Terminating SWA in Meter Box?

[JohnW2]

The gas bond goes from the black box MET to the service pipe, then to the consumers installation at the other side of the meter. ... The water and AN Other (I assume it goes to the bathroom) connect to the MET of the consumer unit, 4m away.

Is that technically allowed by the regs? Should there not be a single MET to which all MPB conductors are connected?

Kind Regards, John

 

[mfarrow]

I don't think there's a regulation that precludes this.

There's been discussions about multiple METs before.

 

[JohnW2]

I don't think there's a regulation that precludes this. There's been discussions about multiple METs before.

OK, I'll have to have a look- both at the regs and for these previous discussions. I have to say that I thought there had to be a single MET (in the sense of the place to which Earthing and MPB conductors were connected) - and I suppose it would make sense in terms of the aim of creating an equipotential zone. With multiple METs, there is the theoretically possibility of the link between them becoming (partially or completely) broken, thereby potentially compromising the equipotential zone.

It actually occurs to me that if multiple METs are allowed, that most of the discussions and arguments about whether joins in MPB conductors or non-continuous looping of MPB conductors between services is allowed (or even is ‘good practice’) might become rather moot – since one could define the join, or a branch point, as being an MET!

Kind Regards, John

 

[securespark]

What's the gas PEB doing on the service pipe?

Complying with 544.1.2 ?

Kind Regards, John.

That reg states that where there is a meter, it has to be on the consumer's pipework.

 

[securespark]

The gas bond goes from the black box MET to the service pipe, then to the consumers installation at the other side of the meter. ... The water and AN Other (I assume it goes to the bathroom) connect to the MET of the consumer unit, 4m away.

Is that technically allowed by the regs? Should there not be a single MET to which all MPB conductors are connected?

Kind Regards, John

AFAICT, it has always been allowable to use an MET external to the board and the earth bar in the board itself as an MET, thus this situation has always existed.

Even if you decided that you were going to group all your earth/ PEB connections in one place (eg inside the board), there would still be a risk, albeit a small one, of a loose connection creating the hazard which you describe.

 

[JohnW2]

Complying with 544.1.2 ?

That reg states that where there is a meter, it has to be on the consumer's pipework.

We've discussed this one at length in the past, and I agree that is one interpretation.

However, I would hope that most of us would agree that such a practice is not really correct in terms of the whole point of main bonding, and in some cases (like the one in the picture we're looking at) could be potentially very dangerous. In the situation we're looking at, if the gas meter did not provide electrical continuity and one attached the main bonding on the consumer's side of that meter, that would leave an unbonded extraneous-conductive part (the incoming gas pipe) just a few inches away from a metal adaptable box connected to installation's earth. That's a potentially dangerous situation and, indeed, the very situation that (proper!) main bonding is designed to prevent!

Even if one believes that 544.1.2 calls for bonding always to be on the consumer's side of the meter, the situation we're looking at in that picture is such that I would personally regard compliance with that regulation as being far too potentially dangerous - wouldn't you?

Kind Regards, John

 

[JohnW2]

AFAICT, it has always been allowable to use an MET external to the board and the earth bar in the board itself as an MET, thus this situation has always existed.

AFAIAA, it has always been allowable to use either of those as the MET. What I'm not so sure about is whether it's permissible to have MPB/PEB connections split between the two of them.

Even if you decided that you were going to group all your earth/ PEB connections in one place (eg inside the board), there would still be a risk, albeit a small one, of a loose connection creating the hazard which you describe.

Sure, any connection can come loose. However, it would not have surprised me if the regs required all MPB/PEB conductors to be connected to the same 'block'/bar, since that at least reduces the number of things whose coming loose could compromise the equipotential zone.

Kind Regards, John

 

[mfarrow]

To me (and I haven"t got my book with me so please excuse any terminology errors) an MET can exist at the end of any eathing conductor, be that the one between the cutout and the CU, or at any other point in between.

There are plenty of situations where you might have intermediate, or star-connected, METs. A peak and off-peak situation being on of them, or a switchfuse to an outbuilding. All perfectly legitimate.

