Termination issues

[cameo]

Just had a new supply fitted to the corner of a field in my small holding, PME 100A SP Located in a small brick building. I need to get a 60 amp supply over some distance about 300 metres, for which I will need 120mm2 xlpe 90 SWA. Buried direct in ground

Anyone got any ideas what switch gear to use at either end capable of this size cable. And then to scale it down and feed a CU at the other end?

 

[ricicle]

How did you manage to work out your cable requirement but don't know which switchgear to use

 

[cameo]

Worked out by using voltage drop calculation from bs7671 including grouping factors and the fact i'll have ill have slightly higher Soil Thermal Resistivity (its going in 1m deep, to avoid land work issues)

 

[GaryMo]

Why XLPE? Can't you use 95mm² SWA?

Volt-drop = (300 x 60 x 0.5)/1000 = 9v

Maximum allowed for a distribution circuit is 5% or 11.5v, the above gives 3.91% or 9v

 

[cameo]

Ok i have no idea why i wrote xlpe!!!! must have it on the brain!

Could get away with 95mm2 Swa but i have further voltage drop issues to consider when it comes to the final circuits connected to the Cu unit im terminating into.

But im still not sure what to do with the switchgear side of things?

 

[electronicsuk]

A switchfuse with large terminal chamber should take care of the DNO supply end, and a similar setup with a SP+N switch at the other end should take care of getting back to a sensible conductor CSA to feed the consumer unit.

Another question is whether or not exporting a PME earth is sensible over those sort of distances... opinions?

 

[sparkyspike]

You're looking at lug connections rather than tunnel connections. 200A switchfuses can take M8 lugs. At the CU end though, I'm not sure. Maybe a small panel board?

 

[cameo]

im not going to terminate the earth at the cu end, im gonna TT it, for the reasons, distance etc

 

[hairyben]

Cheap option is to make a custom enclosure from a steel cabinet with some terminals inside. Crimp lugs bolted together or to a short busbar would be a normal way to join such different cable sizes, assuming you're dropping to say 25mm. Once you get to this kind of size stuff tends to be more bespoke.

Off the shelf option is to use a 200 or 300 amp switchfuse, check with the manufacturers who should be able to advise it's compatibility with 120mm and 25mm

 

[cameo]

so i need a 60amp switch fuse with 120mm2 terminals? know any contenders??

Like the idea of steel cabinet and small busbar option could then run my 25mm tails straight through to CU.

But decided to carry the PME earth right through and add earth rod also "belts and braces"

many thanks

 

[RF Lighting]

Are you aware that in BS7671, TN-C-S supplies are not recommended for some agricultural and horticultural installations?

 

[cameo]

Yes, the land is neither, its just a piece of land i hope that one day will have house on it, right over the little brick shed that houses the new supply which i hope one day will be under my stairs.

Id rather have TN-C-S now than call them back later, any STORAGE SHEDS, that i build for the moment If I want to distribute power to them. I could make them TT by not terminating the earth? But since none of them have any services (water/gas/oil), and there is strictly spaeaking no equipotential zone yet, I wont worry. Anyway I think 120mm2 CSA would easily satisfy my bonding needs in the future (im putting in three core)

Incidentally RF do you have any suggestions on what switchgear i could use??

 

[RF Lighting]

As was suggested earlier.

Either a 200A or 300A switch fuse at each end.

Fit probably 63A or 80A fuse(s) in the supply end to provide discrimination from the suppliers fuse, and solid link(s) in the CU end.

Using a switch fuse with a big rating will have bolt on connections for lugs, and lugs are available for every size cable.

You will be able for example at the supply end, to lug 25mm² tails onto the incomming side, and your 95.0mm² onto the outgoing side.

It also provides you with a handy isolator at the CU end.

Also be aware, if you did decide to TT the supply to your new house, the cable must not be terminated into a metallic enclosure, until it is RCD protected.

 

[cameo]

cheers for that, sounds good,

I realised that before, I will have no choice but to carry on the PME earth, it would be pretty difficult to terminate that size of cable into anything but metallic and that would be an earthing issue anyway, unless I snook an s' type rcd into the 25mm tails at the supply end (to discriminate between my consumer unit) , now its getting complicated!!

cheers anyway

 

[RF Lighting]

S'pose you could fit a 100mA type S in the supply, but exporting the earth is the best bet.

I'm not sure of the ohms per meter value for steel wire on a cable that size.

You may need to run a copper earth in parallell with the SWA to achieve the desired Ze values.

Is the supply SP&N or TP&N?

How many cores SWA cable are you planning on using?