No CPC in submains cable

[kamkam]

Hi all just done pre site survey on a job found the following (never come across this before)

2 ccu's in the property 1 located next to the incoming head (A) and the other (B) is located approx 15 meters away

Both CCu's are feed via henley block. (A) tails are 25mm however the cable feeding (B) is 16mm 2 core non armoured double insulated PVC with no cpc. The run goes thought the fabric of the building there is no cpc run along side this cable!. There is also no isolator for that.

The cut out is 100amp the earthing arrangement is TNS.

Ze at (A) is 0.35

Ze at (B) is 0.63 (Just picking up paralel paths as there is visually no earth on that side of the building) all Zs on the (B) side are as expected very high 1.23 on sockets as an example.

The building is very old prob 200 years old with a newer extension added approx 1970's hence two dbs. The property is also in the middle of know where ie old farm house.

I want to ditch the 16mm feeding (B) and install 25mm swa via isolator etc etc. I am trying to find the reg (if it exisits) regarding the requirement of cpc in cables run though the fabric of a building so can back up my reason for running a new cable. OR would it be ok to run 16mm earth from the MET along side the current 16mm and install a fused 60amp double pole isolator. Haven't calculated VD on the 16mm yet just wanna run this by you guys for your opinion.

Let me know if I have missed any info that might help the answer...

Cheers

Kam

 

[loop3]

The reg defo exists, maybe in several parts, sorry ain't got a bgb to hand . Is the b board in the same building? I'd personally recommend pulling in an armoured. Whether you use the armour as cpc or have a dedicated core is down to you design/calculations.

 

[kamkam]

Hi Loop3... yes the building is joined together. Seen different regs regarding cpcs but no 'one' reg that will be easy to explain the the customer (he's bit of a know it all - well thinks he is)

 

[loop3]

Also if the cut out is 100a feeding both ccu by henley, then 16 should be 25mmsq.

 

[loop3]

That's defo in the regs . Ib<in<it...

 

[kamkam]

Also if the cut out is 100a feeding both ccu by henley, then 16 should be 25mmsq.

Not if via a fused 60amp double pole isolator

 

[loop3]

Go through thermal constraints and short circuit protection calcs.

 

[loop3]

Ok. With fused isolator. Fine. But cpc is a must. I'll check it when I get in.

 

[loop3]

Don't you just love the customer who knows it all!!?? I like inquisitive people, but not tight fisted inquisitive people. Lol

 

[loop3]

The type that will encourage you to cut corners but are ready to sue at the first opportunity!

 

[loop3]

Give no leeway.

 

[telectrix]

as an alternative, you could TT the sub main board, as long as all circuits are RCD protected.

 

[loop3]

Good call tel.

 

[kamkam]

TT was my first thought but then was concerned about the cable going though the building with no cpc

 

[Engineer54]

as an alternative, you could TT the sub main board, as long as all circuits are RCD protected.

Why would anyone in they're right mind TT part of an installation that has a perfectly sound TN-S at the cut-out, 15 metres away!!!

Good call tel.

No, it's a very Bad idea/call!!!

 

[wildgoose]

How can it be "a very bad idea" if it's compliant??

Agreed, the ideal situation would be to extend the main earth off the existing MET to maintain TNS earthing but for cost or practical reasons, this may not always be possible. Therefore the TT suggestion which is a safe and compliant is still valid option.

 

[MarkieSparkie]

I with E54 on this one, TTing this installation, although possible, seems bonkers in this case with a TN-S earthing arrangement in good condition only 15m away. Just export the TN-S earth by following the existing sub-main route with a 16mm[SUP]2[/SUP] earthing conductor from the MET at the origin (A) to the EMT at the sub-board (B), and fit 60A DP fused isolator at the origin (A) to supply the original sub-main.

 

[hasel5]

Surely just lack of earthing to the bird is enogh to issue an EDN ?

 

[Richard]

If you can run an additional 16mm2 earth along side the existing submain, then there's your answer

 

[malcolmsanford]

Not sure why everyone is advocating a 16mm CPC for this sub main

 

[Engineer54]

How can it be "a very bad idea" if it's compliant??

Agreed, the ideal situation would be to extend the main earth off the existing MET to maintain TNS earthing but for cost or practical reasons, this may not always be possible. Therefore the TT suggestion which is a safe and compliant is still valid option.

Yeah right, with an Ra of 200 ohm's is gonna be really safe!!! A TT earthing system should ALWAYS be your LAST resort, not used as a get out of jail quick fix!! Got to be easier to run a CPC in following the the sub-main cable....

 

[kamkam]

Its actually going to be easier (but not cheaper) to run a new submains, so that's what I will suggest first, however if it comes in to high a cost for them then I will run a cpc from the MET to the db (B).
All via fused double pole isolator. Job done. Thanks for all your thoughts on this

Ta very much