No main earthing terminal

[endecotp]

Dear Experts,

I have a TN-C-S system.
I don’t have a main earthing terminal; there is an earth wire from the supplier’s fuse to the consumer unit, and then wires from the CU to the water and gas supplies.
Is this allowed? It’s clearly equivalent in some sense, but I know that normally an earthing terminal outside the CU is used. What’s the story?

 

[EFLImpudence]

Yes. It is fine.

You do have an MET; it is in the CU.

 

[danechip]

As EFLI said, just fine. TT would be a different answer.

 

[JohnW2]

As EFLI said, just fine.

Indeed.

TT would be a different answer.

What 'different answer'? The MET can be in the CU with TT just as it can with TN.

Kind Regards, John

 

[endecotp]

Interesting you mention TT; I have a 17th ed on-site guide and its diagrams (in chapter 2) show external METs for TN-C-S and TN-S, and in-CU earthing for TT. It’s not clear if they’re trying to say that those are the required arrangements or just examples of what is allowed.

 

[sparkwright]

External MET where the supplier provides earth.

In-CU MET for TT where the consumer unit provides earth.

I suppose that sounds very reasonable thinking about it.

But that's not the way it happens though, is it - often the earth terminal is inside the main cut out, and the consumers electrician connects to this, usually having to break out a bit of plastic. Breaking the plastic out now is rare now, since the cut out installer breaks it out to do his Ze.

 

[JohnW2]

Interesting you mention TT; I have a 17th ed on-site guide and its diagrams (in chapter 2) show external METs for TN-C-S and TN-S, and in-CU earthing for TT. It’s not clear if they’re trying to say that those are the required arrangements or just examples of what is allowed.

Well, it wasn't me who introduced the concept of TT.

What you describe is common (probably 'the norm'), given that the DNO will usually connect their TN-C-S or TN-S earth (particularly the latter) to an 'external' MET, not the least because opening up a CU is beyond their scope, but there is absolutely no reason I can think of that the same cannot be done with a TT earth if one so wishes.

Kind Regards, John

 

[JohnW2]

External MET where the supplier provides earth. ... In-CU MET for TT where the consumer unit provides earth. ...

As I've said, that is 'the norm' but ...

... I suppose that sounds very reasonable thinking about it.

Well ... (a) the OP apparently has a TN-C-S earth connected to an in-CU MET, although goodness knows who connected it there, and (b) as I've just written, I see absolutely no reason why one can't have an external MET with TT (I have!).

Kind Regards, John

 

[EFLImpudence]

I have a 17th ed on-site guide and its diagrams (in chapter 2) show external METs for TN-C-S and TN-S, and in-CU earthing for TT. It’s not clear if they’re trying to say that those are the required arrangements or just examples of what is allowed.

That's the trouble with the OSG.

Beware of taking what it says as the way things should be done.

 

[SUNRAY]

i've just changed 2 CUs in identical flats, the main TN-C-S earth lead in the first was not long enough to reach the earth bar in the bigger CU so added an earth bar beside the cutout and terminated water and gas bonds there. Second property the main earth did reach so the additional earth bar wasn't needed.