House probably not earthed

[clifford1]

After living without accident in my house for 26 years, I am now becoming slightly worried because it appears that none of the electrics are earthed.

It is an isolated house supplied by overhead cables from a transformer in an adjoining field. The transformer is on a pole carrying cross-country power lines, which I believe are 500 volts. Higher than 230 anyway.

Two cables cross from the transformer to the house, attached at big insulators on a bracket on the corner of the house. At this point the non-live cable divides into two, one entering the house to become the neutral supply, the other supplies the common earth connection.

There are no other earth points in the house. There is no gas, no steel water pipes (private supply) and no earthing spike. The consumer unit is a traditional one with plug-in holders using fuse wire.

So it appears to me that "earth" as understood at a typical wall socket is just the same as neutral - both are connected by say 30 feet of cable to the same junction point outside the house.
Is this acceptable?

 

[chivers67]

No RCD at all? No earth rod on a TT? No in short.

 

[JohnW2]

Two cables cross from the transformer to the house, attached at big insulators on a bracket on the corner of the house. At this point the non-live cable divides into two, one entering the house to become the neutral supply, the other supplies the common earth connection.

That would be normal for a 'TN-C-S' supply (often called 'PME'), but there's no way we can be sure that's what you've got - unless there are some stickers talking about 'PME' or 'Protective Multiple Earthing' in the vicinity of the supply entry or consumer unit. If not, you'd have to check with the 'DNO' (Distribution Network Operator) who provide your supply.

There is no gas, no steel water pipes (private supply) and no earthing spike.

It may be a private water supply, but if the supply enters your house in any sort of metal piping, that needs to be 'bonded' (connected) to the 'earth' system of your electrical installation with a cable of at least 10mm².

Kind Regards, John

 

[ricicle]

Photos, photos, photos !!

 

[westie101]

PNB system!

 

[JohnW2]

Photos, photos, photos !!

Unless they reveal one of the PME stickers which I've already asked about, I'm not sure that photos are going to help much, are they?

Kind Regards, John.

 

[ban-all-sheds]

PNB system!

vs TN-C-S?

 

[westie101]

Won't be PME as the neutral/earth connection mentioned will be the only one, rather than multiple points

 

[JohnW2]

Won't be PME as the neutral/earth connection mentioned will be the only one, rather than multiple points

If the transformer is supplying only one property, if the neutral is connected to earth at the pole/transformer, is that not considered functionally similar to PME, even without the 'M'? In terms of the installation, is it meant to be treated as TN-C-S (which sounds as if what has been done), or should it be treated as TT? Indeed, if the householder added a TT electrode, would that put an 'M' back into PME?

Kind Regards, John.

 

[westie101]

The M says what it means!

Why would the cons need to convert it to TT by fitting an earth rod?

The CPCs should be connected to the neutral at the supply position as with PME (TN-C-S) bonding should match as well.

In other words what you said with the exception of the neutral/earth location

 

[JohnW2]

The M says what it means! Why would the cons need to convert it to TT by fitting an earth rod?

I wasn't talking about converting it to TT. rather, given that the M says what it means, I was asking whether addition of an additional earth electrode at the consumer end would provide the 'M' to render it 'PME' (functionally/literally it obviously would - so it's almost a question of regs and semantics).

The CPCs should be connected to the neutral at the supply position as with PME (TN-C-S) bonding should match as well.

To, to be sure, are you essentially agreeing with me when I wrote "...is that not considered functionally similar to PME, even without the 'M'?" - i.e. 'treat as PME' ?

Kind Regards, John

 

[EFLImpudence]

So it appears to me that "earth" as understood at a typical wall socket is just the same as neutral - both are connected by say 30 feet of cable to the same junction point outside the house.
Is this acceptable?

Yes, that is all the 'earthing' is.

There are other ways of doing it but perfectly normal

 

[clifford1]

Thank you very much for those replies.

There is no PME sticker.
There are no buried pipes entering the house (other than a plastic one from the well to the pump in the kitchen).

So the set up is perfectly normal for a rural supply to a single property?

 

[JohnW2]

So it appears to me that "earth" as understood at a typical wall socket is just the same as neutral - both are connected by say 30 feet of cable to the same junction point outside the house.

In your case, indeed - and that 'junction point' will presumably be connected to 'real earth' (via a spike/rod in the ground) at the nearby transformer. In the much more common (less isolated!) situation, in which the transformer supplied many properties, the neutral conductor would also be earthed at serveral locations as it was distributed to the various properties (hence the 'M' of 'PME') and the 'junction points' would exist separately within each of the supplied properties - but neutral and earth will still be joined, at those 'junction points', just as with your installation.

Kind Regards, John.

 

[westie101]

and that 'junction point' will presumably be connected to 'real earth' (via a spike/rod in the ground) at the nearby transformer

The earth connection referred to will be the only point the LV neutral is earthed (I am assuming at this point there is an earth rod).

On a power network at a transformer position two connections to earth have to be provided: -
1/ An HV earth that connects all steelwork and the transformer tank to earth, this is to enable the HV protection to operate, this must have a minimum value of 40 ohm
2/ An LV neutral earth, this must have a minimum value of 20 ohm.

However to reduce the risk of voltage appearing on the LV system from an HV fault (voltage gradient etc.) they MUST be separated by at least 10m.
So if you are building an overhead network, to excavate a minimum 10m trench at each transformer becomes an issue in a lot of ways. So to counter this, it is common and the norm to earth the neutral at a point remote from the transformer.

It is possible to use a single combined earth but this must have a value of below 1 ohm, which in a rural area can lead to huge amounts of excavation.

The above is why it is unlikely to be PME and more likely to be PNB when supplying a single customer.
(PME requires an earth at the transformer position, so in this case if the customer provided their own rod connected to the neutral it could still not be PME)