house earth vs garage earth spike

Only if the SWA was of adequate size to form part of a bonding conductor other wise the installation would be non compliant.

 

Right, so you can look on a chart and check can you not? So where does 85mm come from and why say this?

"other wise a separate bonding conductor would of have to been installed or an adequately sized 3core SWA utilising the 3rd core for earthing/bonding. "

 

As stated by leesparkykent that is simply for use as a cpc. However it should be pointed out that this chart is simply based on selection. If using calculation with the adiabatic expression then you may find that other cables are acceptable for your particular setup. None of this is relevant to use as a main protective bonding conductor though.

 

What chart are you referring to? The 85mm comes from the conductivity ratio from copper to steel based on the fact the steel has to offer at least the same conductivity as copper based on a 10mm copper bonding conductor selected from table 54.8. Risteard explained it in his post pretty well IMO but Ill have another go though. for example You are running a supply to an outbuilding that has extraneous conductive parts that require bonding and the suppliers neutral has a CSA of 35mm or less so using table 54.8 we understand that a 10mm bonding conductor is required so you have a few options here....you either use a 10mm 3 core SWA and use the third core as a combined earthing and bonding conductor, you install a two core SWA and use the steel armour as a combined earthing and bonding conductor the CSA of the armour has to meet the requirements of an earthing conductor as well as a bonding conductor. looking at table 54.8 which is for bonding with PME it says if using another type of metal i.e other than copper it has to offer at least equivalent conductance....looking for guidance on equivalent conductance we look in guidance note 8 (earthing and bonding) which provides us with a ratio of equivalent conductance from copper to steel. we then apply this ratio which gives us the required CSA of steel to offer equivalent conductance. Next option make the outbuilding TT so the extraneous conductive parts within the outbuilding can be bonded to the MET of the TT installation we have created or finally run in a separate 10mm bonding conductor from the extraneous conductive parts within the outbuilding back to the MET. note that if the separate 10mm bonding conductor was to be buried in the ground it would have to protected against mechanical damage or would have to be upgraded to 16mm to comply with table 54.1

 

I think @leesparkykent has explained it very clearly and I am suprised some cannot understand.

Stinks as if some members are trying to justify past installations that they have carried out in which they have now discovered are non-compliant.

 

This is my reasoning...

Table 54.8 says 10mm min. cpc required for a <35mm neutral supply on a PME. Looking at table 54.7 I select k1/k2 x S as I want to use the swa as my cpc, so looking at Table D9 Thermoplastic /SWA/thermoplastic cables to BS 6346. Insulation operating at 70°C to comply with table 54.7 I can see that the Conductor csa (mm2) Minimum csa of steel cpc for the smallest 2 core cable (1.5mm) has a csa area of 15mm (Actual armour csa from BS 6346) so it meets and exceeds the requirements for use as a combined cpc and main bonding conductor if there are extraneous parts in the remote supply. I can then select any larger 2 core swa cable as the swa csa is always going to be > 10mm.

So what formula are you using to end up needing 85mm cpc size?

 

STEEL AND COPPER ARE COMPLETELY DIFFERENT METALS WITH DIFFERENT CONDUCTIVITY PROPERTIES.

 

NO SH!T SHIRLOCK?

Look I've taken the time and effort to post up what I think on this with reference to 7671 - rather than just yelling at me why don't you show your working or point me to the relevant reg? Jeez.

 

Don't worry I've figured it out myself.

 

Hi, what have you figured out yourself?

 

I read the right pages in GN8, there is a section on it. I never new about it at all tbh, so thanks for pointing it out. Seems it's not just PME that have this requirement either. And cheers for not getting all busted up over it. Lucky for me I've never had to deal with this kind of situation before I guess. There but for the grace of God go I.

 

Does table 54.8 state the 10mm must be copper regarding <35 neutral, sat in a pub so can't check, also can't be arsed. Personally I would always use a 3 core.

 

The crux is that if the cpc is also the mbc then it has to have the same resistance value as copper if you use anything other than copper, csa doesn't matter as it doesn't come into it. There fore you might well end up with massive cpc if you use swa. I've always thought cpc/mbc was based on K values.

 

Happy days....Every days a school day

 

Glad I only looked at this thread after it was sorted.
The conductance ratio between copper and steel is about 8.5:1.
So 10mm copper = 85mm steel.

 

Yea me too, close shave that...

 

lol

 

Out of interest Coloumb, which pages?

Kind regards

 

I've only got the 16th edition GN8 so it's pg 41 to 42. It's in the main equip. pot. bonding sec under 5.2.1 so it can't have gone far.

 

Thanks Col. Me too. Will have a read.

Kind regards