Yurt supply

[Soulsurfer]

Hey all, just been asked by someone to go see his ff's mums place where she needs a supply for a yurt is it semi-permanent tent thing in the garden. She does work from home and sounds like some kind of therapy.

It's staying as a permanent structure in the garden but am unsure if this would be under the umbrella of perhaps 708 or 721 possibly in BS7671 BYB ?

Just after a rough idea of what kind of thing you would look at or not as the case may be ?
It's only one so not technically a camping park, there has apparently been a supply exported to a shed already with a small sub board, heater and lights etc. that's being used as an office so I can look at a feed from that possibly when I visit next week but never been asked to supply a Yurt before.

Thanks for any info.

 

[Richard Burns]

If you are looking at a tent like structure then the likelihood is that the floor will be in contact with the outside earth.
I.e. the floor will not provide an insulating layer to separate the inside of the tent from earth.
Therefore you would be looking to have full 30mA RCD protection on all circuits and to ensure that the installation earth is as close as possible to true earth.
Possibly providing a local rod to the MET to lower any possible differences.
Obviously TNCS supplies are not recommended to supply an effectively outside location due to potential loss of supply neutral.
TNS and TT supplies should not have this limitation.
I would probably be looking at having a specific earth for that part of the installation.

 

[Soulsurfer]

Thanks Richard, I get the dropped N from TNCS etc, but do not know what type the supply is of yet and have been down that route before with true ground etc.. But do I need to keep earth which would be connected to rod in garden seperate from the MET as was the case from a hot tub install which was TNCS supply ? Or take the rod earth back to the MET ? & not export an earth from the house either way ?

Thanks again.

 

[Richard Burns]

That is where you have to make your professional judgement on the safety of the installation and the risks involved.

Having an earth rod connected to the MET on a TNCS system means that if the neutral supply was lost then the installation would still have an earth via the rod (but might also be earthing several other properties through this rod). Also this rod would provide a local earth reference that would be closer in voltage to the earth around the yurt than the supply earth.
Having an earth rod specifically for a TT system for the Yurt, i.e divorced from the supply earth, might be my choice in this case.
However it would depend on how much power (volt drop is increased by increasing current) was being used and how that power was employed: sockets, lights, water heaters, etc. to see what the risks may be.

 

[UKMeterman]

Hi,
Have a look at BS7909, but you could get a suitable sized (32A?) commando socket with built in RCD and provide a local rod earth to it, you now have a TT supply for the Yurt.

 

[davesparks]

Depending on the proximity of the shed to the Yurt you may be better off putting them both onto a TT system if you feel that it is necessary.




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[Soulsurfer]

Thanks all, still yet waiting to view the job but am going with the TT separate for the Yurt as after reading up Guidance note 8 it's pretty well shown and examples shown exactly along the same lines.

 

[Soulsurfer]

So finally had the call about the power to this yurt ! It's a big one too ( the Yurt ! )
Lady only called as the two extension reels she was feeding across garden and pathways to the tent from the shed / office were tripping the breaker all the time !
So basically an ex friend spark has run from TNC-S supply in main house out to a shed and wired it up via small c/u and rcbos for lights and computer / printers low loads really.

She now wants rid of extension leads for yurt power which works out 45 mtrs of 4mm swa and split into a few socket outlets inside and also some lighting. Then 1.5 swa for couple of LED floods on a wall to light the driveway / parking for clients.

Now I have only ever attempted one TT and am therefore apprehensive with it as far as they go.
In theory if I supplied on the 4mm swa and used an internal core as cpc then only connected at shed end but separated from the yurt end and then the yurt end was earthed to a rod for the TT system would this seem an option ? To avoid the PME issue of exporting PME and or dropped PEN conductor etc..

 

[Richard Burns]

Not knowing the layout or the cable runs means it is not clear how you want to run the supply.
However, check your distances and volt drop requirements for the size of cable. At 45m for sockets, lights and floodlighting I might move up to 6mm². Especially if the driveway has long lengths of cable.
I think you want to maintain the TNCS to the shed and along the Yurt supplying SWA and then not connect the armour or cpc to the earthing system at the Yurt end.
The earthing system at the Yurt would be supplied by a local earth rod.

If this is the case then this sounds OK.
Make sure your earth rod has a nice low resistance.

 

[HandySparks]

Couple of questions:
Is 4mm² big enough to keep VD within limits over a 45m run? (Will it be supplied from near the origin of the installation?)
Why use 3 core SWA if you're TTing the far end?

(I see RB is on the case.)

 

[Soulsurfer]

Thanks for the input guys, basically the shed itself is around 5 mtrs from the service head, concrete garden wall to cleat the swa to running alongside the yurt at bottom of garden. So in reply to RB I was going to connect the steel armour to the yurt end box or similar possibly small c/u then separately connect sockets cpc etc to a separate single earth and use that to run back to the rod. Biggest issue locally is very sandy / granite based soil and extremely hard to get low readings for earth electrodes ! Yet to calc all v.drop etc before quoting. Thanks

 

[Soulsurfer]

Looks like around 9.9V Vd on the 4mm swa.

 

[Richard Burns]

The earthed armour of the SWA should be inaccessible at the yurt end so terminate the SWA in a stuffing gland at the CU/box or into gland normally into a plastic box prior to your CU/box.

