Single concentric cable use

Desie · May 24, 2014

Hi guys.
I am a new member to this site and boy do I have the need for some advice.
recently got handed a job of upgrading a supply to a site from single phase to three phase. The site is a Mushroom Farm with many outbuildings. The existing wiring at the main incomer is atrocious. All sub circuits have been wired in straight concentric, no earth. The cables terminate in various distribution boards which have earth electrodes connected and RCDs for protection. The cables are installed underground and covered in concrete for the most part but there are some overhead installation. Protection to these cables would be purely by mcbs back at the main distribution dis board.
Can this system be acceptable under BS 7671 and can I reuse these cables.

telectrix · May 24, 2014

test the cables . if they test out OK, your only concern is the lack of earthed screen on these cables as the DBs each have their own earthing system ( TT ). how good this is , you'll need to get Ra values. personally, i'd replace the lot with SWA in ducting.

Richard Burns · May 24, 2014

Welcome to the forum; jumping straight in there with an interesting question, well done.

The concentric cable used underground should be in a protective duct if there is no earthed sheath.
The overhead concentric cable should be installed at such a height that there should be no likelihood of the cable being normally accessible from the ground, normally about 6m.
Other than that so long as there is an earth at each point of utilisation then you may be OK, subject to what Telectrix has said above.
And SWA in ducting is much better!

Desie · May 25, 2014

Thanks lads. Yes I to would like to replace the cables with swa but in this situation it is not an option. It's not an ideal position to be in but by ensuring that the cables are ok and that the ze/zs at each dis bd is with in the regs and that the RCDs are properly specified and test ok then we will be ensuring adequate protection. Stillllll not happy.

telectrix · May 25, 2014

you can only do as much as the customer is willing to pay for. make sure you add a comment on the cert. that you recommend replacing the cables.

wozz · May 26, 2014

Are you talking about split con or straight con? I read it as straight con.

Desie · May 26, 2014

Hi Wozz.
It is straight concentric as used by DNO in TNCS supplies but it is being used as a sub main supply to distribution boards on this site. The bare concentric conductors being used as a neutral. There are no earth conductors associated with the cable. Each distribution board has its own earth electrode and RCDs. It's obviously been a system that has been installed by a non qualified person. The funny thing is it would not have been very much cheeper than installing the proper cables when you consider the additional work of installing earth electrodes at each board. We are now recommending upgrading to swa but it will be expensive as it's a fairly large operational site.

Desie · May 26, 2014

You are so right Telectric. It's sometime difficult to get across to a client that you are not trying to wipe his eye and that his system,although working,is not the way it should be done
Thanks for your reply
regards Desie

wozz · May 26, 2014

Then I think you are screwed, if you want to comply with 7671 you have got to rip it out.

Desie · May 26, 2014

Yea Wozz in this type of cable the neutral is bare conductors wrapped around an insulated conductor. BS7671 considers a neutral conductor as a ' live ' conductor and as such should have an insulation sheath. So from this point of view the cable cannot be considered or used for a twin cable. But this is just one of the many problems so either the client accepts a rewire or a walk away is on the cards.
Thanks Wozz.

Schneker · May 27, 2014

Desie said:
Yea Wozz in this type of cable the neutral is bare conductors wrapped around an insulated conductor. BS7671 considers a neutral conductor as a ' live ' conductor and as such should have an insulation sheath. So from this point of view the cable cannot be considered or used for a twin cable. But this is just one of the many problems so either the client accepts a rewire or a walk away is on the cards.
Thanks Wozz.

Says a lot about your standards mate that you'll walk away from what I'm assuming would be a decent earner rather do a bosh job!

Whats the betting that if you walk away the job still gets done by another "competent" contractor ??

Desie · May 27, 2014

Schneker said:
Says a lot about your standards mate that you'll walk away from what I'm assuming would be a decent earner rather do a bosh job!

Whats the betting that if you walk away the job still gets done by another "competent" contractor ??

You gotta know when to hold them
Gotta know when to fold them
Gotta know when to walk away
lyrics from a jinny cash song but sometimes good advice