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Topic Title: Buried supply to a shed/garage
Topic Summary: List of requirements
Created On: 11 May 2009 02:57 PM
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 01 June 2009 03:18 PM
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AJJewsbury

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Hang on - was this the post you're referring to?

Extraneous parts - yes, metal clad switches (I assume that the cladding is not considered part of the installation).


"cladding" - I'd taken this to mean the metal cladding of the switches (definitive vocabulary notwithstanding) - are you suggesting that the building itself is metal clad?

- Andy.
 01 June 2009 03:39 PM
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spinlondon

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Originally posted by: OMS
I think you'll find that I pointed out to the OP that the switches etc were not extraneous conductive parts.

I know, I do read the posts. I notice that you did not respond to the OP's statement. "(I assume that the cladding is not considered part of the installation)".

There is no reason not to export a PME earth to a building just that if that building does contain extraneous conductive parts they need to be main equipotentially bonded - in this case in minimum 10mm of copper or other metals offering equivalent conductance - essentially the basis of our discussion of the last 200 posts Spin

As I have said before, unless there is a reliable path to earth for those extraneous-conductive-parts, there is a possibility of introducing a difference in earth potential between those extraneous-conductive-parts and the PME earth.
This difference in earth potential could possibly prove fatal in the event of damage occuring to the supply neutral.
I think I have established my view regarding copper equivelance in the 200 posts. To date there has been no evidence produced that would make me want to alter it.


Regards



OMS


 01 June 2009 04:00 PM
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OMS

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I know, I do read the posts. I notice that you did not respond to the OP's statement. "(I assume that the cladding is not considered part of the installation)".


Well, like Andy, I assumed the cladding was part of "metal clad" ie the OP was talking about metallic accessories and containment etc. It's not clear to me that this garage does actually contain any extraneous conductive parts just that if it does and the OP exports the PME earth he needs to be cognisant of the requirements of bonding - hence my initial comments about under engineering the solution

As I have said before, unless there is a reliable path to earth for those extraneous-conductive-parts, there is a possibility of introducing a difference in earth potential between those extraneous-conductive-parts and the PME earth.


Spin, for them to be extraneous conductive parts they would generally be at earth potential wouldn't they - that's why we bond them.

This difference in earth potential could possibly prove fatal in the event of damage occuring to the supply neutral.


Hold on Spin - we bond the extraneous conductive parts to minimiuse touch voltages, BS 7671 doesn't require us to take action to prevent a broken PEN, that is the function of ESQCR.

Put in perspective, a broken supply neutral alone is a fairly rare occurance and even if it were to occur the potential difference between bonded metal work and true earth would be at most several 10's of volts if the DNO have installed the required electrodes on thier PME distributor - hardly instantlly fatal is it if you consider even BS 7671 alows a voltage of 50V to persist without risk.

I think I have established my view regarding copper equivelance in the 200 posts. To date there has been no evidence produced that would make me want to alter it.


So it's still K1/K2 for you then Spin - did you actually read 5.2.1 in Guidance Note 8.

Regards

OMS

-------------------------
Failure is always an option
 01 June 2009 04:43 PM
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spinlondon

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One of the problems of not reading posts etc. and making assumptions, is that you don't get a clear picture of what is going on.
Do we bond to minimise touch voltages or to provide a sufficient path to earth for fault currents? If it is just to minimise touch voltage, why then is such a large CSA required?
I am not suggesting that BS7671 requires us to take action to prevent a broken PEN.
However I do believe that BS7671 requires us to take into consideration the effects that a broken PEN would entail. Hence the requirement for 10mm² main bonding.
The potential differnce to earth would depend upon where the break to the PEN occurred. Making an assumption that the touch voltage would be < 50V would not be a problem within the equipotential zone.
However making that assumption could IMHO prove fatal when the PME earth has been exported.
Yes it still is.
I do not posses GN8.
I am a little suprised that you advocate the use of the GNs, considering your stated opposition to at least one of their authors views.
 01 June 2009 05:07 PM
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OMS

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One of the problems of not reading posts etc. and making assumptions, is that you don't get a clear picture of what is going on.


Really

Do we bond to minimise touch voltages or to provide a sufficient path to earth for fault currents? If it is just to minimise touch voltage, why then is such a large CSA required?


