Main Earth Conductor

[Paul Barker]

On a house I am working on (new CU few new circuits fusing down of diy 2.5mm radial circuits which were on 30amp)

The main earth block is right beside the cutout on the tncs supply. The 10mm water and gas Maind eq bonding is to it, and a 16mm is from it to the main fused switch.

Then from this switch the entire supply is taken to the consumer unit via 16mm t and e about 9 meters routed in such a way it doesn't require rcd protection.

All the cpc's terminate in the CU.

My question is which of these?
1/ should I supplement the earthing conductor in the 16mm t and e to make it up to 16mm?
2/ should I run a seperate 16mm?

3/ do nothing and make a note of it under departures?

 

[sparkyspike]

What size is the main fuse and what size is the switchfuse?

 

[Lectrician]

If it was a quick and easy run I would just through a 16mm GY in and be done with it.

If the route was tricky, I would check the earth size required with the adeabatic equation instead of using table 54blur blur.

 

[securespark]

I agree. The feed from the switchfuse should be sized according to the size of the fuse in that device. Also, what type of cable is supplying the CU via the switchfuse? If armoured, is the armour the CPC or is it via one of the conductors? Or both?

You have two choices:

1. Do the calcs and check that the current set-up is sufficient.

2. Throw in a 16mm² for good measure.

 

[Lectrician]

He said is was 16mm T+E which has a 6mm earth.....

 

[Paul Barker]

OK so check my working please I can't get back to the job to check fuse but say hypothetically it is 80a 1361.

I do know the pfc of the cooker circuit so we could work to that for the moment 350 amps (Id don't expect pfc to be much different at CU. k is 143

so I get 10 second (correction 20, I did my calcs for 20 just typed 10 here by mistake. )before fuse blows.

s comes out at 10.9mm

So theoretically I could run 6mm to supplement the 6 mm in the t and e. Is that allowed though?

just looking at the costs as the owner is very cost conscious though not unreasonable. He would go for 16mm but I don't want to waste his money unnecessarily..

 

[Spark123]

You need to measure the EFLI at the consumer unit and ensure that the Zs is sufficient to disconnect the switch fuse within 5s. Then you can do a quick calc to make sure that the 6mm is big enough. K=115 for a cpc in twin and earth.
As long as the 6mm CPC in the twin and earth isn't being used for main protective bonding then I don't see a problem with that side of things.
The "Earthing Conductor" is the cable which connects the MET to the DNO head, the conductor which connects from the MET to the consumer unit is a "Circuit Protective Conductor" so unless as above, it can be sized as a CPC.

 

[Lectrician]

As above.

But do not supplement the earth, add a new conductor that is sized as required. The difference in the cost does not justify supplementing as you suggest.

 

[Paul Barker]

Thanks guys that's answered all the questions.