TN-C-S earth bonding

[harveysmith]

Hi! Brand new poster. Just found your site and what a fascinating one it is - Well Done!

I've spent most of today browsing around and have learnt a lot. Many things to ponder over but here's one thing to start with;

Should my domestic TN-C-S system (you've taught me not only what that means but that there are possible alternatives as well) have an earth rod? It does and I'm not sure whether it should or not.

Mike G

Technically competent novice

 

[Jim2287]

TN-C-S should not require/have an earth rod, as the earth is supplied by the Electricity supplier, via the Neutral conductor (Protective Earth & Neutral combined)

If you have had a recent Periodic inspection carried out, then you should be able to make reference to External earth loop impedance (Ze) of your electrical installation, this should be lower than 0.35 Ohms.


Im sure you understand, as you have identified your system as TN-C-S, but to confirm .... there is a Green and Yellow cable going into the supply neutral from your "Fuse Board" or to an Earth Terminal block (and then to your Fuseboard)



however ..An earth rod is not an earth bond

You should have a 10mm2 green&yellow from the fuseboard/main earth terminal to your water supply and to your gas/oil supply (if applicable)

bonds should be made within 600mm of the services entering the building and on the consumer side of any stop cocks/meters.


bonding cables are only 10mm2 if your main earth conductor is 16mm2, which it most probably is for domestic premises (for a =<100 amp supply)

 

[nozspark]

Carrying on from the above


............although some electricians have and still do fit them to TN-C-S (PME) supplies as a safe guard in the event of a suppliers neutral failure.

 

[harveysmith]

Thanks for your prompt reply Jim2287. Yes there is the green/yellow earth wire coming out of an accessible terminal block which is an integral part of the 100A fuse box. Also, talking to a neighbour I get the impression that (before my time here) Norweb (ahh ... happy days) wrote to the village en masse about some "change involving earthing" and some people "got electricians in". My predecessors clearly didn't.

By the way, its an overhead rural line from small pole transformer, 3phase (5 wire) to my DP and then one of the phases to my house. This is why I have a pucka 3 phase supply to my garage! I wonder if they decided they couldn't guarantee the earth connection

 

[Spark123]

The sizing of the main equipotential bonds needs to be verified with the distribution network operator (United Utilities) for TN-CS, they may have additional requirements to those in the IEE regs.

 

[davy_owen_88]

TN-C-S is also known as PME (Protective Multiple Earthing) Having an earth rod still connected is not dangerous and was probably just left when the TT arrangement was converted. The rod will provide a backup should the neutral go open circuit but this is very unlikely as the neutral on a PME supply is earthed at regular intervals by the DNO, so if needs be, you can safely remove the electrode.

 

[harveysmith]

God I'm so slow at this typing!

Perhaps in light of your latter comments Im being a bit harsh. Still it's clearly time to get things checked properly.

Plastic water pipes and we can only dream of gas! and cable tv and +1Mb broadband and...

 

[harveysmith]

Many thanks to all.

 

[Spark123]

What led you to conclude it is TN-CS?

3 phase 5 wire makes me think it could possibly be TN-S.

 

[securespark]

PICTURES!!!

 

[harveysmith]

Spark123.

I perhaps overdid the country yokel response of my neighbours. I concluded that it had been TNS in the past as they felt that (without having a name for it) something was not being provided by the electricity board which had formerly been provided. This I took to be the separate earth and I thus concluded that bonding (a la PME) had to be provided by the consumer' hence the electricians. Probably a load of BS - what do you think?

By the way, who should I contact to find out or will an inspecting electrician be able to tell on site?

 

[Spark123]

If there are earth electrodes it was probably TT in the past, this is where the only form of earthing is by the earth rod (aka earth electrode). If you have an earth connection from UU then it will be either a TN-S or a TN-CS. An electrician should be able to tell what sort of earthing arrangement you have.

 

[harveysmith]

Securespark

If you only knew what that involved!! I will go away and attempt that but the CU + 100A fuse etc is 12 feet up in the air above a landing. After that I will have to get the snaps posted somehow. However, as you're showing such gratifying interest I'll risk life and limb.


Mike G

 

[Spark123]

How to post a pic:
//www.diynot.com/forums/viewtopic.php?t=73320

 

[securespark]

Securespark

If you only knew what that involved!! I will go away and attempt that but the CU + 100A fuse etc is 12 feet up in the air above a landing. After that I will have to get the snaps posted somehow. However, as you're showing such gratifying interest I'll risk life and limb.


Mike G

Then HTF do you get access to it in an emergency?