Shared earth conductor

[JohnW2]

Does 543.1.1 not require any standalone 'protective conductor' (which I presume includes a SB conductor) to have a CSA of at least 4mm² unless it is 'mechanically protected'? ....

Also 544.2.1, 544.2.2 & 544.2.3 .....

544.2.1 A supplementary bonding conductor connecting two exposed-conductive-parts shall have
a conductance, if sheathed or otherwise provided with mechanical protection, not less than that of the smaller
protective conductor connected to the exposed-conductive-parts. If mechanical protection is not provided, its crosssectional
area shall be not less than 4 mm2.
544.2.2 A supplementary bonding conductor connecting an exposed-conductive-part to an extraneousconductive-
part shall have a conductance, if sheathed or otherwise provided with mechanical protection, not less
than half that of the protective conductor connected to the exposed-conductive-part. If mechanical protection is not
provided, its cross-sectional area shall be not less than 4 mm2.
544.2.3 A supplementary bonding conductor connecting two extraneous-conductive-parts shall have a crosssectional
area not less than 2.5 mm2 if sheathed or otherwise provided with mechanical protection or 4 mm2 if
mechanical protection is not provided, except that where one of the extraneous-conductive-parts is connected to an
exposed-conductive-part in compliance with Regulation 544.2.2, that regulation shall apply also to the conductor
connecting the two extraneous-conductive-parts

 

[JohnW2]

Indeed. Which is the point of supplemntary bonding, the supplementary bonding conductors are much shorter than the CPCs. So the 'touch voltage' between bonded objects is much lower than it would be with CPCs alone.

Indeed.

However, I think that folk probably tend to get more excited/concerned about SB (and when it is desirable/needed) than they probably need to be. For a start, for exposed-c-ps of things on the same circuit, the CPC paths between them (within a single room) will usually already pretty short. As for exposed-c-ps of things on different circuits (if there are any) and/or extraneous-c-ps (if there are any), there is really only a need to consider SB in relation to any 'simultaneously touchable' things, and in many rooms there will usually be few, if any, such pairs of things.

 

[SUNRAY]

What hasn't been asked yet is: The circuit feeding the CU has an CPC to cover all of the outgoing circuits, on that basis why should the outgoing circuits not be on a single CPC too?

 

[JohnW2]

What hasn't been asked yet is: The circuit feeding the CU has an CPC to cover all of the outgoing circuits, on that basis why should the outgoing circuits not be on a single CPC too?

Well, whilst 'in service' (and with everything intact) there would seem to be no reason (to not have a single CPC for all circuits). However, as I've said, when I raised this issue here some 14 years ago, the main 'anti' argument were that it would 'frustrate' subsequent testing of the individual final circuits.

I suppose there's also the question of the nature of the final circuits. If any were ring finals then, as I often say, I see one (quite probably the only one!) of the 'advantages' (over a radial) is the 'CPC redundancy' - something that would obviously be lost if there were only a single CPC for the final circuits.

 

[ebee]

Does 543.1.1 not require any standalone 'protective conductor' (which I presume includes a SB conductor) to have a CSA of at least 4mm² unless it is 'mechanically protected'? ....

View attachment 399676

Indeed yes

 

[JohnW2]

Indeed yes

Quite - but EFLI seemed to think (at least, that's what he implied) that such a regulation did not exist.

 

[EFLImpudence]

Where did I imply that?

 

[JohnW2]

Where did I imply that?

Apologies - looking back, it wasn't you - it was ebee himself, but he now seems to accept the truth! Sorry!

 

[ebee]

What hasn't been asked yet is: The circuit feeding the CU has an CPC to cover all of the outgoing circuits, on that basis why should the outgoing circuits not be on a single CPC too?

Yes indeed, and yes they could be if the designer 9and installer?) is happy about the resilience.
One such example could be one decent metal conduit with a multitude of individual conductors for individual circuits.
Is resliance assurred? is the conductor the correct size consider all scenarios.

 

[JohnW2]

Yes indeed, and yes they could be if the designer 9and installer?) is happy about the resilience. One such example could be one decent metal conduit with a multitude of individual conductors for individual circuits. Is resliance assurred? is the conductor the correct size consider all scenarios.

As I said, one of the main arguments against it seems to be that it to some extent 'frustrates' subsequent testing of individual final circuits. However, I don't think that 'frustration' is necessarily a reason for not doing t - and, as you point out, when the only CPC is metal conduit, one has no choice but to 'work with it'!