Cable termination of concentric cable

[picklesquirt]

Background: I am not an electrician, I am a retired professional engineer high power broadcast transmitter design , but am dabbling outside my sphere of knowledge, so help please. It is along time since I have looked at the 16th edition.

I am not in paid work but am helping a friend who is a farmer.

The question is we are running a single phase supply from a barn to a newly constructed barn, only a few lights and some 13A sockets. 2.5 sq mm SWA cable would do the job protected by a 30A fuse from the existing barn which has an 80A rcd 100mA trip. Around the farm are numerous lengths of SWA cable but are only 1.5 sq mm . We have found a drum , nearly full of a cable I am not familiar with, it is a single core concentric cable. Dimensions are

Center conductor 6.45mm dia solid aluminium eq 32.7 sq mm
Spiral copper neutral 25 strands 0.99m dia eq 19.2 sq mm
Plus sheath
THis cabel will feed a small consumer unit fitted with 10 mA trip
My question is how should it be terminated ? There are other places on the farm where this cable has been used and the spiral copper outer has just been twisted , bound with tape and put into a 100A terminal block. This does not look elegant or professional . Is there a marketed fitting for terminating this type of cable?
I might add this installation will not be inspected or signed off but must be safe for both people and fire risk.
Earthing is provided by earthing rods.

 

[pc1966]

A photo would help!

If it is concentric cable (i.e. inner line, outer is combined E & N) then you can't use it as only the DNO are allowed TN-C supply connections.

If it is split concentric, so the outer has both bare E and sleeved N you can use it, but the regs only allow for burying that if it is in suitable duct for protection since upon damage due to a spade, etc, the neutral is exposed and it is "live" as in current-carrying. That is different from SWA cable that can be buried directly.

Now I am a great fan of using duct for buried cables anyway as (a) they have a bit more protection against stones, etc, and (b) you have a sporting chance to replay without digging up should a fault occur. Also you can put other stuff through as well, such as fibre networking. But it adds cost and you might not get the savings you expect.

 

[loz2754]

The best way you can help your farmer friend is to get him to employ an electrician to properly design this installation, even if you want to do the work yourselves.
You state with confidence that 2.5mm² SWA is suitable to be protected by a 30 amp fuse. It is not. Unless it is suspended in free air or on a perforated cable tray throughout its run.
You also admit this installation will not be inspected or signed off. This is in direct contradiction to the wiring regulations (which is now in its 18th edition), which state that all electrical works should be properly designed, inspected and tested and certified as being fit for use.
This is especially true in special locations, such as agricultural buildings, for which dedicated sections exist in the regulations.

 

[picklesquirt]

Thanks for taking the time to reply. I would still like to know how this cable should be terminated, but take your points and will go shopping for some 2.5mm SWA and a 20A fuse.
Many thanks

 

[UKMeterman]

If you are exporting a TT arangement then use the traditonal brass gland and do use some SWA and do the cable calculations. If the supply is TNCS (PME) or in come cases TNS then you will need to use a NYLON (plastic gland) at the load end and provide suitable local earthing arangements not useing the DNO earthing faclity. In short you need to consider TT and cross bondings so the earth resisitance gives a touch voltage during a fault of 25V or less, it is very easy to kill a four legged animal.

 

[Marvo]

Is the concentric cable solid copper or copper coated aluminium? Some pics would be helpful. Concentric cable shouldn't be used buried direct, it's not an armoured cable so it's prone to underground damage from stones, rocks and subsidence. Concentric can be terminated using crimp-on lugs or even fit some tails onto it using spilt bolts (with insulating covers), if it copper coated aluminium I'd definately recommend using short tails of standard copper wire to terminate. It really depends what you're terminating it into.


As stated above I'd suggest you get some help at least with the design, especially with the earthing which is critical.

 

[picklesquirt]

Is the concentric cable solid copper or copper coated aluminium? Some pics would be helpful. Concentric cable shouldn't be used buried direct, it's not an armoured cable so it's prone to underground damage from stones, rocks and subsidence. Concentric can be terminated using crimp-on lugs or even fit some tails onto it using spilt bolts (with insulating covers), if it copper coated aluminium I'd definately recommend using short tails of standard copper wire to terminate. It really depends what you're terminating it into.
View attachment 111340

As stated above I'd suggest you get some help at least with the design, especially with the earthing which is critical.

