Earth to neutral fault????

When you turn the main switch off it disconnects the live and neutral conductors.

On a TN earth system the earth and neutral are joined before the switch on the suppliers side.

Strima said:

When you turn the main switch off it disconnects the live and neutral conductors.

On a TN earth system the earth and neutral are joined before the switch on the suppliers side.

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So does that mean I don't have a fault on my side of the fusebox?

no it doesn't

But I am still testing the same neutral and earth circuit even with the box turned off so if the fault disappears does it not mean it is in the fusebox or just the mains earth/neutral bond showing up when switched back on

Hi, sorry but it would be a bit brave to say there isn’t a fault. The test equipment is not what is normally used and it’s not really possible for us to confirm how you have used it. What you have is a possible explanation for what you are reporting, if that makes any sense.

As mentioned above, earth and neutral are solidly connected at the source of supply in all public supply systems, and may in fact be the same conductor part or all of the way from there to your house.

On TN systems there will only be a fraction of an ohm of copper between your consumer unit and the connection. If it is TN-C-S that will be a very small fraction indeed because there will be a solid connection within your own service head. On a TT installation with the earth obtained via a rod / electrode, there might be a hundred ohms or more between earth and neutral at your end, but the connection is just as solid at the supply end.

The connection at source keeps the neutral near earth potential and is what makes it a neutral (as far as single-phase installations are concerned.) The other wire, the one that is not connected to earth, then becomes the line.

If the continuity disappears when you turn off the main switch, there is no indication that your installation is faulty, this is what you would expect. However, despite being near earth potential, neutral is considered a live conductor (it is part of the energy supply circuit) and you should not go poking around at it with the continuity tester with the main switch on. In any case, whatever reading you get is somewhat arbitrary because there is usually some AC voltage present (due to voltage drop along conductors) and most continuity and resistance tests are not supposed to be carried out with stray voltages present.

Thanks for the illustration, it does not have the electricity co isolator switch but the earthing looks about right and all the neutral wires come off the same terminal in the fusebox, worried it is going to trip the new fusebox, although have not had any problems with the current wiring.

What we can say is that with the main switch on, with the 2 most common earthing systems in urban areas you can expect to see continuity between earth and neutral.
The diagram above from @Wilko showed TN-C-S which is the most popular for new installs and what many others are slowly being converted to (I won't get into a rant about this). It would be a reasonable bet you have this system if you don't live in the middle of nowhere. On this system the Earth starts at the Neutral at your supply, hence the link.

The fact you lose continuity with the main switch off further supports the likelihood that this is what you are picking up.

So at this point, no definite fault. The electrician will check all of this stuff with a more sensitive testing device before he starts so I wouldn't be concerned.

Lucien, point taken about poking about, so that is reassuring with the fusebox turned off, then, I was starting to worry!

wrf12345 said:

Thanks for the illustration, it does not have the electricity co isolator switch but the earthing looks about right and all the neutral wires come off the same terminal in the fusebox, worried it is going to trip the new fusebox, although have not had any problems with the current wiring.

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Ahead of any consume unit change, especially if going from old to new, the electrician will conduct some tests with a much more sophisticated test instrument to minimise this risk. The idea is that before he starts tearing everything out he makes sure that it's going to work afterwards!
Having checked the fixed wiring is ok, the other possibility that can cause issues afterwards is fixed equipment, e.g. immersion heaters, cookers, showers, as sometimes they still work but the elements have deteriorated enough to cause a little earth leakage. In this situation the new consumer units notice that some electricity is going the wrong way and will disconnect the supply out of concern it's going through you! This is comparatively rare though.
Hope that helps.

Here are some diagrams showing the 3 commonly used earthing systems in the UK, shamelessly stolen from the "IET Wiring Matters | Autumn 11" article elsewhere. Here PE = "protective earth" and PEN = "protective earth neutral" and the 'T' part in the designation comes from 'terre', the French for earth.

It was in some other article that Lucian pointed out it is the connection somewhere between the Earth and one of the current-carrying conductors that defines it as neutral.

So when energised you would expect to see a low resistance between N & E, but that should always be on the supply side of the installation. It is not permitted to have N & E common in an installation in the UK (known as a TN-C supply, seen in some older European places) and for any RCD device it would see that as a fault.

So ultimately you are right that the N & E needs to be checked for insulation quality as part of an upgrade, just as L & E/N needs checking, but that would be done by any competent electrician before energising the modified system at 250V or 500V DC to be sure all is OK.

For interest only (earth to neutral fault), I have a 1972 install still with the old fuse box install, a 30 ma RCD was installed many years ago with whole house protection and worked perfectly for years without any nuisance tripping. A 9 kw shower is separately protected with a 40A/30ma RCBO. One day, about 10 years ago the RCD kept tripping for approx 3 hours, I found a full E to N fault, ("0" ohms with a multimeter), the shower still had power (as expected) during this period. After 3 hours the fault disappeared, never again to return, I often think about this as I really did nothing to track it down. My system is TNCS.

johntheo said:

One day, about 10 years ago the RCD kept tripping for approx 3 hours, I found a full E to N fault, ("0" ohms with a multimeter),

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To be honest I think this might be a case of 2+2=5
You had an Earth leakage fault for 3 hours, and during that time you measured 0 ohms between neutral and earth.
If you repeat the measurement today with the same meter you will still find the 0 ohms as Neutral is joined to earth in the suppliers cutout. A more sensitive test instrument would show that there is resistance present of fractions of an ohm.
I think this measurement was a red herring regarding the temporary fault.