MPB - regs or common sense?

[EFLImpudence]

It's just wiser to have it unbroken.

Scenario: you have the main bonding coming from the MET to the water pipe and then a separate bonding cable on the same clamp goes to the gas supply

A plumber arrives to work on the water pipes, he disconnects the clamp and leaves both bonding cables dangling.
During the time it takes him to do his work none of the pipes in the house is bonded. Whereas if it were a continuous bond, they would be. Obviously the gas but probably the water, too, through the pipe connections.

Then when the plumber has finished he may put them both back on or he may have forgotten there were two.

 

[JohnD]

yep, good practice but not required by regulations

What was the question again?

 

[JohnW2]

IMO the "screw loose" problem is not so worrying if it happens at one of the ends, as if someone knots a few pieces of 10mm G&Y together and hides it under the floorboards (a builder once suggested that to me and was astonished when I fetched my crimping pliers)

Definitely agreed, but discussions about this 'unbroken MPB' business always seem to major on the issue of 'looping' the MPB between water and gas pipes.

Everyone seems agreed that, despite what one reads, the regulations appear to impose no insistance on 'unbroken' MPBs. What would be your view of a crimped or soldered join somewhere along the length of an MPB?

Kind Regards, John.

 

[JohnD]

I have always considered a sound permanent joint by crimping or brazing (I find it difficult to solder large cables) perfectly correct.

Doesn't everyone?

 

[1john]

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.

Large cables are easy to solder if they arn't on show .

 

[JohnW2]

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.

Indeed so, but if I understand correctly what you are saying, I don't think anyone has suggested that this would be acceptable - it would mean that the bonding path to the second service would include a length of the first service's pipe - and although that would be fine so long as everything remained intact, some Devil's Advocate is going to point out that someone could cut that length of pipe (and/or replace it with plastic) at some point in the future!

Large cables are easy to solder if they arn't on show .

Indeed. many things are 'easier' when not available for inspection .

Kind Regards, John.

 

[SgtTrojan]

Haven't you finished this job yet?!!

 

[JohnW2]

A metal pipe coming out of the ground is at a potential.
The earth coming into your house may be at a different potential.
If these are not bonded together then there may be a potential between them which would be dangerous.
That is why bonding is carried out, and that is why it is done as close to the point of entry to your property as possible.

Yes, agreed, and for a TN-S or TN-C-S system, it certainly makes sense to achieve as good (low impedance) as possible bonding to supply pipes as close as possible to their entry into the property; with such systems, it is extremely unlikely that the supply pipes would be introducing a potential other than earth potential, whereas the DNO's terminal could, under fault conditions, be at almost any potential - hence possibly very different from the earth potential introduced by the supply pipes. Two points arise:

Firstly, if one accepts that "it ... makes sense to achieve as good (low impedance) as possible bonding to supply pipes as close as possible to their entry into the property", then the sort of multiple connections to the supply pipes I was talking about could, in fact, be advantageous (i.e. theoretically safer). That is particularly true of an installation like mine. There is a 10mm² MPB cable travelling to a bonding point close to the entry point of the supply - and I accept that to be necessary just in case someone cuts the pipe or replaces it with plastic. However, the fact is that, in order to get to that bonding point, the MPB cable travels several metres alongside the (22mm) pipe to which it is ultimately bonded. Given that the effective copper csa of 22mm (0.9mm wall thickness) is around 30.5 mm² (i.e. much larger than the MPB cable), the impedance of the bonding would (so long as the pipework remains intact) be lower if the MPB was additionally bonded to the pipe at the first point the cable and pipe meet.

Secondly, I forgot to mention that I have a TT system, and I find more difficult to understand the concept of MPB with such a system. With such a system, it would require pretty extraordinary circumstances for either the supply pipes or the earth electrode/conductor to introduce any potential other than earth - which seems to make it less obvious why one needs to bond them together with a cable - but, even accepting that as a 'just in case' requirement, it is again not obvious (at least, not to me) as to the basis one would use if attempting to determine (from first principles) an appropriate sizing for the MPB conductor. I accept that TT systems often get changed to other systems - but if that happens, the whole bonding situatiion needs to be reviewed, anyway.

Kind Regards, John.

 

[JohnW2]

Haven't you finished this job yet?!!

It's not a 'job'. Rather, it's part of a discussion, across an evolving number of threads, regarding a critical review of an installation, and my attempts to gain an understanding of how people interpret the (often surprising non-explicit) regulations.

Kind Regards, John.

 

[ban-all-sheds]

They are often clearer than you think, if you take them as written rather than going in for deep analysis of how they might be "interpreted".

IMO a good rule is that if you can see two interpretations, and one results in illogicalities and inconsistencies, that's not the one to go for.

 

[bernardgreen]

whereas the DNO's terminal could, under fault conditions, be at almost any potential - hence possibly very different from the earth potential introduced by the supply pipes.

Which would bring the supply pipes up to the potential of the DNO's terminal and different from ground so creating a risk of electric shock when standing on the ground and touching the gas meter or an outside water pipe if there is a fault on the DNO's network. Or when there is a large unbalance on the three phases in the local network.

 

[JohnW2]

They are often clearer than you think, if you take them as written rather than going in for deep analysis of how they might be "interpreted".

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. Albeit in fields far divorced from electrical wiring, I have in my time been involved in the writing and/or reviewing of such documents, and I have always striven to work to that principle.

IMO a good rule is that if you can see two interpretations, and one results in illogicalities and inconsistencies, that's not the one to go for.

That's a very reasonable rule of thumb to apply if one finds oneself having to consider interpretations, but it's no panacea. 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. The problem arises when two or more interpretations are perfectly credible, probably depending upon individual opinions, and not illogical or inconsistent, but simply different.

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.

Kind Regards, John.

 

[JohnW2]

whereas the DNO's terminal could, under fault conditions, be at almost any potential - hence possibly very different from the earth potential introduced by the supply pipes.

Which would bring the supply pipes up to the potential of the DNO's terminal and different from ground so creating a risk of electric shock when standing on the ground and touching the gas meter or an outside water pipe if there is a fault on the DNO's network. Or when there is a large unbalance on the three phases in the local network.

Well, that surely 'depends' (on lots of things). If, as is far from impossible, the supply pipes provide a very low impedance path to true earth potential, then the DNO's fault may well not be capable of raising the potential of the pipes very far - and the situation would be of the pipes trying to pull the DNO's supply down to earth potential. How well it achieved that would be a function of (amongst other things) the impedance and current-carrying capacity of the bonding - hence, I presume, the requirement for the MPB to be sized in relation to the size of the DNO's supply cable.

However, I'm not quite sure what point you are making - are you trying to argue against MPB?

Kind Regards, John

 

[Spark123]

MPBs are not sized in relation to the DNO supply cable in a TT system, they are sized relative to the earthing conductor.

 

[JohnW2]

MPBs are not sized in relation to the DNO supply cable in a TT system, they are sized relative to the earthing conductor.

Exactly - and that's why I said in the earlier part of my sentence, only the latter part of which bernardgreen quoted:

and for a TN-S or TN-C-S system ..... with such systems .... the DNO's terminal could, under fault conditions, be ....

.. 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.

Kind Regards, John