TN-S Earthing

[Gardner]

Are earth rods or earth electrodes required for TN-S supply?

 

[123]

Are earth rods or earth electrodes required for TN-S supply?

If its a TN-S supply no.

 

[Gardner]

If its a TN-S supply no.

But we would bond the plumbing, and foundation. If no metal plumbing and no re-bar foundation is present, can earth electrodes still be omitted?

 

[PEG]

...They are...but you are not putting them in...:icon12: ,that honour goes to a higher power...

 

[Engineer54]

Generally no, but it's not unheard of. In the 50's all LCC (London County Council) properties many of which were in Kent, Essex Herts etc... were provided with either an auxiliary earth rod or lattice grid on TN-S supplies....

 

[Gardner]

But code minimum under BS7671 does not require them unless the supply is TT?

 

[Engineer54]

But we would bond the plumbing, and foundation. If no metal plumbing and no re-bar foundation is present, can earth electrodes still be omitted?

But you don't test Ze, in the UK and Europe it is required that the Ze value generally has to be known/measured. In the UK it is generally accepted that the max TN-S value to be 0.80 ohm, but in reality is much lower'

Oh and main bonding metal service pipes gas/water, are NOT considered as part of the main earthing system, main bonding serves a completely different purpose....

But code minimum under BS7671 does not require them unless the supply is TT?​

Were going back 60 years or so, at that time there was no BS 7671, it was i think then the 12th or 13th edition of the electrical Reg's. The above was basically a LCC requirement for it's properties... nothing more!!

 

[Gardner]

But you don't test Ze, in the UK and Europe it is required that the Ze value generally has to be known/measured. In the UK it is generally accepted that the max TN-S value to be 0.80 ohm, but in reality is much lower'

But with an RCD, that value can be higher? I dont know the value in this case, so an unknown value would necessitate RCD?

Oh and main bonding metal service pipes gas/water, are NOT considered as part of the main earthing system, main bonding serves a completely different purpose....

Can you give more detail, at least the theory behind it?

Were going back 60 years or so, at that time there was no BS 7671, it was i think then the 12th or 13th edition of the electrical Reg's. The above was basically a LCC requirement for it's properties... nothing more!!

Ok, that makes sense now. My mistake to assume.

 

[Knobhead]

Trying to reconcile the NEC section 500 with the UK’s BS7671 is just going to give you a headache.

The NEC recognises a metal water pipe as an electrode, BS7671 doesn’t.

The last time I got into this discussion was on an American site, I’m not going along that path again, it dragged on for ages.
Some thoughts they held about TT systems would make them hero’s to our Electrical Trainee’s. A high resistance ground was to limit fault currents???

 

[davesparks]

The NEC recognises a metal water pipe as an electrode, BS7671 doesn’t.

It does if certain conditions are met, including the size of the pipe being suitable and the owners permission being granted etc. So basically any metal water pipe on your land which you own could be used.

 

[DPG]

It does if certain conditions are met, including the size of the pipe being suitable and the owners permission being granted etc. So basically any metal water pipe on your land which you own could be used.

Interesting. Which reg mentions that a water pipe could be suitable as an earth electrode? Daz

 

[Engineer54]

Interesting. Which reg mentions that a water pipe could be suitable as an earth electrode? Daz

It's been on the books for decades now. The water supply system needs to be from a privately owned/derived system and permission granted from the owner. If the other criteria are met, the system can be used as a means of electrical earthing.
These systems though are as rare as hens teeth and would mainly be found in rural areas...

In all other situations, extraneous water pipes would be required to be ''main bonded'' to the installations MET and not relied on as a means of electrical earthing for the installation...!!

 

[Leesparkykent]

Interesting. Which reg mentions that a water pipe could be suitable as an earth electrode? Daz

I haven't got my BGB round me as on site, but think it could be 542.2.4 or thereabouts.

 

[DPG]

It's been on the books for decades now. The water supply system needs to be from a privately owned/derived system and permission granted from the owner. If the other criteria are met, the system can be used as a means of electrical earthing.
These systems though are as rare as hens teeth and would mainly be found in rural areas...

In all other situations, extraneous water pipes would be required to be ''main bonded'' to the installations MET and not relied on as a means of electrical earthing for the installation...!!

I just assumed it wasn't in the current regs. Cheers. Daz

 

[DPG]

I haven't got my BGB round me as on site, but think it could be 542.2.4 or thereabouts.

Yep, you're right! Didn't realise it was still in the regs. Ta. Daz