Bonding Gas Supply in an Outbuilding

[Leesparkykent]

Reg 542.1.3.3

Where a number of installations have separate earthing arrangements, any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow through them or be earthed within one installation only and insulated from the earthing arrangements of any other installation. In the latter circumstances, if the protective conductor forms part of the cable, the protective conductor shall be earthed only in the installation containing the associated protective device.

 

[spark 68]

To be honest this type of situation is one of the few times TT ing the far end is worth considering on economic grounds due to the cable sizes involved, probably not in domestic where the distances are fairly short, but certainly in cases where you may only want say a lamp in an outbuilding containing plant that may be some considerable distance from the main building.

 

[Deleted member 26818]

Has the type of supply been established?

 

[GEOFF S]

Then why the blanket statement ?

543.2.5 The metal covering including the sheath (bare or insulated) of a cable, in particular the sheath of a mineral insulated cable, trunking and ducting for electrical purposes and metal conduit, may be used as a protective conductor for the associated circuit, if it satisfies both requirements of items (i) and (ii) of reg 543.2.2

543.2.2 this reg is actually more about switchgear and bus bar trunking etc. but the salient points here are (i) and (ii)

(i) Its electrical continuity shall be assured, by construction or by suitable connection, in such a way as to be protected against mechanical, chemical or electrochemical deterioration.

(ii) Its cross sectional area shall be at least equal to that resulting from 543.1, or verified by test in accordance with the appropriate part of BSEN 61439 series.


What this is basically saying is that if the armouring is of insufficient csa to satisfy its use as a protective conductor, you cannot make up the difference with another undersized protective conductor, either the armouring has to comply on its own for its intended purpose, or the other protective conductor has to comply the same way. as an example of what would not be allowed, consider you need a CPC of say 10mm, and you calculate your armouring works out as a copper equivalent of 6mm, you cannot just add another CPC or core of 4mm to make up the difference, you would need the additional CPC or core to be 10mm on its own.

You can use both where for instance the armouring would comply as a CPC and a core for the MPB purposes, or vice versa.
This is one reason why you sometimes see a 10mm G/Y cable running along side an armoured cable.

Of course you can, you just need to calculate the current sharing between the two cpc.

Cheers

 

[Deleted member 26818]

Of course you can, you just need to calculate the current sharing between the two cpc.

Cheers

That's fine for determining the CSA of a CPC, but will it be suitable for determining whether the armour and core are acceptable as a bonding conductor?

 

[GEOFF S]

That's fine for determining the CSA of a CPC, but will it be suitable for determining whether the armour and core are acceptable as a bonding conductor?

I dont see why not, you will have all the required information to determine acceptance.


Cheers

 

[Deleted member 26818]

Well, if you use a 6mm 3 core SWA cable, the minimum armour CSA would have to be 36mm.
That's the CSA of the armour on a 6mm 4 core.
Might just as well use two of the 6mm cores instead.

 

[davesparks]

Well, if you use a 6mm 3 core SWA cable, the minimum armour CSA would have to be 36mm.
That's the CSA of the armour on a 6mm 4 core.
Might just as well use two of the 6mm cores instead.

Calculating the steel CSA required for a main bond is not the same as calculating the steel CSA required for a CPC. The requirement is that the steel CSA must give the same conductance as the required copper main bond. The calculation for working this out is given in guidance note 8 page 55, but it is basically the steel CSA must be greater than the product of the required copper CSA and the ratio of the resistivities of copper and steel.

The ratio of the resisitivities of copper and steel is given as 8.5, so the required minimum CSA of steel to be equivalent to a 10mm copper main bond is 8.5x10=85mm

That 6mm 4 core isn't looking so good really is it!

 

[Engineer54]

There is no way you can use/combine two different metals types to make up any shortfall in required minimum CSA/mm. I can't believe that someone is actually talking about calculating the fault current share between between copper and steel CPC's.


If neither the copper conductor or the Steel conductor has sufficient CSA (eg A SWA cable using a 3rd core as the CPC etc) singly/in it's own right to fulfill the requirements of it's CPC or CPC/Main Bonding Conductor. Then you CANNOT combine the two CSA's of the copper conductor and the steel armouring!! I'm quite amazed really, that any experienced electrician would even think that this was an acceptable solution!!

 

[GEOFF S]

There is no way you can use/combine two different metals types to make up any shortfall in required minimum CSA/mm. I can't believe that someone is actually talking about calculating the fault current share between between copper and steel CPC's.


If neither the copper conductor or the Steel conductor has sufficient CSA (eg A SWA cable using a 3rd core as the CPC etc) singly/in it's own right to fulfill the requirements of it's CPC or CPC/Main Bonding Conductor. Then you CANNOT combine the two CSA's of the copper conductor and the steel armouring!! I'm quite amazed really, that any experienced electrician would even think that this was an acceptable solution!!

It may smack of bad design, but there are no technical reasons not to do so, nor do the regs prevent it.

Calculating current sharing is well documented, and there is a specific guidance for SWA and a parallel Protective conductors.


Maybe you could give a technical reason for your opinion?

Cheers

 

[GEOFF S]

Calculating the steel CSA required for a main bond is not the same as calculating the steel CSA required for a CPC. The requirement is that the steel CSA must give the same conductance as the required copper main bond. The calculation for working this out is given in guidance note 8 page 55, but it is basically the steel CSA must be greater than the product of the required copper CSA and the ratio of the resistivities of copper and steel.

The ratio of the resisitivities of copper and steel is given as 8.5, so the required minimum CSA of steel to be equivalent to a 10mm copper main bond is 8.5x10=85mm

That 6mm 4 core isn't looking so good really is it!

