Neutral-Earth Voltage 120V

[RK_London]

I'm decorating my upstairs bedroom and decided to replace the light fitting (it's been there for 25yrs). I disconnected the old fitting which was only connected to live and neutral. (See pictures below). I checked the voltages and noticed the neutral- earth voltage was 120V which doesn't look right. Live- Neutral was 240V as expected.

I've switched off circuit from fuse box and trying to figure out next steps. Should I call in electrician or are there some simple checks I can do?

Any help/advice appreciated.

Thanks.

 

[pete01]

What about live to earth? Maybe the earth isn’t really an earth

 

[RK_London]

Just checked - Live to earth is 103V

 

[plugwash]

Yes this needs to be addressed.

The next step would be to test the continuity of the earth wiring from that fitting location back to the main earth terminal. This is not teriiblly difficult but does requilre a long temporary cable to be set up. If that is bad, then the fault is within the lighting circuit and it becomes a matter of searching and testin different parts of the circuit until you find it.

If the connection to the main earth terminal is good, then this suggests a serious problem with your supply earthing setup. If that's the case post a picture of the CU/meter setup for further advice.

 

[pete01]

I dont think you have an earth at the point, maybe it's come loose in the previous light fitting?

 

[CountryFan]

Can you confirm your earthing arrangement? If not, maybe post a photo of consumer unit area. Maybe test something else, like a socket?

 

[Murdochcat]

The Earth isn’t connected back to the CU is my best interpretation of what’s being reported

 

[plugwash]

The Earth isn’t connected back to the CU is my best interpretation of what’s being reported

I agree, that is the most likely cause, but it needs to be confirmed. It's also possible (but less likely) that the issue affects the entire installation.

 

[endecotp]

You have a multimeter, right?

Turn off the main switch.

Get an extension lead, plug it in to a socket near this light fitting.

Set meter to continuity (low resistance).

Poke one probe into the extension lead’s earth hole.

Toch other probe on light’s earth.

There should be continuity, but presumably there won’t be.

Repeat at other light fittings and light switches. Work out the pattern.

 

[SUNRAY]

You have a multimeter, right?

Turn off the main switch.

Get an extension lead, plug it in to a socket near this light fitting.

Set meter to continuity (low resistance).

Poke one probe into the extension lead’s earth hole.

Toch other probe on light’s earth.

There should be continuity, but presumably there won’t be.

Repeat at other light fittings and light switches. Work out the pattern.

Start by measuring the voltage between the extension lead earth and the light fitting earth.

 

[endecotp]

Start by measuring the voltage between the extension lead earth and the light fitting earth.

No. The voltage will either be zero, or it will be some confusing floating voltage induced by something.

 

[SUNRAY]

No. The voltage will either be zero, or it will be some confusing floating voltage induced by something.

But at least you will know you are shoving mains across your ohmmeter.

Personally when I'm doing this test I go round looking for voltage and when it's zero I know it's good then work outwards from there and to be honest I'd probably be starting it with the power on and a LED screwdriver, not a wander lead as it's so much easier and less cumbersome. Then moving on to the dead tests.

On one job which was very confusing due to the machinary around, I powered the faulty earthwire with a 24V transformer to earth via a small bulb and measured for 24V to the water pipes as my most reliable reference but that was an exceptional situation

 

[endecotp]

But at least you will know you are shoving mains across your ohmmeter.

Step 1 is turn the mains off.

 

[SUNRAY]

Step 1 is turn the mains off.

Well you could make it that complicated or you could learn how to use other devices to simplify the fault finding process.

 

[ericmark]

Personally when I'm doing this test

This is the problem, I have a loop impedance tester, so I just use that.

On the odd time I don't have one, I have to look around me, and decide what reference point is valid, no good using a radiator tap with plastic pipes for example. And found with mother's house it simply had no earth to whole house. How I don't know, as a lad I made an error line to earth and ruptured a 13 amp fuse, so it did have an earth, but hunting around the house, all bonded together, but no earth, only thing I could find was an earth stake from the old party line telephone.

Long story short assumed earth had been from water pipes, and new plastic water main resulted in no earth, house wired 1954. So got the DNO to give me a TN-C-S supply.

So without a loop impedance tester, or earth tester

how does one start? Until found no earth in mother's house never even considered it would not be at the consumer unit. Old fuse box had already been changed to a consumer unit, but I could find no sign of either a DNO fuse, or an earth rod.

I can think of methods, like using an old tungsten bulb, but there would be some danger, and I am not willing to detail how I would do it for anyone to read, it could be a pre-1966 installation when earth was not always taken to the lights. But until @RK_London confirms that other outlets do not show a floating earth, then all tests with a multi-meter are invalid.