Electrical earthing question

[John Rumm]

Since it is a BS7671 requirement that main equipotential bonds be present on all installations, that is not something you can avoid doing generally.

[John Rumm]

One of the reasons for the "Multiple" in PME supplies. They are connected to (real) earth at many places along the route making a full disconnection of the PEN conductor from earth very unlikely.

[John Rumm]

Yup, that is correct for meters inside the building. If internal, then the connection should be within 600mm of the union on the outlet of the meter. If it is an external meter, then it should be within 600mm of the point of entry into the building. These are qualifies with a "where practical". The guidance also applies to isolation points and unions in addition to meters (when say in large installation with one central meter, but individual isolators into each dwelling)

544.1.2 applies.

[Brian]

Absolutely.

That said, there was a time poorer quality cables were used and some cases of corrosion have led to failures between the consumer a the ‘last’ bonding point.

[Mike Clarke]

Diverted Neutral Current. This relates to a break in the neutral conductor of your incoming supply, not the neutral within your house.

John Ward has a thorough explanation of the problem and its dangers at

and

[Scott]

Thanks. I shall watch with interest once I have watched the final episode of Sex Education.

[Vir Campestris]

Shouldn't a modern box have an RCD that trips when live and neutral currents mismatch?

Andy

[Andy Burns]

RCDs (each device typically covering half the circuits from a consumer unit) or RCBOs (one device per circuit from a consumer unit) only care about mismatch on the circuits that are downstream of the device. With a PEN fault is upstream of them on the supplier's side, hence not detected.

[John Rumm]

More than likely yes... However if you have a PME / TN-C-S supply the bond between earth and neutral in on the main cutout, before your CU.

So if the neutral current is displaced and flowing through the earth instead, the RCD in the CU will still see that as in balance since the current is not flowing in the installations earth - only the supply earth.

[nib]

The PEN-fault detector in my EV charger connection I think just measures live to local earth voltage, so will trip if enough current flows back to the supply through earth rather than neutral to cause the local earth to rise more than a few volts.

nib

[Scott]

Forgive my ignorance on the subject, but if the current is going down the brown wire then returning (or not) via the earth there must be a huge mismatch between the brown and the blue, which I thought was what an RCD detected?

[Andy Burns]

The fault isn't within the house circuits, the live/neutral currents there are balanced.

The problem (with a PEN fault) is that the neutral current having arrived at the neutral bar in the consumer unit, then leaves over the neutral meter tail, but "finds" it can't leave the house via the main incoming neutral as there's a break somewhere externally.

However the neutral is joined to the earth in the supplier's cutout, so it leaves via the earth, or anything metallic bonded to the main earth terminal, possibly a neighbour's pipes ...

None of this "finding any way via earth" is visible to the RCDs/RCBOs.

[Scott]

Thanks. I mostly get it now.

[John Rumm]

It would - but that imbalance would have to be on the load side of the RCD.

So for example pick up the cut end of a mower flex and make accidental contact with live. You get a shock since the current flows through you to earth bypassing neutral. That means the RCD will see the imbalance between current in live and neutral and trip.

In this case the property is supplied by a Protective Earth & Neutral (PEN) conductor. At the main cutout (i.e. before your electric meter and your consumer unit) it is split to provide a neutral connection and a main earth connection.

That main earth connection will also be connected via your main equipotential bonding to things like gas and water services.

Now someone goes and cuts the PEN conductor to your property. That would mean you have no neutral or earth to your property - and might expect everything to just stop working.

However because the PEN conductor is bonded to lots of other earthed things at your cutout, there may still be an indirect path for the neutral current connected to the CU. So as long as there is no earth fault on one of its circuits, the RCD will see balanced current in the L & N, and it is fat dumb and happy.

What it does not know, is that the neutral that is connected to the CU, is no longer the intended one, but is now being made by your water and gas mains. And probably the neutral connection to your neighbours, since there is likely a conductive path between their Neutral / Main Earth terminal to yours. (e.g. if you share a metal water pipe with a neighbour, and you both have main bonding to that water pipe, then you have a direct connection to their neutral).

