of the switch may have been long enough to make a connection with the back box, but certainly not long enough to meet the two bare wires - and the back box doesn't appear to be earthed so what good would earthing switch to back box be??

It would do no good at all.

If you are lucky it will be just this one switch and you can carry out the test to prove the earth connection, and then connect the wires into a connector block along with a new 1.0mm earth wire which you can then connect to both the metal switch plate and the back box.

It is essential that you prove that one of the wires is earthed using the test method above. The other wire may not be earthed as it goes on to the next part of the circuit. If there is no proper earth connection, connecting these wires to the front plate will create more of a hazard than before. The bare wires once proved need green and yellow sleeving fitted.

If you find all the CPCs are cut out and want to connect them back in yourself, you must ensure that every single CPC on the circuit is connected, and then you will need to carry out the R1+R2 test above at every single switch and light fitting on the circuit to prove every point is properly earthed before switching the power back on. It's easy to get caught out by a hidden junction box for example.

If you can't get the circuit properly earthed, or don't feel confident you can carry out the tests safely, you will need to call in the services of an electrician to do it for you.

The other less desirable option, is to replace the metal light switches with plastic ones which don't require an earth connection, and fix them back with nylon screws so there is no chance of getting a shock from the plate screws should a live wire get trapped in the metal box behind. If you have metal fittings that are not correctly earthed you will have to check they are built to classII standards or replace them with classII fittings.

I bet you wish you never asked!

 

[ericmark]

I seem to remember 1964 as the date when we were told to earth light fittings, but the wiring regulations then was not the Bible it is regarded as today and it is still not law. The change was because of the number of metal fittings. You as said can use nylon screws or recessed screws with a plastic blank which cover them. Some dimmers do have plastic shafts others a metal and the chances of the switch becoming live are quite low, but as you have seen wires do become lose.

I have in the past tried to chase around and find all disconnected earth wires and found one out of the 100's disconnected which I can't assess so the whole exercise was in vain. So step one power off of cause is to look at other switches and ceiling roses if they also have earths disconnected then go down the double insulated route. If however it's the odd one out then yes connect them up.

There are two ways to test an earth, you have been directed to the dead test which is really the only option for DIY. However electricians have a meter called a loop impedance meter which allows one to test an earth live. This is a very quick test and you may be better getting an electrician with his meter to test it for you. This meter tests all the way back to the sub station in one go so is very much faster but does have some danger involved when using is so does need the user to be trained.

 

[CollC]

Thanks very much for the replies folks. I am going to put a plastic faceplate/switch on for the time being. This is the only metal faceplate light switch in the house so its no problem to fix it that way.

My electrician got back to me, he is coming at end of feb to install RCD at the consumer unit as well as other things so will get his advice on whether to chase the grounding in all switches. Seems a big job.

I opened another light-switch just out of curiosity - yep, no earth connected in there either. Cowboys!

I'm going to get some silicon sealant for the screwheads on all the light switches as well.

I've not opened a plug socket yet....they will have ground attached, right? Surely nobody is that mad?

Thanks again folks! Very much appreciated

 

[nebjamin]

I am not a qualified electrician, however if your supplementary bonding is up to scratch (i.e. your water pipes are bonded to the electrical earth at your incoming supply), and you have all copper pipework then a quick way to test without requiring you to remove the cover of the CU is to measure the resistance between the metal face plate on your switch and a nearby radiator.

If the resistance is very low (less than a few ohms) then you know that the metal faceplate is earthed. The principle is the same as the test method 2 in that doc but using the CH pipework as the wander lead.

You could also use the earth pin of a nearby socket.

 

[Taylortwocities]

Usually, you do not need to open up the socket. If the two fixing screws are metal then they will be connected to earth.