Is the concentric cable solid copper or copper coated aluminium? Some pics would be helpful. Concentric cable shouldn't be used buried direct, it's not an armoured cable so it's prone to underground damage from stones, rocks and subsidence. Concentric can be terminated using crimp-on lugs or even fit some tails onto it using spilt bolts (with insulating covers), if it copper coated aluminium I'd definately recommend using short tails of standard copper wire to terminate. It really depends what you're terminating it into.
View attachment 111340

As stated above I'd suggest you get some help at least with the design, especially with the earthing which is critical.

Hi Marvo
Thanks for answering my original question with the split bolt clamps. I must say I expected an insulated housing with provision to clamp the center conductor and the spiral outer to give provision to then connect tails to it.
Thanks to all for their comments. and see attached picture, it does have a raw aluminium central conductor.
It's a bit disappointing not to be able to use it as we have a nearly full drum.
I assume the spiral outer finds it's way back to the star point of a pole transformer somewhere across the fields TP&N

 

[Marvo]

Yeah.... I've never seen that particular concentric arrangement but I'm not UK based and it's something we don't get locally. There are special crimp on lugs for single solid aluminium conductors, also mechanical compression ferrules with an internal tongue and shear screws. There's a cleaning and deoxidising process you use and also a special grease you apply that prevents corrosion in the ferrule or lug, I've still got some somewhere but I haven't used it in decades and I can't remember the name of it..

If I was using this as a buried cable I'd probably bring it into a connection box at both ends where it surfaces and go from there to the DB using SWA or some other kind of cable thats easier to run and easier to terminate using standard methods. I think most of the reasons not to use it have been mentioned already but very wide bend radius limitations is another, also high loop impedance and fault disconnect times if it's a long run might be a couple of others to add to the list.

 

[pc1966]

Yeah.... I've never seen that particular concentric arrangement but I'm not UK based and it's something we don't get locally.

Is is a DNO-only cable here, while the split concentric can be used, the type shown here has a combined N & E and that is not permitted in an installation under UK wiring regs, but it is allowed for the DNO who work to different rules.

AFIK they used to have aluminium outer as well, but that was prone to corrosion and open circuit which is REALLY bad for the PEN conductor, so now it seems copper outer for corrosion resistance, but aluminium for centre conductor for cost/weight. As you say, for Al you have special care for terminations and it is far more brittle than Cu.

I looked in to using 4C 50mm Al cable once but eventually decided the cost-savings were just not worth the hassle for the length needed (around 120m).

 

[nicebutdim]

It's definitely an odd type of cable to have a drum sitting around.

 

[DPG]

It's definitely an odd type of cable to have a drum sitting around.

Wonder where it came from?! Not the local wholesalers I'll warrant.

 

[oscar21]

Probably had a dodgy meter move and got some cable off him as well. I've only ever tried to use it once (split con) and didn't like it, unless the central conductor is going into a round hole like a henley block it doesn't grip properly in say something like a main switch terminal which are mostly square, the conductor won't crush at all so the surface area isn't much.

Going back to the original post though, I think the OP said something about an earth rod so he would only be exporting the L&N so technically he wouldn't need the earth core anyway.

 

[timhoward]

Going back to the original post though, I think the OP said something about an earth rod so he would only be exporting the L&N so technically he wouldn't need the earth core anyway.

While it 'would work' it can't be installed to regs without a CPC, and that's before we get into impact protection considerations.

 

[picklesquirt]

Is is a DNO-only cable here, while the split concentric can be used, the type shown here has a combined N & E and that is not permitted in an installation under UK wiring regs, but it is allowed for the DNO who work to different rules.

AFIK they used to have aluminium outer as well, but that was prone to corrosion and open circuit which is REALLY bad for the PEN conductor, so now it seems copper outer for corrosion resistance, but aluminium for centre conductor for cost/weight. As you say, for Al you have special care for terminations and it is far more brittle than Cu.

I looked in to using 4C 50mm Al cable once but eventually decided the cost-savings were just not worth the hassle for the length needed (around 120m).

Hi All,

Thanks for all your comments about this odd ball cable, found in a barn and no idea where it came from.
Decided to abandon it and to stay with the 30A supply end fuse and use 4mm swa.
For you chaps who live and breathe the regs is it ok to use twin and use the armour as the earth. At least the twin I have found has the correct single phase colours. Most 3 core has only phase colours.
Many thanks for your thoughts

 

[MJPD29]

Hi All,

Thanks for all your comments about this odd ball cable, found in a barn and no idea where it came from.
Decided to abandon it and to stay with the 30A supply end fuse and use 4mm swa.
For you chaps who live and breathe the regs is it ok to use twin and use the armour as the earth. At least the twin I have found has the correct single phase colours. Most 3 core has only phase colours.
Many thanks for your thoughts

For us chaps who work within the rules, we tend to sleeve cores to suit ourselves.