Not at all - the armouring usually makes a very fine protective conductor - but we are discussing bonding as far as I'm aware.

Table 54.8 is quite clear, for a TN-C-S system the minimum bonding conductor size is 10mm2 or equivalent conductance. The dominant case here if the armour is going to be both CPC and bonding conductor is that of bonding conductor



Because that is about the ratio of conductance between steel and copper - you appear to making some equivalence based on thermal capacity not conductance



My pleasure

Regards

OMS

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

Edited: 11 May 2009 at 07:29 PM by OMS

Edited: 12 May 2009 at 12:08 AM by spinlondon

We are - we are exploring the scenario where the TN-C-S earth is to be used and that the garage contains extraneous conductive parts that require equipotential bonding.



The use of buried SWA is perfectly acceptable - if you wish to use the armouring as both the means of earthing and as a bonding conductor the dominant case is that of bonding.

That leads us to Table 54.8 which requires a minimum conductor size of 10mm2 if made of copper or of equivalent conductance if another metal.

As the armouring is steel we need the equivalent of 10mm2 of copper which using a conductivity ratio of 9:1 leads to the use of a minimum of 90mm2 of steel. Using XLPE/SWA we find the nearest cable would be a 95mm2 2 core.



Sorry, I don't follow that - Table 54.1 tells me that the protective conductor (in this case the cable armour) which is protected against corrosion by a sheath needs to be a minimum of 16mm2 of coated steel - in our case we have around 110mm2 of steel so clearly we comply.

Regards


OMS

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

I suggest you go and re-read the part of the Regs. pertaining to Protective Conductors.

OK, done that - which reg in particular were you thinking of and which part of my assessment above don't you agree with

OMS

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

By exporting the Earth are we changing the TNCS into PME?
Would the SWA be classed as a protective conductor or an equipotential bonding conductor?
How would you calculate the CSA of the SWA as a protective conductor?
Why would you use a diferent calculation when calculating the CSA of the SWA as an equipotential bonding conductor?

Take a look at the little drawing in Fig 2.4 (Page 33) and read the third sentence below that drawing



It would serve the function of both a protective conductor and a main equipotential conductor.



By means of the adiabatic expression or by reference to Table 54.7 depending on the particular circumstances.



Because the armouring is acting as both a Circuit Protective conductor and as a bonding conductor.

Therefore the dominant case is the requirements for a bonding conductor in a PME installation. I would refer to Table 54.8 which tells me that a 10mm2 copper equivalent is needed, as this requirement is more onerous than the requirement for a CPC it clearly needs to meet 54.8 and would also meet 54.7 by default

Regards

OMS

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

Edited: 12 May 2009 at 07:29 PM by spinlondon

You are are kidding I hope - since when did K factor translate to conductance.

K factor is a dimensionless number that attempts to bring together resistivity, temperature coefficient and volumetric heat capacity.

OMS

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Failure is always an option

It is what is used to determine the CSA equivalance for a protective conductor. Why should it not also be used for an equipotential bonding conductor?

Read the note to table 54.8 then google "conductivity of metals" - ignore any hits that relate to "thermal" and search within results for "electrical".

Regards

OMS

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

Sorry, which Reg. is it that tells me I shouldn't calculate the CSA as per the formulae and tables in the Regs., but should Google?

If a conductor is to act as a bonding conductor (as well as a CPC in this case) in a TN-C-S (or PME) system then you would need to refer to regulation 544.1.1. Read the second paragraph as PME conditions apply, that will direct you to Table 54.8. You will note the "*" after the text"Minimumm Copper equivalent........".

When you refer to "*" at the foot of the page you should see that the figures used in the table are for copper but it is allowable to use another metal affording equivalent conductance.

If you had googled conductance as I suggested you would have discovered that the relationship between the conductivity of steel and the conductivity of copper is around 9:1. From this you should recognise that for every 10mm2 of copper you would need to use around 90mm2 of steel to comply with the requirements of the regulation.

To answer fully, you should also recognise that main equipotential bonding conductor sizes are usually not calculated - they are selected based on knowledge of the distributors supply neutral size in a deemed to comply fashion in accordance with 544.1.1

Regards

OMS

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

543.1.1 "The cross-sectional area of every protective conductor, other than a protective bonding conductor, shall be ...."

- Andy.

I've tried telling OMS that, but he states that we shouldn't use that. He states that we should use the formula 9:1.

and as I said,



and as Andy kindly pointed out if you approach the problem the other way:





Well the point to this post was to point out that your comment:



was a little erroneous if the garage contained extraneous conductive parts.

As for long winded, that's one opinion, although if you believe I haven't answered your question then I can only conclude you don't understand what the question is.

Perhaps if you take a look at figure 5.16 in Guidance Note 8 and examine building B3 then you may get the point.



I actually said, around 9:1 - I couldn't tell you the exact figure without knowledge of the specific material although it won't vary much.



Because we are talking about the bonding conductor sizing being the dominant case - and as I tried to point out, we don't calculate that - we select it.

OMS

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

Have you read it - it refers to the adiabatic expression or selection in accordance with 543.1.4 (ie Table 54.7) and it specifically tells you that the adaibatic expression or selection in accordance with 54.7 is not applicable to bonding conductors - the point Andy was subtly trying to make I suggest

OMS

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Failure is always an option

It's not often I'm accused of being subtle!

Yes, I was trying to point out the reg which specifically says you can't use the calculation you'd usually use for CPCs to size a bonding conductor.

I'm with OMS on this one - 544.1 is what's needed for main protective bonding conductors, for PME it says to refer to 54.8. Table 54.8 says 10mm2 Cu or equivalent conductance if other metals. AFAIK BS 7671 doesn't give the relative conductance of copper and steel, so we have to look elsewhere. The Boy's own Almanac of Metallurgical Data might be one source, but if you don't have a copy to hand and happen to be in front of a internet connected PC, then google might be quite convenient.

- Andy.