Supply to Greenhouse

[rick]

Would appreciate any advice taking power to a greenhouse. Greenhouse about 10 metres from house. Intended loads are lighting and sockets for 3 kw heater and propagator. Supply is TN-S. Run 3 core SWA from FCU off downstairs ring main (on RCD board) inside house with armour tied to earth at house end only. Inside GH terminate SWA in double socket and from there take rubber cable to light switch and on to luminaire mounted at apex of roof. All fittings inside will be IP65 / IP66.

Does that sound about right, and if not any corrections happily received.

Also, does the GH need to be bonded, and if so, bond to armour, CPC or TT spike? I have read many conflicting opinions on this...

 

[spark 68]

I'm puzzled, S68. Why would TTing the greenhouse mean that an additional RCD is required? Just trying to understand your reasoning. Thanks.

Hi HS,

Good point, I was assuming (fatal error lol) you would need an RCD for the TT supply, the OP was talking about feeding the GH from a ring final in the house which is already RCD'd at 30mA.

I missed a bit (thanks) I was looking at the drawings in GN7, which says that all circuits other than socket outlets need at least 300mA RCD protection for effective protection against fire, but final circuits protected by 30mA will be deemed to also protect against fire.

I was assuming that he would need two RCD's for the two different earthing systems, the drawings in GN7 show the feeds either from a non RCD'd supply or a 300mA RCD supply.
It may be possible to use the house 30mA RCD on the TT side, but this could cause unwanted tripping of the main house supply.

Edit: another point if TT is used, then DP switching also has to be used for everything.

 

[kenny james]

i would assume an additional RCD would ensure that the circuit would not interfere with the house circuits if it tripped. the TT question tbh not sure.

 

[Knobhead]

To be honest I’d go for a separate feed from the CU, be it RCBO or a RCD in a housing.
Just going from experience with the father in laws GH it did trip on a regular basis. Fed from the bedroom FRC I can see you being late for work when the ring RCD trips and you’re happily in the land of nod with no alarm clock.

 

[rick]

Hi Spark68

Many thanks for all your help here – I think I get myself a copy of GN7 tomorrow!

If using the armour of the SWA for the bonding, how do I work out if the CSA of the steel armour is sufficient – is there a table for that somewhere as if so I do not know of it. And if the armour is sufficient to use as the bonding what when it gets inside first adaptable box where it exits the building – connect to CPC or take separately back to MET ?

Ref earth Rod / TT idea… if I understand correctly, you are suggesting using the house earth for the armour of the SWA and an earth rod for the GH frame and the earth to appliances in the GH? If this could cause nuisance tripping, why could one not use the earth rod to earth the frame of the GH and use the house earth for earthing the appliances? A post above even suggested doing both and connecting everything together … so you can understand the confusion!

Kenny, hi – if both RCDs are 30mA surely either could trip first?

Also I’ve just looked at the IP ratings of the heater in question – IPX4 – Reg. 705.512.2 states IP44 as a minimum unless enclosed… and this is a £200+ product sold specifically as a greenhouse heater designed to be hung from the ceiling or stood on the floor – no mention of needing to enclose it…!

 

[spark 68]

Hi Rick,

I suggested TT may be a possibility to save you having to bond back to the MET in the house, as you could then use a smaller armoured supply cable, as then the armour would not be required to be large enough to suffice as an equivalent to 10mm copper, also it would only need to be two core, this could have significant cost savings.

 

[rick]

To be honest I’d go for a separate feed from the CU, be it RCBO or a RCD in a housing.
Just going from experience with the father in laws GH it did trip on a regular basis. Fed from the bedroom FRC I can see you being late for work when the ring RCD trips and you’re happily in the land of nod with no alarm clock.

Hi Tony - RCBO - hadn't thought of that and like the idea. Despite it being a nearly impossible cabling job I am coming around to the idea of putting it on it's own circuit.

Although I do have a nice old wind up alarm clock with bells on the top which would wake up Rip Van Winkle !

 

[spark 68]

Rick,
I cross posted with you, see post #20.

I would definitely consider putting it on it's own supply, with a small extra CU in the house and henly off the tails, use a small shower type CU and consider a 20A or 32A MCB and RCD or RCBO, with a seperate 6A and 16A MCB in another small IP rated CU in the GH.

