Main Earthing conductor max resistance?

[dokkan1080]

I know main bonding to water and gas need to be 0.05 ohms

But what about the main earthing conductor?

Does it have a maximum resistance value from MET to the CU?

 

[pc1966]

So would it be fair to say that the main earthing conductor only runs as far as the fused switch and from there on it becomes a cpc which has to meet the max Zs requirements of the fuse in the isolator which in turn dictates its max resistance?

Maybe, maybe not:

• It has to allow the fused-switch to meet its Zs for 5s disconnection at the end of the sub-main (as you just said).
• It has to allow all final circuits to meet the Zs requirement.
• It has to meet the adiabatic limit for all OCPD clearing (supply fuse & finals circuits, but obviously they will be smaller and can be ignored if supply is met)
• And also any possible extraneous bonding on TN-C-S (which ought to be back to the MET, but if sub-main to the remote board, such bonds might well be connected there) so you might have the (typically) 10mm minimum.

If you have no reason to doubt the design, then the resistance check is basically to verify it is working as designed, so based the GN3 figure would be:
R measured <= 0.05 + expected cable R2

 

[dokkan1080]

So would it be fair to say that the main earthing conductor only runs as far as the fused switch and from there on it becomes a cpc which has to meet the max Zs requirements of the fuse in the isolator which in turn dictates its max resistance?

Maybe, maybe not:

• It has to allow the fused-switch to meet its Zs for 5s disconnection at the end of the sub-main (as you just said).
• It has to allow all final circuits to meet the Zs requirement.
• It has to meet the adiabatic limit for all OCPD clearing (supply fuse & finals circuits, but obviously they will be smaller and can be ignored if supply is met)
• And also any possible extraneous bonding on TN-C-S (which ought to be back to the MET, but if sub-main to the remote board, such bonds might well be

16mm to fused isolator, then 16mm to CU
Or it might have had 16mm from the met and a separate 16mm to the isolator