Confirming continuity of earth

[majordanby]

Hi,

I’m replacing some of my light switches with metal face plates, before I do I’d just like to confirm the continuity of the earth to the back box.

I don’t have the equipment or expertise to perform a full continuity test from the CU so I just wondered if there is anyway I can do this for the purpose of the individual switches, I.e use my multimeter to check the resistance between the live and ground with the circuit turned off or if I need to call the experts in.

Thanks in advance

 

[EFLImpudence]

I.e use my multimeter to check the resistance between the live and ground with the circuit turned off or if I need to call the experts in.

Not live and ground.

There should be continuity between Neutral wire and the Circuit Protective Conductor (Earth wire), or

between the CPC in the light switch and the CPC in a nearby socket (the socket screw if metal).

Not 100% verification but probably good enough.

Is there an earth block with green and yellow wires near the consumer unit? You can use this with a long wire without having to enter the consumer unit.

 

[majordanby]

OK, great. Thanks for the response. I’ve no reason to suspect there wouldn’t be continuity but we’ve just moved in and with kiddies running around want to be doubly sure.

Cheers

 

[EFLImpudence]

I should have said -

if you turn of the Main Switch or Two pole RCDs that will disconnect the Neutrals from Earth so just turn off the MCBs.

 

[Taylortwocities]

I’ve no reason to suspect there wouldn’t be continuity

I've been round a heap of electrical installations. People do stupid things, and thats not just DIYers, so-called electricians can be rubbish too.
My advice is to suspect everything.

 

[JohnW2]

There should be continuity between Neutral wire and the Circuit Protective Conductor (Earth wire) ....

I think that will only work (or, at least, work reliably) with a TN-C-S supply (and, as you subsequently noted, with the Main Switch 'on'). With TN-S or TT, there will usually be a PD between neutral and CPCs, and that will almost certainly screw up any attempt to use a meter to measure resistance/continuity between them.

Kind Regards, John

 

[333rocky333]

A majority of premises have an Exposed external earth bar or some piece of exposed metal part of the electrical installation near the meter, unless the house is massive just use some old wire as a wander lead and use your meter on continuity, will give you a basic idea

 

[majordanby]

Thanks for the responses guys. Novice question here but would measuring the voltage between the L1 and CPC not give an indication that there was continuity of the CPC, I.e should show 240v approx. If the CPC was not connected it would show 0V?

 

[Taylortwocities]

It’s not a reliable method.
It depends on what you are measuring it with. Digital multimeters can pick up stray/induced voltages and make you think something is there, when it isn’t.

Resistance measurement between a known earth and the circuit’s earth/cpc is the way to do it.

 

[bernardgreen]

I’d just like to confirm the continuity of the earth to the back box.

Metal faced switched should have an "Earth" terminal on the switch and the CPC should be taken to this terminal to ensure the faceplate is earthed. Important if the switch is loosened from the back box for any reason.

A short lead from the back box Earth terminal to the front plate Earth terminal will earth the back box,

 

[Taylortwocities]

Yes. But he wants to test to confirm if the earth conductor is actually connected to earth.

 

[majordanby]

Ok, thanks all.

The CU doesn’t appear to have an exposed earth so for simplicity’s sake I’ll test resistance between the switches CPC and an adjacent plug sockets earth.

Thanks

 

[JohnW2]

The CU doesn’t appear to have an exposed earth so for simplicity’s sake I’ll test resistance between the switches CPC and an adjacent plug sockets earth.

That's not actually a foolproof way to do it. Although unlikely, several, maybe all, of the accessories (switches, sockets etc.) could have their CPCs (correctly) connected together but without any of them actually being connected to earth.

Without test equipment which you won't have, I don't think that any 'resistance' measurements could totally prove that the CPCs were actually connected to an effective earth. Although others may disagree, ironically I think that your own suggestion (measure voltage between L and CPC) is perhaps the 'most reliable' thing you could do. Whilst, as TTC says, a digital meter can pick up and indicate stray voltages from a wire which is 'connected to nothing', the voltage you see due to that will be nowhere near 230/240V - so I would say that a measured L-CPC voltage of around 230/240V would provide a reasonable indication that the CPC was actually connected to earth.

Kind Regards, John

 

[majordanby]

Haha, too many options. Thanks Jon.

Another option, isolating power to CU, connecting lighting ring phase to CPC at CU and them testing resistance between L and CPC at switch???

 

[JohnW2]

Another option, isolating power to CU, connecting lighting ring phase to CPC at CU and them testing resistance between L and CPC at switch???

[before someone else points this out, your lighting circuit is almost certainly not a 'ring'].

Once you'd got the CU open, you might as well just use a 'long lead' to measure continuity between the CPC at the CU and the CPC at the switch - but what you suggest would achieve the same, and might be more convenient. However, that will again only prove that the CPC at the switch is connected to the CU, not necessarily that it is connected to an (effective) earth. As I've said, that could only be proven with an electrician's test gear - but, as I also said, finding 230/24V between L and CPC at the switch would be a reasonable 'secondary' method of 'confirming' an actual connection to earth (after initially doing the resistance measurement).

Kind Regards, John