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The way I view that Regulation is that if you have an installation where the source of supply is PME then it cannot be applied. If I have remote building from such a supply and the requirements for main protective bonding is 25.0 then this applies throughout and as such any distribution cable must be able to support it.
If "except where PME conditions apply " means the same as 'except where the supply to the installation is PME"
then where is the size of bonding conductor defined within the regs?
It would appear to be totally wrong if the regs don't actually provide details for bonding size in the most common type of earthing arrangement used, yet do provide suitable guidance/rules for the less common versions.
For years most have used10mm^2, but if you do apply the "except where PME conditions apply " to mean 'except where the supply to the installation is PME" then there would be no justification for that, or any other size as both 54.1.1 and 54.8 both include the note that they do not apply where PME conditions apply.
Even using 25mm^2 wouldn't have any justification, especially as the temperature rise of CNE cable provides different loading compared to single core, hence DNOs have different cable ratings.
If the main supply type is a TNC-S (PME), then selection of main protective bonding conductor sizes is completed by reference to table 54.5 of BS7671.
To summarise this table, sizing is completed by reference to the size of the PEN (Protective Earth Neutral) conductor of the supply. PEN conductors of anything less than 35mm2 require a minimum of 10mm2 bonding conductors. PEN conductors between 35mm2 & 50mm2 require a 16mm2 bonding conductor to be used. The sizes of the required bonding conductors increase significantly as the size of the PEN conductor rises.
PME main bonding conductors are not subject the 25mm2 maximum requirement of any other supply type. The largest required bonding conductors on any PME supply is with an incoming supply PEN conductor of more than 150mm2 which requires a considerable 50mm2 bonding conductor to be installed!
That's exactly my point, if this table applies to PME (and other TN arrangements) then it does mean that "except where PME conditions apply " doesn't mean that just because the supply is PME that this exception applies.
To me it's simple
If the "except where PME conditions apply " means the same as 'except where the supply to the installation is PME" then there is no sizing provided for installations supplied via PME within the wiring regs. (Which doesn't make sense.)
On the other hand if "except where PME conditions apply " means exactly that, then you would use table 54.8/544.1.1 in their entirely for all TN supplies including PME.
If you were to deal with a "where PME conditions apply" situation, such as street furniture etc. direct from the utility supply, (i.e. before the CNE is split in the service head) then you would need to obtain that sizing from the utility - which is typically 35mm^2 for most applications, but perhaps 16mm^2 for street lighting and consumer services.
This latter aspect is not contained within the wiring regs.
Sorry I am not reading the way you are.
For pme it's not the c.s.a. of the meter tails that is used for Table 54.8, but the c.s.a. of the PEN conductor in the supply cable to the service head
For a supply on a standard cutout of 100A or less, the supply conductor will be 35mm or smaller. As such, the sizes for the earth conductor to the MET and onwards to the CU will be 16mm for a PME supply, and 10mm from the MET to where the gas / water enter (assuming the water is metallic). For a supply greater than 100A you should be asking the DNO
More than somewhat confused!
Has there just been a change in the relevant regs, or is the argument that we have been applying them incorrectly all along?
#32 shows the amended Reg see the blue book to compare.
That is the way I see it.
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But both 544.1.1 and 54.8 have the caveat "except where PME conditions apply"
That is the crux of the matter.
I take it to mean if the conductors are part of the PME system (i.e. the combined part) then table 54.8 and 10mm^2 or 6mm^2 etc do not apply, and no figures are specified within the wiring regs.
In all other cases - where the conductors have been separated (after the service head) then the conditions in table 54.8 and 544.1.1 are applicable.
If it did just simply mean "except for PME" then that would be the wording.
Why write the clause as particular to PME conditions, if they didn't mean where it is part of the PME (TN-C ) aspect.
Taking "except where PME conditions apply" to mean you can't use that table or regulation when dealing with part of the PME system fits the words, and the fact that the bonding size isn't specified.
Taking "except where PME conditions apply" to mean that the bonding size in the regulation and table isn't applicable to an installation supplied by a PME system begs the questions 1) what cable size should be used as 54.8 is "except where PME conditions apply" so what size for PME; and 2) why the "odd" wording, why not just say 'except for installations supplied from PME"?