[Scott]

On Sat, 28 Oct 2023 16:29:19 +0100, John Rumm [e-mail] wrote: [snip]

Thanks for the clear explanation. Does it matter if the earth on the ring main is connected to the kitchen sink or cold water pipe? Can the returning current make its way back through (or past) the meter into the 'inside' earth and into the kitchen sink? If the earth for the water pipe is inadequate, can you then get an electric shock from the cold tap?

[John Rumm]

No, that is fairly typical since the main water pipe will be connected to the main EQ bond, and the kitchen sink probably has a connection to that.

In normal circumstances the load will be between the L and N, and the RCD will be happy. If there is a fault, and some current flows to earth (and that does not matter if the it is the "proper" circuit earth, or something that is bonded to the main earth terminal, or just some independent local earth) the RCD would see an imbalance, and trip.

This is the concern with loss of the PEN conductor. Your loads were strung between L & N. If N is cut, in the worst case (where the N in the property is left unconnected to anything or "floating"), its voltage will rise to mains voltage. The appliances won't work - since the floating N can't sink or source any current - but it would be a shock risk if you could touch the earthed but now live thing, and an independent actually earthed thing at the same time.

That is why you have EQ bonding to create an equipotential zone. By joining all the things that could introduce a potential into the zone together, you limit the voltage difference, and hence reduce the shock risk. So in a case where the incoming cold water pipe is actually plastic for its underground run, the only metal conductive bits might be the pipes in your house, they would would also now rise to mains voltage.

However it would be the same mains voltage as that which your main earth has risen. So you could safely lean against an earthed (but now live) cooker, and touch the cold tap, since they would both be at the same voltage and hence no current would flow through you.

Now in many cases those EQ bonds will not only create an equipotential zone (i.e. what they are designed to do), they will also give you what is called fortuitous earthing (and by extension an accidental neutral as well since they are joined at the cutout).

How good that earthing will be can vary. If it were to just an earth rod, it could be tens or hundreds of ohms. If it were to a incoming metallic a cold water pipe or gas pipe, It might actually be a fairly decent earth - although the resistance will vary with soil conditions etc. In a town with close adjacent properties, then there is a fair chance those EQ bonds could actually be a pretty solid connection to the (still connected) PEN in the property next door (via your Neutral -> Main EQ bond -> Water pipe -> Neighbours Main EQ bond -> Their Neutral). This could even hide the fact that your PEN conductor was disconnected. Obviously this is less than ideal - especially since the main EQ bonds are unlikely to be more than 10mm^2, but your meter tails will often be

25mm^2. So under high load, that earth is going to get hot!

Only if it is not included in the bonding...

[SH]

Its all very well bonding a metal water pipe such as after the house's stop valve to earth,

for many years all hot, cold and heating pipes were done in copper.

There is now heavy use of PEX plastic pipe. given that plastic pipe cannot be bonded to earth, shoulding the metal kitchen sinks and the wall radiators be bonded to earth as they are common touch points for humans?

[The Natural Philosopher]

If they are not connected to any other metal object that might become live, why bother?

>

[John Rumm]

No, not as a general rule. The question you need to ask, is is the thing in question capable of introducing a potential into an equipotential zone...

So in the case of a house with TN-C-S earthing you consider the whole thing as one EQ zone - so anything that comes in from outside like services etc gets bonded.

The you move down to rooms - some of which may have supplementary bonding[1] because they are considered to be high shock risk environments (like bathrooms etc). So in a bathroom the metal pipes coming into it (be they water, drainage, or CH), the earth wire of any circuits that serve the room would get bonded because they could bring a potential into the room. However something like a stand alone metal bath, sink or a radiator with plastic plumbing would not need bonding since it can't introduce a potential into the zone.

[1] Note that supplementary bonding does not require a connection back to the main earth terminal - it just needs things to be connected together. (you may get a connection back to the main earth by virtue of a circuit earth, which will usually be included in the bonding)

[tony sayer]

Wot if your water main incomer and gas are plastic?...