PME and earth stake?

[delindevon]

Hi all,
I have just been to price a CU change. The problem is the CU is about 20m from the mains position and is run in 16mm T&E. Here's the good bit the 16mm runs through a neighbours flat, so it can not be replaced and I can not run an earth next to it. I can, though not easily, run another earth and a stake. If I do I think I should disconnect the PME, although I dont want to. What about a RCD to protect the 16mm if I fit one I can't seperate the circuits inside the flat and a RCD 20m away is not ideal.
Any ideas, thanks Del.

 

[nickblake]

one you do not disconnect a PME system if the cu is about 20 meter from the mains position you need to install a seperate db with a 63 amp mcb ,then change the cu as normal with a 17th edition ,as for the 16mm in a neighbours flat this needs to be relocated , on a PME TNCS installation then the max Ze is 0.35 , once you have installed the 16 mm check the Zs the max for a 5 sec disconnection time is 0.73 you should be able to reach the max ZS for the other circuits if this cannot be reached add an additional earth , and dont forget the bonding to water gas etc this has to be installed at the point of entry of the service and then to the mains incomming DB

 

[delindevon]

as for the 16mm in a neighbours flat this needs to be relocated
That's the nub of the problem I can't run a new cable, the neighbours flat is between this flat and the mains position. The cable will have to go through or around the neighbours flat neither of which is an option.

 

[nickblake]

is the mains supplying all the flats,if so this should be installed with an isolator so all the flats can be isolated with one switch , the only thing i can then suggest that the dist board in the neighbours flat is labled warning of the supply form your flat , as you can imagin giving advise is difficult with out seeing the installation or more info

 

[delindevon]

Nick, all the flats have seperate meters and a switch fuse each in the meter room. The problem is the size of the earth,and the fact that the 16mm is not protected by an RCD. I can measure the run properly I think it's more like 10m to be honest.

 

[nickblake]

the biggest problem is protecting the 16mm as if it trips you loose the whole supply , 16mm twe has a 10mm earth , i know the installed size should be 16mm but if all the Zs readings can be met then there shouldnt be a problem ,its also on a switch fuse so the cable is protected ,if the cable is more than 50mm from the surface or installed in a position where it cant get damaged then its fine , where its exposed you could installit in a earthed steel trunking then it will not need RCD protection

 

[andyb]

Do you need to run a separate earth? Have you done your calculations to see if the 6mm earth in the 16mm t&e is ok?

 

[delindevon]

Thanks for your input Nick, it's nice to get other peoples ideas.
The big problem here is that short of buying the next door flat I can not move check or alter the 16mm. If there were no regulations and presuming the Zs is OK I would fit a 100 m/A RCD at the mains position and 30 m/A RCBO,s in the flat, there is only 4 circuits, then Robert would indeed be your Fathers brother. Alas its not as easy as that. I have wondered if I were to put a 30 m/A RCD at the mains position then disconect the earth when it gets to the flat and fit a stake for the flat would that cover the regs. An earth fault in the flat would not trip the mains position RCD but the 16mm would be covered. Its odd and I dont like though as it would mean 2 earthing systems, all be it seperate on the same instalation. Is there another way?

 

[Guest123]

16mm twe has a 10mm earth

I dont mean to split hairs but 16mm T&E has a 6mm CPC which may have a bearing on any calcs he might do.

 

[J@mes]

from what I have read it is the kind of job I would walk away from unless you can get the full co-operation of the other flat owner in order to do the work correctly. Is it worth the hassle for £300 max? !

 

[Jimmy Boy]

I dont mean to split hairs but 16mm T&E has a 6mm CPC which may have a bearing on any calcs he might do.

I noticed that Lenny but you beat me to it.

Where do we find a reference guide to number of strands and csa of each strand in cpc's ? Are the number of strands and csa the same in a bare cpc in a T&E as they would be in a stand alone Earth cable..Just wondering

 

[nickblake]

I dont mean to split hairs but 16mm T&E has a 6mm CPC which may have a bearing on any calcs he might do.

yes that was my error