house earth vs garage earth spike

544.1.1 states copper or a CSA affording equivalent conductance in other metals and table 54.8 is for copper which also states a bonding conductor of another metal affording equivalent conductance.
0.004 is the simplified resistance coefficient per degree C at 20 degree C given by BS 6360 for copper and aluminium conductors. I'm not 100% sure of the resistance of steel but one of the guidance notes gives us a ratio of 1:8.5 between copper and steel conductors.

 

Enlighten me then.

 

couldn't edit my post but just looking through my books and the resistivity of steel changes with carbon content...and its guidance note 8 that suggests the ratio given above for the resistivity of copper to steel.

 

So if the csa of the swa meets the requirements of 54.8 why would you need 85mm csa if you where bonding using 10mm?

 

because the resistivity difference between copper and steel.. If 10mm copper was deemed to be of an adequate size for bonding and you wanted to use the steel armour as a bonding conductor or part of then due to the resistivity difference it would have to be of at least 85mm. BS7671 gives you the sizes in copper but also mentions if a different metal is used it has to be of equivalent conductance.

 

I really don't get what you mean mate. If it can be shown the cpc size of swa is the same or greater than that of it's copper equivalent then why would it not clear a fault in < 5s or be suitable to use as a cpc and bonding conductor for a remote supply? As I said before, it would depend on the size of the origin of supply for the size of the bonding conductors if there are extraneous parts in the remote building if it's pme. I simply fail to see what difference it would make if the cpc for the supply is swa or copper or even trunking/conduit so long as the cpc size is equivalent to that of copper.

Would you be able to show your working for the 85mm bonding conductor?

 

but half the time you only need 10mm for mechanical protection, a bit of 2.5 earth might satisfy things electrically in some instances, so as long as it meets this whats the issue.

 

I don't think we are singing from the same hymn sheet here. what I'm saying is that the steel armour would be adequate as a CPC but not as a bonding conductor.

 

But if the supply is remote then if the cpc is swa it would be both the earth and the bonding. Surely you didn't mean stripping out the steel and using it for just the bonding?????

 

I think he is saying that to get the same conducivity from swa as you do from 10mm copper then the swa needs to be 85 square millimeters. Sounds ****** if you ask me.

 

no I didn't mean stripping out the steel and using it as a bonding conductor.....The op has 2core SWA, the armour is used as the CPC, the armour is adequate as a CPC, the CPC is not adequate as a bonding conductor. regulation 544.1.1 and table 54.8 both say that if a different metal is used other than copper then it has to offer at least equivalent conductance. To achieve equivalent conductance the CSA of the armour has to be bigger than copper due to the properties of the different metals. The ratio of conductivity from copper to steel is stated in guidance note 8.

 

yes...why does that sound ****** to you?

 

I really can't see it's going to make any difference what you stick your current through so long as the cpc is the same size. The type of metal is irrelevant. That's why there is more steel in the swa than the equivalent copper size to get the equivalent cpc size. It's simple physics.

You really need to put up your calculations to show otherwise.

 

Well clearly the type of metal is not irrelevant to limit touch voltage. The maths is simple the ratio given is 1:8.5 to offer equivalent conductance from copper to steel. for example if 10mm copper is deemed adequate for a bonding conductor for the installation but you was using the armour or the armour was forming part of the bonding conductor then it would have to be at least 85mm. 10mmx8.5=85mm

 

Leesparky is absolutely correct. Either use a larger SWA, or run another conductor in parallel (or an extra core within the SWA if 10mm^2 or above). Main protective bonding conductors in a TN-C-S system must be at least 10mm^2 copper (and sometimes larger). Therefore the armouring is going to have to be much bigger (leesparky states 85mm^2 and without looking it up that sounds about right).

 

Obviously copper conducts better than steel, that's why they use more expensive copper in th first place, but 10mm = 85mm, I cant see it myself. Especially when a bit of soil will do the job as well.

 

Anyway Lec posted a chart the other week which showed which csa's of armoured complied and which did not, most did according to the chart. Yet I doubt even the armoured csa of a 2 core 85mm cable is 85sq mm.

 

Yes that's because that chart would of been for using the armour as a circuit protective conductor not a bonding conductor.

 

The bonding and cpc size needs to be based on the table given 54.8 for a remote supply if it's pme or an adiabic for a tns. According to GN1 for 16mm bs6346 2 core cpc is 46mm. 85mm sounds like a random number to me. So I would be interested to see some calcs.

 

A remote supply, the cpc is both the bonding and earthing conductor IF there are extraneous parts.