I'm at a disadvantage here, as I'm hanging out for a breakfast roll which comes with a free Amendment 2 book, and so don't have the new book yet.
Trying to summarise, for my own benefit, and because this is getting highly confusing, and first looking at the blue book, we have:
-a paragraph about NOT PME, saying main bonding should be 50% of earthing conductor and bigger that 6mm
-a sentence about PME (except highway / street stuff), saying look at 54.8
-a sentence about more that one source of supply.
From the scan above, the new book has exactly the same structure, with one extra bit tacked onto the end of the first point.
The extra bit adds a bit more leeway for the size of bonding in submains.
Am I understanding correctly that the complex debate is whether the new bit applies to all installations, or only 'NOT PME' installations?
I'm not seeing the caveat for 54.8 in blue book or in the scan above - can you help me out?
Trying to summarise, for my own benefit, and because this is getting highly confusing, and first looking at the blue book, we have:
-a paragraph about NOT PME, saying main bonding should be 50% of earthing conductor and bigger that 6mm
-a sentence about PME (except highway / street stuff), saying look at 54.8
-a sentence about more that one source of supply.
From the scan above, the new book has exactly the same structure, with one extra bit tacked onto the end of the first point.
The extra bit adds a bit more leeway for the size of bonding in submains.
Am I understanding correctly that the complex debate is whether the new bit applies to all installations, or only 'NOT PME' installations?
I'm confused - in blue book you are referred to 54.8 specifically "where PME conditions apply"
I'm not seeing the caveat for 54.8 in blue book or in the scan above - can you help me out?
PME conditions is the nature of your supply and the condition is the earthing and main protective bonding requirements satisfy these conditions hence that Regulation does not apply.
I am confused also!
The heading for table 54.8 says 'minimum csa of the main protective bonding conductor in relation to the PEN conductor of the supply' so surely that is the csa of the DNO supply cable combined neutral earth, which as was mentioned previously in a domestic supply would be 35mm or less?
So in @cliffed s case above I would have thought 10mm or equivalent in steel, the combined swa of the cable and cpc would be sufficient to cover that? But then what is the supply connected to wasn't it an existing ring final, in which case the earthing conductor from there would only be 3mm and as such undersize?
My brain hurts!
Ps @mainline I think pages 55 and 56 osg do a better job.
The heading for table 54.8 says 'minimum csa of the main protective bonding conductor in relation to the PEN conductor of the supply' so surely that is the csa of the DNO supply cable combined neutral earth, which as was mentioned previously in a domestic supply would be 35mm or less?
So in @cliffed s case above I would have thought 10mm or equivalent in steel, the combined swa of the cable and cpc would be sufficient to cover that? But then what is the supply connected to wasn't it an existing ring final, in which case the earthing conductor from there would only be 3mm and as such undersize?
My brain hurts!
Ps @mainline I think pages 55 and 56 osg do a better job.
I think this is wandering slightly, from early on in the thread the interpretation of the new Regulation 544.1.1 is what is being brought into question.
The heading for table 54.8 says 'minimum csa of the main protective bonding conductor in relation to the PEN conductor of the supply' so surely that is the csa of the DNO supply cable combined neutral earth, which as was mentioned previously in a domestic supply would be 35mm or less?I am confused also!
The heading for table 54.8 says 'minimum csa of the main protective bonding conductor in relation to the PEN conductor of the supply' so surely that is the csa of the DNO supply cable combined neutral earth, which as was mentioned previously in a domestic supply would be 35mm or less?
So in @cliffed s case above I would have thought 10mm or equivalent in steel, the combined swa of the cable and cpc would be sufficient to cover that? But then what is the supply connected to wasn't it an existing ring final, in which case the earthing conductor from there would only be 3mm and as such undersize?
My brain hurts!
Ps @mainline I think pages 55 and 56 osg do a better job.
Correct
So in @cliffed s case above I would have thought 10mm or equivalent in steel, the combined swa of the cable and cpc would be sufficient to cover that?
It may very well do
But then what is the supply connected to wasn't it an existing ring final, in which case the earthing conductor from there would only be 3mm and as such undersize?
His problem is using the cpc as bonding to the greenhouse.
Page 55 blue osg is switching and page 56 is blank
Last edited:
It isn't a new Regulation it has been reworked so you need to compare both you will then understand the core debate here.
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