Are you asking a question here Spin - bonding has nothing to do with providing a fault path for earth fault currents - that's the function of the earth fault loop - you will notice that the bonding conductor doesn't form part of that loop - essentially that would be earthing not bonding.

Bonding is established to reduce potentials in a zone of protection.

The large CSA is there primarily to ensure a robust conductor is used that is likley to stay connected - the sizing is based on input from the supply industry who would have taken into account the impact of a broken PEN and advised IEE accordingly.

I am not suggesting that BS7671 requires us to take action to prevent a broken PEN.


As I said, the supply industry have generally done that for us - hence the process of selection rather than calculation

However I do believe that BS7671 requires us to take into consideration the effects that a broken PEN would entail. Hence the requirement for 10mm² main bonding.


See Above

The potential differnce to earth would depend upon where the break to the PEN occurred. Making an assumption that the touch voltage would be < 50V would not be a problem within the equipotential zone.


It would generally depend on where the electrodes were positioned in relation to that break.


However making that assumption could IMHO prove fatal when the PME earth has been exported.


I see - so why was your initial post to this thread :

If you are going to export the Earth, then you will need 10mm² SWA. However, if you TT the Garage, you could probably get away with 2.5mm². Do the cable calcs. for volt drop etc.
Bond the SWA at the house, but don't bond at the Garage.
Run a bond from the Earth bar in the Garage CU to a MET along with a bond from the cladding. Then from the MET to an Earth rod.
The Garage CU should have a main switch, and either two RCBOs or one RCBO and one MCB for the lights.


I can only presume you have had a road to damascus moment over the last 10 pages or so.

Yes it still is.

I do not posses GN8.


Ahh well - here is an extract:

"For a non copper main equipotential bonding conductor the "equivalent conductance" requirement will be met if the CSA(Sm) is not less than that given by:

Sm => Sc x (Rho M/Rho C)

Where Sm is the minimum CSA for the main bonding conductor (in a metal other than copper)

Sc is the minimum required for a copper conductor
Rho M is the resistivity of the metal from which the bonding conductor is made
Rho C is the resistivity of copper

For a steel main equipotential bonding conductor the ratio (Rho m/Rho C) is approximately 8.

You will recall I advocated a ratio of 9:1 in the absence of any further information as a conservative approach. I believe you are advocating about 2.5:1

I am a little suprised that you advocate the use of the GNs, considering your stated opposition to at least one of their authors views.


And your point is ?

OMS

Edit to correct formula above

-------------------------
Failure is always an option

Edited: 02 June 2009 at 10:16 AM by OMS
 01 June 2009 11:22 PM
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spinlondon

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Originally posted by: OMS
One of the problems of not reading posts etc. and making assumptions, is that you don't get a clear picture of what is going on.


Really

Yes really.

Do we bond to minimise touch voltages or to provide a sufficient path to earth for fault currents? If it is just to minimise touch voltage, why then is such a large CSA required?


Are you asking a question here Spin - bonding has nothing to do with providing a fault path for earth fault currents - that's the function of the earth fault loop - you will notice that the bonding conductor doesn't form part of that loop - essentially that would be earthing not bonding.

Bonding is established to reduce potentials in a zone of protection.

Not necessarily so. Supplementary bonding is required for ADS when the CSA of the CPC is not large enough.

The large CSA is there primarily to ensure a robust conductor is used that is likley to stay connected - the sizing is based on input from the supply industry who would have taken into account the impact of a broken PEN and advised IEE accordingly.

If the primary requirement is just for a robust conductor, what does it matter if the conductor is copper, aluminium or steel?

I am not suggesting that BS7671 requires us to take action to prevent a broken PEN.


As I said, the supply industry have generally done that for us - hence the process of selection rather than calculation

So why are you suggesting that I am stating that we are required to take action in the event of a broken PEN?

However I do believe that BS7671 requires us to take into consideration the effects that a broken PEN would entail. Hence the requirement for 10mm² main bonding.


See Above

Your point being?

The potential differnce to earth would depend upon where the break to the PEN occurred. Making an assumption that the touch voltage would be < 50V would not be a problem within the equipotential zone.


It would generally depend on where the electrodes were positioned in relation to that break.

Yes, and if the break was between the electrodes and the installation...



However making that assumption could IMHO prove fatal when the PME earth has been exported.



