2 CUs 1 earth

[PrenticeBoyofDerry]

Question
If a house had been renovated and extended, the new part having a seperate CU with a seperate incoming supply to the old CU.
If the old CU had no connected in-coming earth at supply, would it be allowable to cross connect the new CU's earth the old CU?
If not what would be the alternatives?

 

[rebuke]

Depends what the earthing arrangement is...

Given no earth I'm assuming the old supply is TT, if the new supply is also TT and a rod has been installed, then I don't think there are any regs about having to use a separate rod or anything like that, so as long as you run a separate cable to it (existing one probably isn't big enough to take the fault current of both CUs). I guess there is an issue about how well it could take the fault current if both CUs faulted - I'll leave that to someone with more knowledge.

If new supply is TN-C-S, then I'm fairly sure you wouldn't be able to use it as an earth for the old CU for a number of reasons (obvious ones that come to mind are that the shared neutral/earth conductor probably wouldn't take the full fault current of both supplys, plus you have various additional risks in the event of a neutral fault...)

Not sure about TN-S, but I think it would come down to conductor size, so you'd probably end up having to put in an earth rod and run the old supply as TT...

 

[PrenticeBoyofDerry]

Thanks for reply rebuke, not sure what supply is as I have not visited it yet, might be a simple fix. The original TN-S or TN-C-S may have never been connected just not come across this before, but sounds like no CPC on old circuits, so re-wire likely. Might get away with installing new circuits to new cu?

 

[ban-all-sheds]

If new supply is TN-C-S, then I'm fairly sure you wouldn't be able to use it as an earth for the old CU for a number of reasons (obvious ones that come to mind are that the shared neutral/earth conductor probably wouldn't take the full fault current of both supplys, plus you have various additional risks in the event of a neutral fault...)

Plus would it make a difference if the two supplies weren't on the same phase?

 

[RF Lighting]

I'm going to say no.

 

[PrenticeBoyofDerry]

Thanks for replies guys, just thought someone may have come across this before and knew of a safe way that complies to get around it,
I'll know better later on in week when had chance to investigate, I wouldn't have thought seperate phase cycles would be a problem.
If the max demands not too big old fuse box out and all circuits on to a new one as I doubt current CU is 17th eds
Cheers again guys

 

[scarypants]

No how to delete post

 

[securespark]

Given no earth I'm assuming the old supply is TT, if the new supply is also TT and a rod has been installed, then I don't think there are any regs about having to use a separate rod or anything like that, so as long as you run a separate cable to it (existing one probably isn't big enough to take the fault current of both CUs). I guess there is an issue about how well it could take the fault current if both CUs faulted

No probs: if it's TT, then 30mA RCD required and the fault current is tiny, so MEC can be as small as 2.5mm².

 

[PrenticeBoyofDerry]

Had a look at job today, seems the earth conductors been removed from Lead sheath, but no CPC on the old fuse box sides lighting circuit neither, which is common to find. So not two individual supplies as mentioned by client on the phone, but a fuse box and a CU using same incoming supply.

 

[Spark123]

Better to get the electricity board out to fix a new clamp to the sheath.

 

[PrenticeBoyofDerry]

Better to get the electricity board out to fix a new clamp to the sheath.

There is one but it's connected to the most recent CU about 9 years old,
my plan was to firstly connect the ring final to this board to assure that at least that socket circuit has earth protection then rewire the lighting circuit to the CU and make the fuse board redundant.
Of cousre I'd do an IR test on ring final to check it was healthy.

 

[Spark123]

Can you provide some photographs of what you have?

If there is an earthing conductor coming from the service head or sheath then there is no reason why it shouldn't be connected to a MET local to the electricity meter and then all main protective eq. bonding cables and the CPCs run back to it.

 

[PrenticeBoyofDerry]

Can you provide some photographs of what you have?

If there is an earthing conductor coming from the service head or sheath then there is no reason why it shouldn't be connected to a MET local to the electricity meter and then all main protective eq. bonding cables and the CPCs run back to it.

I'd love to show photos but the securities on my PC won't let me and I don't understand why, some updates have prevented me from uploading photos about a month ago.

The earth conductor does go from the service head to the MET though, I do intend removing the old fuse box but would it be permittable to connect and earth conductor via MET to old fuse box or would there need to be two earth conductors from service head protecting both CU and fuse box? Just for future ref.

 

[Spark123]

Yes, that is the way it is normally done.

 

[PrenticeBoyofDerry]

Yes, that is the way it is normally done.

Cheers Spark123