Dave, thats one interpretation, not hte only one though! Read the first paragraph and note the full stop. So excluding PME, the csa need to be half that of the earthing conductor. Where does it state equivalent conductance to that of copper, what about steel, aluminium etc?

Then you have the second paragraph which states you need not exceed 25mm if Cu or a csa in another metal offereing equivalent conductance.


Cheers

 

[Engineer54]

It may smack of bad design, but there are no technical reasons not to do so, nor do the regs prevent it.

Calculating current sharing is well documented, and there is a specific guidance for SWA and a parallel Protective conductors.


Maybe you could give a technical reason for your opinion?

Cheers

Maybe, if both parallel conductors are of the SAME size and Same metal, but current sharing across two different sized conductors is a no, no!! ...And if then, they are made up of two different metals, Nah, bloody ridiculous!!


Perhaps you can post this specific guidance for SWA and parallel protective conductors... Because something is very wrong somewhere if it's supporting your criteria of a mix and match set up....

This is not MY opinion, just think about it for a nano second, what path is a fault current liable to take, given the choice of a copper or galv steel conductor?? Do you actually believe, the current will divide itself in accordance with any calculations?? lol!!

 

[Engineer54]

Dave, thats one interpretation, not hte only one though! Read the first paragraph and note the full stop. So excluding PME, the csa need to be half that of the earthing conductor. Where does it state equivalent conductance to that of copper, what about steel, aluminium etc?

Then you have the second paragraph which states you need not exceed 25mm if Cu or a csa in another metal offereing equivalent conductance.


Cheers

Hell's Bell's!!

It's an interpretation that just happens to be correct, not that, that should make any difference to you....

 

[GEOFF S]

Hell's Bell's!!

It's an interpretation that just happens to be correct, not that, that should make any difference to you....

Well, why dont we need to check the thermal constraints of a bonding conductor? Well it because the assumption is there will be little current flow, we just need to transfer potential, so if the assumption is little current flow, why do we need to consider conductance or inversely the resistance?

Now the GN Author, goes along with Daves interpretation, but many Engineers incuding some at the IET interpret this a little differently.

So, as i said if you could offer a technical response, that would be much appreciated.

Ive also PM you the report on SWA and parallel protevtive conductors, enjoy your read!

Cheers

 

[Engineer54]

Now the GN Author, goes along with Daves interpretation, but many Engineers incuding some at the IET interpret this a little differently.

Ive also PM you the report on SWA and parallel protevtive conductors, enjoy your read!

Cheers

Any of these many Engineers published anything on this other interpretation??

Already sent my reply on this report, which has been discussed here before...

 

[GEOFF S]

Any of these many Engineers published anything on this other interpretation??

Already sent my reply on this report, which has been discussed here before...

Not that im aware of, ill have a peruse.

With regard to the report, ERA are responsible for the current ratings in appendix 4 of BS7671, and as i said cable manufacturers also support the reports.

But you have yet to offer any technical supports for your stance.

Cheers

 

[Engineer54]

Not that im aware of, ill have a peruse.

With regard to the report, ERA are responsible for the current ratings in appendix 4 of BS7671, and as i said cable manufacturers also support the reports.

But you have yet to offer any technical supports for your stance.

Cheers

Strange then that manufacturers Current ratings are generally found to be different (generally higher) than those listed/tabulated in BS7671. In fact i've rarely used BS7671 current ratings for several years now, preferring to use for obvious reasons manufacturer's published ratings....

No real need for Tech support is there, surely common sense should be smacking you round the head, around about now??

 

[GEOFF S]

Strange then that manufacturers Current ratings are generally found to be different (generally higher) than those listed/tabulated in BS7671. In fact i've rarely used BS7671 current ratings for several years now, preferring to use for obvious reasons manufacturer's published ratings....

No real need for Tech support is there, surely common sense should be smacking you round the head, around about now??

Correct, manufactures use the ERA ratings, which have factors applied to them, if you apply the same conditions that the ERA set out to the BS7671 ratings you will get the same ratings.

They are responsible for most of appendix 4, grouping factors etc. No common senese required just good engineering.

Teh cable manufacturers, the IET support many of the ERA work, thats god enough for me.

As isaid if you can provide technical support for you stance tehn we can continue the debate, otherwise we will have to agree to disagree.

Cheers

 

[Engineer54]

Correct, manufactures use the ERA ratings, which have factors applied to them, if you apply the same conditions that the ERA set out to the BS7671 ratings you will get the same ratings.

They are responsible for most of appendix 4, grouping factors etc. No common senese required just good engineering.

Teh cable manufacturers, the IET support many of the ERA work, thats god enough for me.

As isaid if you can provide technical support for you stance tehn we can continue the debate, otherwise we will have to agree to disagree.

Cheers

Hey, if you wish to believe anything and everything that has been put in print, who am i to tell you that would be foolish!!

Manufactures use their own specifications and data, that includes current carrying ratings!! Do you think that the cable manufactures only supply the UK, or that their current ratings for a given cable type/size are based solely on or around ERA/British Standards?? Now that is being naive!!

 

[davesparks]

Dave, thats one interpretation, not hte only one though! Read the first paragraph and note the full stop. So excluding PME, the csa need to be half that of the earthing conductor. Where does it state equivalent conductance to that of copper, what about steel, aluminium etc?

Then you have the second paragraph which states you need not exceed 25mm if Cu or a csa in another metal offereing equivalent conductance.


Cheers

Why are you asking where equivalent conductance is mentioned when you go on to mention equivalent conductance yourself in you second paragraph.

I have no issue with their being other interpretations or however you wish to describe it. If you can provide links to the published documents/guidance which could be referenced if I did not follow the IETs guidance notes and I did have to defend my installation in court then I will consider using that guidance in my work.