Volt drop is too high for lighting at 9.9V

 

[Soulsurfer]

The earthed armour of the SWA should be inaccessible at the yurt end so terminate the SWA in a stuffing gland at the CU/box or into gland normally into a plastic box prior to your CU/box.

Volt drop is too high for lighting at 9.9V

Great that helps, only calc'd vd for the main swa run then was thinking of splitting off 2-3 sockets and fuse spur to lower the feed to lighting via switch in yurt afterwards, or would you fuse down the 1.5 swa from a separate mcb in a small cu in yurt as better practice ? There is a small electric heater in yurt permanently so main reason for the sockets there.

 

[Richard Burns]

You need to up your csa if the drive lights are fed from the yurt, 10m of 1.5mm² SWA running less than 230W of light would be OK with 6mm² SWA supply to yurt.
Splitting the circuits down is up you, when having an earth rod it is often easier to have a CU so you have an easy earthing connection for the rod earthing conductor, but if using DPSFCUs for the lights then I would tend to use one for inside lights and one for outside lights so you can isolate them separately once the outside lights get full of water!

 

[Soulsurfer]

Just under 3V vd for separate 1.5 swa for lights outside fed from yurt.

Ok so small cu in the yurt sounds like the best way to go, outside lights most likely going to be 2 x 20 watt LED's or similar and around 15 mtrs or so of 1.5 swa for them. At present she has low voltage led festoon lights just plugged into extension reel for inside lighting in yurt. Only need to cover 20A max for everything from mains to yurt. Likely less than 18A total really.

 

[Soulsurfer]

You need to up your csa if the drive lights are fed from the yurt, 10m of 1.5mm² SWA running less than 230W of light would be OK with 6mm² SWA supply to yurt.
Splitting the circuits down is up you, when having an earth rod it is often easier to have a CU so you have an easy earthing connection for the rod earthing conductor, but if using DPSFCUs for the lights then I would tend to use one for inside lights and one for outside lights so you can isolate them separately once the outside lights get full of water!

Morning Richard, just a quick ask, if a small consumer unit fitted in yurt to supply power & seperate lighting out to driveway combined with Rcbos x 2, would I still best feed the sub main swa out to yurt with a time delayed 100mA Rcd ? As at moment circuits in shed are all on own rcbos at 30mA and I am going to test if I can TT the yurt end consumer unit first with tester & electrode test probes etc.. because if I can't get low enough readings for TT electrode I'm going to have another problem.

 

[Richard Burns]

If the SWA is supplied from a TT supply then you would need to fit the 100mA RCD; if, as I expect, the SWA is supplied from the TNCS supply then the 100mA RCD would not be required, but could be fitted if you wanted to have an additional back up.

 

[Soulsurfer]

If the SWA is supplied from a TT supply then you would need to fit the 100mA RCD; if, as I expect, the SWA is supplied from the TNCS supply then the 100mA RCD would not be required, but could be fitted if you wanted to have an additional back up.

Thank you for that, and yes supply via TNCS sub that feeds cu in shed / office then out from that to yurt cu and TT at that end if results are low enough when testing rod / rods

 

[The Ghost]

Btw you can get soil conditioning agents, done by Furse I think that will increase conductivity for the sandy soil if you are hard put to get low ohms on your rod. Soil Conditioning Agent - Kingsmill Industries (UK) Ltd - http://www.kingsmillearthing.co.uk/products/soil-conditioning-agent/ for instance.

 

[Soulsurfer]

Btw you can get soil conditioning agents, done by Furse I think that will increase conductivity for the sandy soil if you are hard put to get low ohms on your rod. Soil Conditioning Agent - Kingsmill Industries (UK) Ltd - http://www.kingsmillearthing.co.uk/products/soil-conditioning-agent/ for instance.

Thanks Vort that's spot on

 

[Soulsurfer]

Just a quick check, as getting sorted today. If submain swa is indeed fed from TNCS source in the office cu and then out to yurt and at that end a metal cu with 2-3 circuits power, interior lights and separately 2 x 20W L.E.D. exterior lights all with rcbo protection and earth from TNCS divorced there through a plastic enclosure or similar and then TT rod earth for the cu in yurt. Would I need a simple mcb for the swa sub main out to yurt, or would that require more ? as only time delayed rcd's I can get hold of locally are double pole main switch type ones which would be not really what I need.

 

[Richard Burns]

The SWA is a distribution circuit on a TNCS supply.
The cable is installed in such a way that it is enclosed by an earthed metal covering that is suitable as a cpc. Therefore the regulations do not require any sort of RCD protection to this cable.
An MCB will be fine (of suitable rating).

It is very hard to find any time delayed RCDs for single phase that are not double pole 2 module breakers.
I think I have seen some mentioned but never used anything other than 2M DP.

 

[Soulsurfer]

The SWA is a distribution circuit on a TNCS supply.
The cable is installed in such a way that it is enclosed by an earthed metal covering that is suitable as a cpc. Therefore the regulations do not require any sort of RCD protection to this cable.
An MCB will be fine (of suitable rating).

It is very hard to find any time delayed RCDs for single phase that are not double pole 2 module breakers.
I think I have seen some mentioned but never used anything other than 2M DP.

Thanks so much for that really appreciate taking your time out. It was a grey area for me as things have kind of gone Rcd/rcbo crazy here with most I know in the trade locally & didn't want to get anything pear shaped on the start ! Thanks again such a help.