size of earthing conductor from switched isolator to CU

[n180]

Domestic single phase supply, TNCS, 25mm tails.

From supply to Fused switched isolator : Earthing conductor is 16mm.

From isolator to CU : Earthing conductor downgraded to 6mm for what ever reason I don't know.

Is there something in the regs that says after an isolator the conductor can be downgraded to 6mm? As far as i'm aware it should be 16mm all the way to CU, with or without isolator in the middle.

Thanks.

 

[Pete999]

If the main Earth doesn't need to go via the switch, then why change it, earth the switch from the MET.

 

[Ian1981]

Domestic single phase supply, TNCS, 25mm tails.

From supply to Fused switched isolator : Earthing conductor is 16mm.

From isolator to CU : Earthing conductor downgraded to 6mm for what ever reason I don't know.

Is there something in the regs that says after an isolator the conductor can be downgraded to 6mm? As far as i'm aware it should be 16mm all the way to CU, with or without isolator in the middle.

Thanks.

It’s a distribution circuit from the switch fuse isolator to the CU the earthing conductor stops at the isolator, it’s just a Cpc to the CU
Problem arises if the bonding conductors go to the CU as the cpc in the distribution circuit needs to act as a protective bonding conductor as well as a cpc.

 

[andyb]

You will need to work out the minimum csa for the protective conductor. From experience I'd be surprised of the 6mm wasn't adequate.

 

[Baddegg]

What’s the earthing arrangement? But as others have pointed out what’s its actual use?

 

[n180]

Thanks guys. Without working out the adiabatic calc i don't think 6mm is enough, besides as pointed out by ian, i've got the water bonding going to CU. So i'm going to save any doubts and headaches and just upgrade it to 16mm, can't go wrong then.

 

[pc1966]

As TN-C-S there is a minimum for bonding but if (as Pete999 pointed out) the MET connections to water/gas/etc are not via the CU it is less of an issue, even if still a bit strange.
[automerge]1590787215[/automerge]
Ah, just missed your reply - agreed.

 

[andyb]

So where would you say was the main earthing terminal?

 

[davesparks]

Thanks guys. Without working out the adiabatic calc i don't think 6mm is enough, besides as pointed out by ian, i've got the water bonding going to CU. So i'm going to save any doubts and headaches and just upgrade it to 16mm, can't go wrong then.

So extend the water bonding to the MET, which is the best place for it to connect, and carry out the calculations to establish whether 6mm is the correct size cpc or not.

 

[n180]

MET (and isolator) is too far in a cupboard outside the building. I'd rather take 16mm in to CU than do the same amount of work to take 10mm water bonding out to MET and still be left with a 6mm from isolator (or do more work to upgrade 6mm as well, which customer won't be willing to pay for).

Anyway, thanks for the suggestions, but as you guys don't have all the site details i've decided what I believe to be the best course of action.