Earthing Conductor as a single core.

[safedepth]

Evening all.

I am running a new submain from a TT install via Henley blocks then a 80a Switched fuse. It will be run in 16mm T&E. The MET is at the service head position.

My question is this, does the earthing conductor have to be a single conductor or can it be formed from 2 seperate conductors. I.E. the 6mm cpc in the T&E plus a seperate 4mm single, combined this would be the required 10mm conductor. I have searched the BGB and GN8 but can find no definitive answer. For that reason alone I suspect it is not permissable but don't want to waste the customers money. Additionally, the BGB tables for values of k do not seem to support this thinking.

I tried ringing the NIC tech line today but missed them.

Thanks in advance.

 

[Andy™]

there is only one earthing conductor, and its not in a submain. providing the 6mm earth is adequate for the circuit as a distribution circuit, your good (but dont forget to allow room for VD & earth loop for outgoing circuits)

but if your connecting gas/water earth to this board, then things are different

 

[safedepth]

Current set up is, tails into RECs isolator, then into henleys then split two ways. One side to small CU for garage area, (where the service head is) and other side into 80a Switched fuse. It's this side that then goes off to the house cu in 16mm T&E. The MET is at the service head. All main bonding to this point, none to the cu's. Are you saying this should be termed a distribution circuit rather than a submain?

Either way the 6mm is not sufficient, adiabatic equation indicates 10mm is needed, actual result was 8.5ish.

 

[Andy™]

distrbution circuit... submain.... same thing

either way, looks like your not getting out of a larger earth. you can have conductors in parallel. its easier if they are the same CSA as each other. im sure someone who knows more about this will be along shortly

 

[safedepth]

Thanks Andy.

 

[NozSpark]

You say this is a TT installation,,,, is that from the head?

If so then ALL circuits need to have RCD protection unless you can get your Zs low enough (I'm guessing you don't have a earth rod tester so it's Ze that you're measuring and not Ra), so I doubt that your switch fuse will be adequate.

Why not just use the earth of the sub main to just protect the cable and put another rod in at the end for the board that it's supplying?

 

[Andy™]

good point about the TT Noz

 

[safedepth]

The TT is at the head. The submain supply is only 15m tops from the head it is surface mounted clipped direct throughout its run. The RCD protection is at each board. Do I need an S Type RCD prior to the split? You are quite right, I am measuring Ze, it is 97ohms.

 

[Andy™]

you must have an RCD on the tails. and if the switch fuse is a metallic case, then the RCD must be before that and in an insulated enclosure

 

[safedepth]

Isolator insulated Recs2. Both cu's are insulated and have rcd's. Submain/distribution circuit is clipped direct and protected by switched fuse only.

 

[NozSpark]

Then it'll need RCD protection I'm afraid

100A 100mA is the norm, but you may get away with 300mA,,,, they'll also need to be time delay (or S type)

Last time I bought one it was knocking on

 

[Andy™]

[No message]

 

[johninlondon1]

I think the CPC has to be able to take the full fault current as one cable. If two are in parallel then each of them should be able to take the fault current individually as it would not be known that the fault current would divide proportionately between the 2 CPC cables in parallel, and one cable, if too small of CSA might not be adequate. The more experienced fellows will have a definitive answer as I am just a student.

 

[Evans Electric]

Parallel conductors , as in "live ones" should be , I believe, of the same size to avoid overloading the smaller one, which I think answers the question .

 

[Sidewinder]

Parallel conductors should be equal in size only exemption is swa armour in parallel with another conductor, internal or external.

If you use a separate g/y with an swa then it must be capable of carrying full fault current.

 

[safedepth]

Thanks for all your help everyone. I have today bought an S type 100mA RCD. I will be going with the single 10mm earthing conductor.