MPB - regs or common sense?

[ban-all-sheds]

If one possible interpretations leads to 'illogicalities and inconsistencies', then I agree that it's pretty reasonable to dismiss that interpretation, in favour of an interpretation which is more logical and not inconsistent with anything.

Can tell you're new here.....

 

[JohnW2]

If one possible interpretations leads to 'illogicalities and inconsistencies', then I agree that it's pretty reasonable to dismiss that interpretation, in favour of an interpretation which is more logical and not inconsistent with anything.

Can tell you're new here.....

Well, since I was merely agreeing with what you said, if it makes me new here, I guess you must be new as well

Kind Regards, John

 

[ban-all-sheds]

What I meant was you have yet to encounter the people who don't agree at all that it's reasonable to dismiss interpretations which lead to illogicalities and inconsistencies in favour of ones which are more logical and not inconsistent with anything....

 

[JohnW2]

What I meant was you have yet to encounter the people who don't agree at all that it's reasonable to dismiss interpretations which lead to illogicalities and inconsistencies in favour of ones which are more logical and not inconsistent with anything....

Ah, I see!! Maybe I am new here (true) and therefore haven't yet directly encountered such people in this forum, but I have spent more years than I care to remember dealing with such people in the wider world Our mutual acquaintance Mr Jobsworth is often a case in point!

Kind Regards, John

 

[DetlefSchmitz]

Bas: Are you going to show John the 'High integrity earthing and ring final circuits.' thread as an example?

 

[ban-all-sheds]

Ooh - I'd forgotten that one. I was thinking of the more recent malarkey about CUs vs DBs, and how the Building Regulations did not apply to you if you had more than 1 CU. Or was it that if you had more than 1 CU you had slipped through a crack in space to another dimension? Something like that.

 

[Paul_C]

Maybe, but well-drafted rules/regulations, standards, legislation, instructions or whatever should be as explicit and complete as possible and hence should seek to minimise the need for any interpretation.

{.....}

Having said that, it seems to me that the issue with BS7671 is often not so much that of alternative interpretations of what has been written but, rather, the need to make decisions about matters of detail when the document is silent on such details.

This is where the Wiring Regs./BS7671 have introduced more and more such instances of different interpretations and lack of detail from the completely new format of the 15th edition onward.

The 14th and earlier editions tended to be much more precise about what was needed to be in compliance.

 

[JohnW2]

This is where the Wiring Regs./BS7671 have introduced more and more such instances of different interpretations and lack of detail from the completely new format of the 15th edition onward.
The 14th and earlier editions tended to be much more precise about what was needed to be in compliance.

I wasn't aware of that (I'm not very familiar with anything prior to "16th"), but it doesn't really surprise me. There seems to have been a tendency in many fields for regulations etc. to move in that direction over the past couple of decades - often in the name of 'flexibility' (which I think is one characteristic which regulations should, in general, not have!).

I also personally feel that problems arise by having 'wiring regulations' which are nothing more than a subtitle to a British Standard. More-or-less by definition, Standards are prospective, relating to future practices (after the Standard has been published). Presumably at least partially as a result of this, the 'wiring regulations' seem to contain remarkably little guidance regarding the very important (and very real-world) issue of what 'old' practices (i.e. undertaken in compliance with 'earlier editions' of the regs) do, and do not, remain acceptable in existing installations, even though they would not be compliant in terms of current/future work. Given the very high proportions of installations which were at least partially 'wired' under earlier editions of the regs, this seems to be a rather unfortunate deficiency.

Kind Regards, John.

 

[holmslaw]

..

 

[Spark123]

so my response to his comments continued to relate spicifically to TN-S and TN-C-S systems. As I also wrote, the theoretical basis for sizing the MPB is far less clear (at least, to me) for a TT system. Basing it on the size of the earthing conductor is, I suppose, a start, but the size of the earthing conductor in a TT system is, itself, seemingly a pretty arbitrarily-decided animal.

The rules for MPB sizing are the same for TT and TN-S i.e. the MPBs must be at least half the size of the earthing conductor and not less than 6mm (Reg 544.1.1).
TN-CS is a bit different where they are in relation to the neutral of the supply and should be verified with the DNO.