 

[JohnW2]

To me (and I haven"t got my book with me so please excuse any terminology errors) an MET can exist at the end of any eathing conductor, be that the one between the cutout and the CU, or at any other point in between.

We may be into terminology issues already, since I thing there is only one 'earthing conductor', namely that which connects an earth electrode (TT) or DNO-supplied earth (TN) with 'the MET'. Perhaps more to the point, 524.4.1 says:

• In every installation a main earthing terminal shall be provided to connect the following to the earthing conductor:
  (i) The circuit protective conductors
  (ii) The protective bonding conductors
  (iii) Functional earthing conductors (if required)
  (iv) Lightning protective system bonding conductor, if any.

Taken literally, that would seem to call for a single MET.

There are plenty of situations where you might have intermediate, or star-connected, METs. A peak and off-peak situation being on of them, or a switchfuse to an outbuilding. All perfectly legitimate.

In my installation, with multiple distribution circuits and multiple CUs, there are 'earth blocks/bars' all over the place. However, there's only one which I regard as the MET, and that's the only one to which the earthing conductor and main bonding conductors are connected.

Kind Regards, John

 

[mfarrow]

You are very fortunate to be in that scenario then John. In many situations though, there are submains to different parts of the intallation which are many meters away. Taking the PBCs of these back to the origin would entail running extra green/yellow cables with the submain, which would in many cases be impracticable.

 

[JohnW2]

You are very fortunate to be in that scenario then John. In many situations though, there are submains to different parts of the intallation which are many meters away.

I'm not sure I understand. My submains are all many metres long, some of them tens of metres long.

Taking the PBCs of these back to the origin would entail running extra green/yellow cables with the submain, which would in many cases be impracticable.

If, as in my case, one is talking about a single dwelling, I'm not sure what PBCs you're talking about 'taking back to the origin'. Certainly in my case and, I would have thought, usually, the things needing main bonding will be relatively close to the 'origin' of the installation, and not in distant parts of the installation where the submains go (all on different floors, in my case). What sort of things were you thinking would need bonding in the vicinity of distant ends of submains?

What you are proposing effectively involves having at least some combined CPC/MPB conductors, and I'm not totally sure that's allowed.

Kind Regards, John

 

[mfarrow]

What sort of things were you thinking would need bonding in the vicinity of distant ends of submains?

- Water mains entering a summer house
- Steel frame of a garage
- Foundations of a lighting column

for example.

 

[JohnW2]

What sort of things were you thinking would need bonding in the vicinity of distant ends of submains?

- Water mains entering a summer house
- Steel frame of a garage
- Foundations of a lighting column ... for example.

Fair enough. I wasn't really thinking about 'outhouses' or things like lighting columns - but, rather, about distribution circuits within the main house/building!

It is, of course, when there are extraneous-c-ps in outhouses that many people advise not exporting the main installation's earth but, rather, locally TTing the outhouse (and bonding extraneous-c-ps to that) - at least partially because the alternative is to ensure an adequate MPB path back to the house's MET.

P.S. FWIW, I do run an MPB all the way back to the house's MET (albeit looped fromthe bonding of other services) from my (just about!) detached garage's frame, and my only other outhouse (a greehouse with a water supply) is locally TTd.

Kind Regards, John

 

[mfarrow]

The concerns about exporting earths to outbuildings is limited to PME, as I suspect you already know

Of course, taking each protective bond back to a single MET does remove a possible point of failure, but that's true of any installation, and of any connection, which is why one must be competent to know what type of joint to use in each situation, and how tight/secure it should be.

 

[JohnW2]

The concerns about exporting earths to outbuildings is limited to PME, as I suspect you already know

I wouldn't say 'limited to' PME. Even without PME, the other considerations, particularly for long runs, are the adequacy of the 'CPC' CSA for (a) achieving an acceptable Zs and/or (b) serving as a PMB conductor if there are extraneous-c-ps to be bonded in the outhouse.

Of course, taking each protective bond back to a single MET does remove a possible point of failure ...

Indeed - and that is why I wouldn't have been surprised if the regs did insist on a single MET (although I've yet to find evidence that such is the case!).

Kind Regards, John