I see - so why was your initial post to this thread :

If you are going to export the Earth, then you will need 10mm² SWA. However, if you TT the Garage, you could probably get away with 2.5mm². Do the cable calcs. for volt drop etc.
Bond the SWA at the house, but don't bond at the Garage.
Run a bond from the Earth bar in the Garage CU to a MET along with a bond from the cladding. Then from the MET to an Earth rod.
The Garage CU should have a main switch, and either two RCBOs or one RCBO and one MCB for the lights.


I made this post because I considered it to be the wisest choice.

I can only presume you have had a road to damascus moment over the last 10 pages or so.

Is that near Walthamstow?

Yes it still is.

I do not posses GN8.



Ahh well - here is an extract:

"For a non copper main equipotential bonding conductor the "equivalent conductance" requirement will be met if the CSA(Sm) is not less than that given by:

Sm => Sc x (Rho C/Rho C)

According to this calculation, a ratio of 1:1 will suffice.

Where Sm is the minimum CSA for the main bonding conductor (in a metal other than copper)

Sc is the minimum required for a copper conductor

Rho M is the resistivity of the metal from which the bonding conductor is made

Rho C is the resistivity of copper

For a steel main equipotential bonding conductor the ratio (Rho m/Rho C) is approximately 8.

You will recall I advocated a ratio of 9:1 in the absence of any further information as a conservative approach. I believe you are advocating about 2.5:1

I am a little suprised that you advocate the use of the GNs, considering your stated opposition to at least one of their authors views.


And your point is ?

You want me to accept the views put forward in a GN, where by your own admission you are at odds with the views of an author of one of the GNs.

OMS


 01 June 2009 11:30 PM
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spinlondon

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Originally posted by: AJJewsbury
Hang on - was this the post you're referring to?

Extraneous parts - yes, metal clad switches (I assume that the cladding is not considered part of the installation).


"cladding" - I'd taken this to mean the metal cladding of the switches (definitive vocabulary notwithstanding) - are you suggesting that the building itself is metal clad?

That would be my reading of the post.
I suppose in view of the fact that there is no other reference to metal cladding, the OP could have been refering to the metal clad switches.
Why he would consider them not to be part of the installation, I do not know?
Perhaps I should take my own advice and read the posts more carefully?


- Andy.


 02 June 2009 10:15 AM
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OMS

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Not necessarily so. Supplementary bonding is required for ADS when the CSA of the CPC is not large enough.


Mmmm - supplementary bonding is required primarily to re establish the "equipotential" reference in order to further minimise touch voltages between exposed and extraneous conductive parts when under fault conditions.

If the primary requirement is just for a robust conductor, what does it matter if the conductor is copper, aluminium or steel?


Well the short answer is BS 7671 tells us there is a difference between the conductor materials - and has done for many a long year.


Yes, and if the break was between the electrodes and the installation...


Well we would need to have specific information to put a voltage against that one Spin - suffice to say that if the distributors main complies with the statutory requirements for PME then worst case would be several 10's of volts.

According to this calculation, a ratio of 1:1 will suffice.


Correct - my dodgy typing I'm afraid. Here it is again (and edited above)

Sm => Sc x (Rho M/Rho C)

Where Sm is the minimum CSA for the main bonding conductor (in a metal other than copper)

Sc is the minimum required for a copper conductor

Rho M is the resistivity of the metal from which the bonding conductor is made

Rho C is the resistivity of copper

Plug in a few values and see what you get

So why are you suggesting that I am stating that we are required to take action in the event of a broken PEN?


Because you stated:

However I do believe that BS7671 requires us to take into consideration the effects that a broken PEN would entail.


Your point being?


That the effect of a broken PEN suffered by the supply side impacts on the consumer side so supply industry has implemented two requirements:

1 - to use a mandated minimum conductor size

2 - to liase with the distributor to ensure that the cable selected above meets local network conditions.

You might have noticed that representatives of the supply industry have sat on the JPEL 64 committee for a number of years


You want me to accept the views put forward in a GN, where by your own admission you are at odds with the views of an author of one of the GNs.


I don't want you to accept anything Spin - I'm just pointing out that what could be considered to be authoratative guidance tends towards my view rather than yours.

Regards

OMS

-------------------------
Failure is always an option
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