Diverted neutral current (2025 Update)

Have a look at your incoming cable. If it has the earth wired to the sheath of the cable that's TNS. If the earth goes into the neutral terminal that's TNCS (or a PME sticker). If you have an outdoor earth rod that's TT.

Yes. Particularly required on TT where the earth impedance can be 200 ohms

- the fault current wouldn't trip an MCB but it would trip an RCD. If next door causes your earth to go live via the water pipe, your RCD won't help but your earth bonding should shunt much of the current to your real earth. On TT your neighbour should have an RCD that trips.

Theo

OK I believe that in reality there are only two systems in use, TT and TN-C-S. Whilst your system may appear to be TN-S I understand you should treat it as TN-C-S as any repairs to the distribution system may result in Earth and Neutral being combined.

On PME Neutral and Earth are combined .....

Dave

RCDs/RCBOs only care about an imbalance which is on your side of your consumer unit, they won't trip due to a fault on the supplier's side.

Details of the types and how to tell which you have here:

Indeed...

However (most) RCDs have no earth connection, so if it is the earth which is introducing a potential into your house, the RCD won't see it, and even if it tripped, it would not disconnect the earth.

(which is why we have equipotential bonding)

Bring me sunshine :-)

Note that an earth rod can also be added to a TN-C-S install - it just becomes another of the Multiple earths of PME.

If the neighbour has a disconnected neutral, but there is a path to your neutral via theirs and yours equipotential bonding, then their RCD may not see a fault since there will be a balance between live and what it is seeing as neutral.

Would it not be safer to have an isolated section on the incoming water pipe?

Possibly, but were do you stop? Gas pipe, oil pipe, building metalwork, that TT earth on an outbuilding submain etc. It is difficult to make sure you catch all possible connections to an unintended earth.

In article snipped-for-privacy@4ax.com, Scott snipped-for-privacy@gefion.myzen.co.uk> scribeth thus

Very simply an RCD looks at the current in on the Live wire and out on the neutral if there is an unbalance caused by the incoming live being diverted elsewhere then once its tripping rating is exceeded then out it goes and will disconnect Both lines so in effect your isolated.

Unless you share the same earth rod or wire with someone else then that can be a bit iffy;!.

Course on a more of less normal supply the earth and neutral are bonded at the substation ..

John.. Do any RCD's have an earth connection i thought it was only the very old e-RCD was it called?..

Yes, some do. They can use it to trip on live/neutral reversal and neutral disconnected. They can also use it to superimpose a low voltage on the neutral wire. I'm racking my brains to remember why, but it might have been to detect neutral/earth shorts even when no load is being drawn. It was a standard feature of one of the RCD ICs.

ELCB - Earth Leakage Circuit Breaker

I regret to say that in the seasonal spirit of Christmas I watched another 'JW' YouTube video last night and he actually said the best way to install a TT system for an existing property is to use a plastic water pipe. I think he also said that a plastic gas pipe can be used outdoors but not indoors.

Yup plastic gas pipes outside are common in the distribution system, and often right up to a premises these days, but less so after the meter on the consumer side. Trackpipe (plastic coated corrugated steel) is quite common though.

You quite often see it on some brands of RCBO for example:

Yup earth connection for those is the whole MO, a different approach to fault detection:

(oddly, those makers that produce them, seem rather shy on promoting the benefits / features)

Up to the premises, but *never* inside the boundary of a building including uninhabited spaces like garages. At least that is the case with many of the semis where I live. These have a pair of semi-detached garages in between each pair of houses but separated from the houses by a metre wide path. The gas meter is in the garage and the yellow plastic terminates under the concrete apron in front of the up/over door and then a run of 3/4 inch iron pipe runs under the garage floor to the meter. Another run of 3/4 inch iron runs from the meter, under the path into the house entering adjacent to where the Baxi Bermuda back boiler was originally fitted.

As built, my house was plastic from the main in the footpath, to "somewhere" in front of the house, then black iron rising in the cupboard under the stairs where the meter was.

When Cadent replaced the main in the street they found the junction between plastic and iron was under the built-on porch, so they had to re-site the meter, coming up outside and in through the wall of the porch.

Then the meter and along the skirting in the hall a couple of feet, back through the house wall into the garage along and then through the wall again to under the stairs and rejoin my pipe that used to be on the output side of the meter.

So externally it looks pretty clean, an 18" stub of white pipe up to a plastic elbow (concealing a stopcock) and out of sight, I need to build some sort of cupboard round the meter.

Not as bad as next doors, who have converted their garage, so it had to go up, over the porch, along the flat roof, along the whole length of the former garage, round to the back of the house ... looks awful!

Gas regulations! Plastic buried underground, is much less likely to be melted by the heat from a fire, and less of an issue, even it it did. A plastic gas pipe subjected to heat in a building, would be much more serious.

My house was built in the 70s, water pipe is plastic. Gas pipe appears as plastic coated steel. I have a substantial earthing system connected from my amateur radio aerials (10mm^2 and at least 10 earth roda and many buried wires). I'd like to convert to a TT system, but if I disconnect my supplied earth and check the earth loop impedence to my system it is about 6 ohms. Is this likely to be the earth bonding to the gas pipe (Unable to disconnect easily due multiple paths through boiler etc) to the earth next door. If so can I get teh gas people to fit an isolating section in the pipe?