 

[JohnW2]

.... As I also wrote, the theoretical basis for sizing the MPB is far less clear (at least, to me) for a TT system. Basing it on the size of the earthing conductor is, I suppose, a start, but the size of the earthing conductor in a TT system is, itself, seemingly a pretty arbitrarily-decided animal.

The rules for MPB sizing are the same for TT and TN-S i.e. the MPBs must be at least half the size of the earthing conductor and not less than 6mm (Reg 544.1.1).

Agreed - but, as I implied, I find the whole concept of MPB (let alone MPB cable size) in a TT system a little difficult to get my head around. If one accepts that it is incredibly unlikely that supply pipes would introduce a non-earth potential, the MPB is merely joining two 'earths' together. Even if there is an argument for having it (the only one I can think of being that vanishingly small probability that a supply pipe might introduce a non-earth potential), I still cannot think of a logical basis for trying to decide what cable size to use (e.g. for deciding that it should be at least half the size of the earthing conductor). Indeed, in that very unlikely situation of supply pipes and a TT earth electrode being at appreciably different potentials, the resultant current due to bonding would obviously flow through both the earthing electrode and MPB, in which case one might logically expect that they would be required to be the same size. Ah well

Kind REgards, John

 

[bernardgreen]

If one accepts that it is incredibly unlikely that supply pipes would introduce a non-earth potential, the MPB is merely joining two 'earths' together.

No it is not joining two earths together, It is joining an "earth" wire in the building to pipes outside the building that might be in contact with true ground.

The confusion arises because the word "earth" is used for what is no longer an "earth". The "earth" wire in a modern install is a protective circuit CPC and is derived in many cases from the incoming neutral. Hence it may not be at ground potential. Only in a modern TT install is the CPC true ground where it is connected to a rod driven into the ground. More confusion comes from the older TT installs where the CPC had to be isolated from ground if the voltage operated "earth" leakage breaker was to be effective as a safety device. It measured voltage between the CPC and the ground rod and if this became too high the trip operated. Connecting the CPC of that type of TT install to a metallic incoming supply pipe that was in contact with the ground would provide a short across the coil of the "earth" leakage breaker rendering it in-operative.

Even if there is an argument for having it (the only one I can think of being that vanishingly small probability that a supply pipe might introduce a non-earth potential),

It is the other way round. The supply pipe may introduce the true ground potential into a installation whose CPC ( "earth" wire " was not at ground potential

I still cannot think of a logical basis for trying to decide what cable size to use (e.g. for deciding that it should be at least half the size of the earthing conductor).

Where the CPC is derived from the neutral and the supply pipe is metallic and in good contact with ground then in the event of a network fault the current in the bonding cables could be very high as it forms an un-intentional but effectively part of the alternative route for current in the network neutral to return to the sub station.

 

[ban-all-sheds]

Somebody wrote a good book about this.

He tells me.

 

[securespark]

A continual bond would suggest that you cannot take the bonding conductor to a pipe clamp and then further down the line on that pipe add another clamp and lead onto another service.

This practice is acceptable for supplementary bonding, but not for PEB's.

 

[securespark]

Maybe, but well-drafted rules/regulations, standards, legislation, instructions or whatever should be as explicit and complete as possible and hence should seek to minimise the need for any interpretation.

{.....}

Having said that, it seems to me that the issue with BS7671 is often not so much that of alternative interpretations of what has been written but, rather, the need to make decisions about matters of detail when the document is silent on such details.

This is where the Wiring Regs./BS7671 have introduced more and more such instances of different interpretations and lack of detail from the completely new format of the 15th edition onward.

The 14th and earlier editions tended to be much more precise about what was needed to be in compliance.

That is indeed the case. The reason subsequent regulations were written in a different style was because they felt the 14th was too much like a set of instructions: Do this, do that etc....

The powers that be felt this made it too easy for any T, D or H to just pick up the book and do what it said, without understanding why.

The idea with future regulations was that you needed a certain knowledge before being able to interpret the good book.

This came from a conversation I had with a certain Mr Cook of the IEE a good few years ago.

BTW wasn't it the same man who wrote a paper on the very topic of all things green & yellow?

I know who'll have a link.... BAN!!! BAN!!!