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So I have been asked to look at a job which includes providing 2 submains (one for a small dwelling, the other for a small workshop/garage). The supply at the farm is PME/TNCS from a pole mounted transformer.

The back story; whilst waiting for planning permission for a dwelling the customer decided to place a static caravan at his father-in-laws farm and requested a 100A single phase supply to an external block built meter cupboard (a couple of metres from the static caravan). The 100A supply was planned to be used in the dwelling which would be built adjacent.

Three years has passed now, and the plans changed considerably. The dwelling is now much smaller, and it's power consumption likely to be significantly lower (biomass heating, LED lighting only, smaller and less final ring circuits, etc.) The customer has now also converted one of the barns/pig houses in to a small office/workshop/garage to basically look after the farm machinery (it's currently powered off a 2.5mm T&E from the original farmhouse- typical farm!!) The customer also wants to keep the static caravan as guest accommodation now that the proposed new farmhouse is smaller). Both the new farmhouse and the workshop are approximately 100metres away from the cutout and joined by a party wall.

Despite what seems like a significant load, looking at the current overload protection/current consumption characteristics, I think the 100a fuse might just work. I propose on using 25mm SWA as the submain to the workshop where I will terminate it in a locked cupboard with tails out to the workshop CU, before continuing the 25mm submain to the new farmhouse CU (when constructed).

I'm trying to thrust bore the SWA under the other barns/sheds on the farm, but I'm not sure it will be possible. Therefore it would have to run 20m underground ducted, 60m through 3 of these buildings (clipped direct) and 20m underground again. My question is, would I be best using the PME to protect the SWA (isolating it from the workshop/house CU) and using electrodes to provide TT at the house and workshop as it is a farm with livestock? I will leave the TT installation at the caravan of course. I believe I will also need to protect the submain with a 300ma RCD for fire protection due to it running through farm buildings, despite the supply not being strictly, "agricultural". I will of course protect the submain from overcurrent with a FSU adjacent to the meter cutout.
 
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  • #2
View attachment 49091 View attachment 49091 Picture of current meter cut out/submain to static caravan.

Aerial photo showing static caravan/meter cutout in yellow circle, with the yellow line the submain, red circle the workshop area, and red blob the proposed new farmhouse. Blue blob is the existing farmhouse on a separate meter and supply (but same pole mounted transformer supply.)

View attachment 49089

View attachment 49090

Caravan Cons unit - general 2.JPG

20190501_000402.jpg
 
Last edited:
D

Deleted member 26818

Thought that was my house for a minute there.
 

suffolkspark

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Arms
Esteemed
Only scanned over that... But 25mm swa on a 100m submain run? Whats your intended fuse size and then check your volt drop......
 
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Only scanned over that... But 25mm swa on a 100m submain run? Whats your intended fuse size and then check your volt drop......
80A Fuse but the DNO have provided test data showing there is 252Volts at the cutout (seems very high!) Therefore I've allowed a slightly higher volt drop than the normal 5% (about 6%). There are no current hungry devices (no lathes, industrial welders, etc). The workshop is just a dry area to tinker with the tractors really, and the house has no electric heating, all LED lights, etc.

Steer clear of the PME the 300mA rcd with be OK for the sub main protection
Would I not need any form of earthing to the SWA sheating at all? Even an electrode at the cut out (though the equipotential zone of the electrode may 'overlap' for want of a better word with the static caravan electrode. I suppose all consumer units at the end of the submains will be protected by 30ma RCD's and TT'd, so the sheating will not be required for the earthing or fault protection anyway.
 

bill01803

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Arms
I assumed there was an electrode as there is a bit of g/y going somewhere, if not put a stake in or bury a length of bare 25mm in a trench
 
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I assumed there was an electrode as there is a bit of g/y going somewhere, if not put a stake in or bury a length of bare 25mm in a trench
Talking to the customer, he believes that the previous installer put a stake in to TT the SWA submain to the static caravan (submain earth isolated from the caravan C.U.), and then a second stake was put in for the caravan C.U earthing and bonding of the chassis. I was unable to see the stake for the C.U. earth/bonding as there is a veranda/decking all the way around. I think I will have to chuck the chickens out and crawl under in to the abyss (great!!!).
 
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I would be interested to know peoples opinions on whether to link all earth electrodes, or isolate them at each C.U.

I'm not sure whether that is opening a can of worms....
 

bill01803

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Arms
Link them
 
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