Current in earth wire to light switch?!

[d000hg]

Replacing a light switch I was testing and discovered the screwdriver lights up (dimly) when touched to the metal face plate even though this is earthed.
Investigating further it seems the earth wire itself is carrying a small current. And presumably always has been... The wiring is quite old, there are RCDs but the fuse box is a total mess

I've heard this is not uncommon and while it sounds bad I have no idea how urgent it might be, if it's an emergency or more quirk of the system? We've been using the old metal faced switch for years without ever noticing a problem!

I own a multimeter, what should I test in order to see how serious this is... I'll report back with findings.

 

[bernardgreen]

discovered the screwdriver lights up (dimly) when touched to the metal face plate

These screw drivers lights up when the potential on the tip is different to the potential on the person holding the screw driver.

The potential of a person standing close to a cable with Live wires can be lifted, by capacitive coupling, to a potential above ground. This potential can be enough to cause the neon in the screw driver to light when the tip is touched on an Earthed conductor or plate.

Use the multi-meter to measure the potential between the face plate and a known good earth. This may requre the use of a wonder lead between the meter and the known good Earth.

A neon Screwdriver can give a false indication of no voltage ( it doesn't light up ) when touching a live wire if the person holding the screwdriver is not at ground potential

 

[d000hg]

Interesting thanks. It definitely lights up fully bright on live one coming into the switch. The earth wire gives just a dim glow, enough to see just.

Finding a good earth might be tricky could I test between earth and the out wire from the switch with it turned off or does that achieve nothing?

I think we have someone here next week on a different matter who has worked as an electrician, fortunately. He'd know though to tell me if we need to get a proper electrician out at the very least

 

[RF Lighting]

That’s an open circuit earth wire. You need to trace the wiring back and find out where it has become disconnected.

 

[terryplumb]

Do you get the same result at power sockets earthed points ?

 

[RF Lighting]

Do you get the same result at power sockets earthed points ?

Good thinking!

 

[d000hg]

That’s an open circuit earth wire. You need to trace the wiring back and find out where it has become disconnected.

as in a wire that goes nowhere? Wouldn't surprise me from what we've seen elsewhere.

Do you get the same result at power sockets earthed points ?

hmm, I'll see if I can take a look.

 

[sparkwright]

On SOCKETS, the two fixing screws are always in contact with earth, as the screw holes are connected via a metal strip to the earth terminal.

So for convenience, you can place this neon screwdriver on the screws.

That's enough of the neon screwdriver - we don't use them.

With the power off, you can do a simple continuity check with your multimeter between the light switch earth, and other known or assumed earth points in the house.

As RF says, there may well be an open circuit on the earth.

This break or loose connection could be anywhere, and the layout of your wiring will govern where to look first, but I suppose at the light fitting of the room in question would be an obvious place to check.

An electrician will have the correct test equipment and experience to do this work.

Work safely, make sure you isolate the power supply before you do electrical work.

 

[muffking]

If you have a multimeter then you could measure the resistance between the earth on the light switch to the socket screw of a mains socket nearby. It won't need to be accurate, but will confirm continuity that both sides have an earth.

 

[d000hg]

I don't really want to do much on this myself beyond establishing it's not going to kill someone touching the light switch before next week - changing a switch or adding a socket is one thing, tracing earths around seems best for a professional to me.

So is an "open circuit" basically saying there is no earth?

Sounds like I can quite quickly and safely go round testing the faceplate screws around the house though to see if this is an isolated case or not?

 

[terryplumb]

Yes ," open circuit" means the earth conductors are not actually connected to earth ,there is a disconnection somewhere.
Metal switches and sockets that are not earthed ,CAN be dangerous in certain circumstances. For instance if a live conductor comes into contact with the metal switchplate.

 

[RF Lighting]

Yes. Basically the earth has become disconnected somewhere within your installation.

As to whether it is going to kill someone, it’s similar to driving a car with no brakes.

 

[muffking]

That's a good attitude. A lot of it is a specialist job for good reason. The main thing you can do is check, within reason, what is safe.

So is an "open circuit" basically saying there is no earth?

Yes. Even if you go round with an extension wire on a multimeter set to measure the resistance between earths then you will know that both are connected together. By doing this between different circuits then you know they are both going back to the fuse board. For example, the mains sockets are not connected to the lighting circuit, but as the earths all go back to the board you are checking for continuity to prove that both are earthed back to the board. If they are open circuit then one of the two is not earthed and you can try elsewhere to track it down.

 

[JohnW2]

As to whether it is going to kill someone, it’s similar to driving a car with no brakes.

I obviously have to agree with you that a disconnected (i.e. ineffective) earth is something that must be corrected, since it has the potential (under certain, rare, circumstances) to result in someone being killed.

However, I think your analogy is perhaps a tad over-dramatic! I would think that it would be far fairer to say that living with such an electric fault would be "similar to driving a car with no airbags". In both cases, absence of the protective measure could theoretically result in a death, but only in very rare circumstances (which many people won't experience in a lifetime) when 'something goes very wrong'

Kind Regards, John

 

[RF Lighting]

The RCD is the airbags. The earth stops you from getting hurt in the first place (the brakes) and the RCD is there to try and limit your injuries when things have gone very badly wrong (the airbags)