 

[rick]

hi again spark68

Why do you suggest a separate CU and not RCBO off main CU (there is room for it)

TBH I don't really understand fully the TT route as it is not something I have much experience of - in particular why not stick a spike in at the GH end and connect it to both CPC from house and armour of SWA?

 

[spark 68]

Thanks for the post chaps.

And in any case no way to run a separate circuit from CU on this one although I like the idea and would prefer that if it were possible.

Because you said it was not possible to run a seperate supply with the existing CU.

 

[rick]

Sorry I understand - I meant the cabling run was a right PITA not that there wasn't a spare way on the board.

 

[spark 68]

hi again spark68

TBH I don't really understand fully the TT route as it is not something I have much experience of - in particular why not stick a spike in at the GH end and connect it to both CPC from house and armour of SWA?

Rick have a look at this, this is for a caravan pitch, but the idea is the same only you would have your RCD/RCBO at the supply end

 

[rick]

Much thanks for that - had better head off upstairs now but will get back to you - and will get GN7 too in the meantime.

 

[rick]

Many thanks to all who have commented so far… I said I would post again as I don’t like abandoned threads. Of course any comments / discussion are more than welcome…

In terms of supply

The most elegant solution: split the incomer to a Henley block, main switch then to SWA. Take SWA to greenhouse and terminate in IP rated CU with RCD, 16A MCB for Socket and 6A MCB for light (or better still two RCBOs)

The next best solution: 16A RCBO in existing CU, from here inside house in T&E to exit point, through wall into adaptable box and convert to SWA. Terminate in socket and provide further power to light by means of switched FCU fused down to 5A.

The least best but simplest to install solution: Spur off power circuit with FCU fused to 13A. Through wall and into SWA as per second solution.

I don’t see the merit in having a separate IP rated CU in the GH except in case of solution 1 as any RCD tripping could trip either RCD as there would be no discrimination (the house CU end having to be 30mA for the T&E run)

However the earthing and bonding arrangements were the original point of this thread, separate post below.

- - - Updated - - -

Earthing arrangements

(note that house earthing arrangements are TN-S – for TN-C-S this may well be different and TT the only way to go)

1. Put earth rod in at GH end. GH electrics (irrespective of supply type as above post) earthed via rod solely i.e. not connected to house earth. Armour of SWA connected to house earth at house end only. GH bonded to earth rod.

2. Export the house earth. For the purposes of earthing, this would be the third core of the SWA (not the armour). The armour would be connected to earth at the house end only. For the purposes of bonding, run G/Y 10mm cable back to the MET at the house supply.

The more I think about it the better I like idea 1. The floor of the GH will be earthy, and I think it’s better that the frame is closer to that earth than the MET earth, which could be at a different potential.

So why should one not export the house earth AND put in an earth rod and connect them together, as suggested by strima and somersetsparks? If so, should we connect the rod to the house exported CPC and/or the armour of the SWA or both?

If we connect the GH frame to the earth rod, and also to the house earth, surely the frame becomes an extraneous-conductive-part (as it would be connected to the house earth and also be well below 22KOhm to MET) and would therefore also require bonding back to the MET. So let’s suggest we do that – but then we have parallel paths to earth which could potentially induce eddy currents in the CPC…?

So I think the best solution is to TT the greenhouse and leave it separate from the house earth. But I am interested to hear opinions on this – if we TT the greenhouse, is it better to separate this from the house earth or not?

A further thought

1. The base of the GH is steel, the frame aluminium, and the bonding copper. Bonding between the three might degrade the quality of the connection over time?

Well, if you’ve made it this far thanks and I look forward to any further discussion.

 

[spark 68]

Hi Rick,

As I said in an earlier Post,

If I was TT ing it, I would segregate the earthing system, the reasons for this are:

No 10mm copper equivalent back to the house Met required, if bonding is required.

If bonding is required, then this can be carried out locally at the GH end, and is much simplified.

You could use 2 core armoured.

I don't really see much point in combining the two systems, as your rod at the GH would need a very low Ra to be of any use, the TNS Ze max is 0.8 ohms, so an Ra much above 1 ohm is going to have very little effect overall, and you are still going to have all the conditions required of continuing the house TNS system anyway.

I would keep it as one system or the other at the GH, both have advantages and disadvantages.
By combining it you are going to make it more complicated than it need be.