Earthing Arrangement

[JohnW2]

Indeed, they were usually fitted where a TT earth existed and in older (pre 60's) installations where the earthing was to a buried service.

Thanks - but to reiterate my question, can we assume that the DNO would not have fitted an RCD (indeed, may well have not allowed the supply to remain connected) after 1966 if there were no earth other than the buried service pipes?

I have also seen them used where we have a PNB service. (which can look similar to a TT installation)

Ah, education time - wotz dat?

Kind Regards, John.

 

[crozzer99]

Thanks for all your replys.

Yes the areas is a bit busy/messy. I think i will give my supplier a call and explain whats going on with the earthing and regaring the RCD and see where we go.

Holmslaw - unfortunatley its not a private supply so plastic will no doubt exist somewhere down the line.

Will let you know how i get on with the phone call.

 

[westie101]

Just spotted

the 'service head' (for want of a better term!) looks all a bit 'messy' to me

Messy is too mild a word, I'd be rather unimpressed if one of our guys did that!

can we assume that the DNO would not have fitted an RCD

Apologies I should have made it clear they were usually fitted by what used to be The Electricity Boards.

Ah, education time - wotz dat?

Used only in one situation in rural areas where a pole mounted transformer feeds one customer only via a single service position.
In this the neutral is earthed at the service position to an earth rod supplied and maintained by the DNO, As a method of earthing it is still permitted in the above situation. (so it looks like TT but the neutral and earth are connected at the main fuse).
We quite often find this earth has been cut or removed by some electricians as they don't quite know what it is!
It does have some advantages in agricultural situations and is specifically mentioned in our advice (CoP) for PME

 

[JohnW2]

Yes the areas is a bit busy/messy. I think i will give my supplier a call and explain whats going on with the earthing and regaring the RCD and see where we go.

Good luck. I think it can be quite a time-consuming mission finding anyone who has a clue what you're talking about, unless you're wanting to pay them some money!!

Kind Regards, John.

 

[westie101]

I think i will give my supplier a call and explain whats going on with the earthing and regaring the RCD and see where we go.

They will be of no use, call your DNO instead

http://2010.energynetworks.org/electricity-distribution-map/

Who can sort the earthing, tidy things up and probably own the RCD

 

[JohnW2]

Messy is too mild a word, I'd be rather unimpressed if one of our guys did that!

Quite - another of those meetings with no coffee or biccies, I suspect!

Apologies I should have made it clear they were usually fitted by what used to be The Electricity Boards.

Ah, OK - but presumably they would also have hicupped if there was no compliant earth provision?

Used only in one situation in rural areas where a pole mounted transformer feeds one customer only via a single service position. In this the neutral is earthed at the service position to an earth rod supplied and maintained by the DNO, As a method of earthing it is still permitted in the above situation. (so it looks like TT but the neutral and earth are connected at the main fuse).
We quite often find this earth has been cut or removed by some electricians as they don't quite know what it is!

Many thanks - and just to complete my education, what does 'PMB' actually stand for?

Kind Regards, John.

 

[westie101]

what does 'PMB' actually stand for?

(you've got my problem, keep hitting wrong keys )

PNB = Protective Neutral Bonding

 

[ricicle]

PNB = Protective Neutral Bonding

It is not only limited to pole mounted Tx's as the college where I work is PNB. The N-E link is inside our switchgear. I will get a photo on here if I get chance.
Although classed as TNC-S it's amazing how many people automatically class it as PME....

 

[westie101]

The N-E link is inside our switchgear

On ground mounted equipment that is often the only place it can be connected if there is no LV equipment on the DNO side (the link cannot be made at the transformer), I know of a lot of sites like this and would not class them as PNB.

 

[JohnW2]

PNB = Protective Neutral Bonding

Thanks, and apologies - just my disobedient typing fingers again!

It is not only limited to pole mounted Tx's as the college where I work is PNB. The N-E link is inside our switchgear. I will get a photo on here if I get chance. Although classed as TNC-S it's amazing how many people automatically class it as PME....

I don't really understand. What is the 'E' than the 'N' links to inside your switchgear? If, as westie described, it's a local DNO-provided earth electrode, that surely couldn't be TN-C-S, could it, since there's no '-C' anywhere?

I must say that I would have thought that the PNB westie described is, in engineering terms, simply TT - the only difference from 'normal TT' being the ownership of the earth electrode and a very slight difference in where the earthing conductor connected to the installation.

Kind Regards, John.

 

[westie101]

simply TT - the only difference from 'normal TT' being the ownership of the earth electrode and a very slight difference in where the earthing conductor connected to the installation.

The difference is that with TT the earth electrode only connects to the earthing conductors in the installation. In PNB it connects to the DNO neutral and the earthing conductors in the installation.
It forms the only point that the neutral is connected to earth

 

[westie101]

What is the 'E' than the 'N' links to inside your switchgear? If, as westie described, it's a local DNO-provided earth electrode, that surely couldn't be TN-C-S, could it, since there's no '-C' anywhere?

Yes it is the DNO earth electrode or on a private network the customer's electrode. This connection would usually be made in the DNO fuseboard but on a lot of older sites the transformer feeds directly to the customer's switchgear often with the metering CT's in the same location.
It should actually be classed as TN-S as that is the only point the neutral is earthed (customers being prohibited from using PME on their own network)

 

[JohnW2]

The difference is that with TT the earth electrode only connects to the earthing conductors in the installation. In PNB it connects to the DNO neutral and the earthing conductors in the installation.

Of course - I was being silly - but I still don't see how it can be called as TN-C-S - since, as I said before, there ain't no '-C- anwhere!

Edit: Ah - I've just seen your second message and that you agree with me that it's TN-S.

Kind Regards, John.

 

[JohnW2]

Used only in one situation in rural areas where a pole mounted transformer feeds one customer only via a single service position. In this the neutral is earthed at the service position to an earth rod supplied and maintained by the DNO, As a method of earthing it is still permitted in the above situation. (so it looks like TT but the neutral and earth are connected at the main fuse).

Westie, what I forgot to ask you last night to confirm why that is done. Do I take it that the earth rod is just a way of achieving 'PME' (which would otherwise quite probably be non-existant with a transformer supplying only a single customer) - i.e. to protect the installation in the case of a fault in the neutral feed?

This also makes me realise that I probably wrote some nonsense last night - about there being 'no -C'. If the neutral is earthed at the transformer (I presume it is?), then there clearly is potentially a '-C', which means that if one derived the installation's earth from the neutral, it would, indeed, presumably be TN-C-S, and I would have thought that would remain the same with the additional ('PME'?) electrode also connected to the neutral-derived earth. Or am I getting even more confused?

Kind Regards, John.

 

[westie101]

Easiest to put it this way, All the connections to earth I am talking about are the sole connection for the transformer.

So in PNB with an overhead supply you could have a few spans of LV 2 wire overhead, an overhead service and the neutral connected to earth by a rod outside the customer's premises c connected at the cut-out.

In the case of the ground mounted transformer, there is no facility to make a neutral to earth connection at any of our transformers. Such connection is made at the "other end" of the neutral cable/busbar, be it in a DNO fuseboard or in some cases a customer's switchboard.

In neither case is it PME until another neutral/earth connection is made remote from these points.

In the case of PNB it can be converted to PME by the addition of a neutral earth connection at the transformer, but only if the customer's bonding is up to spec.