Extraneous Parts Earth sizing

[Neon Vagabond]

Customer wants to take power from an existing garage (with board) to supply a greenhouse, to be fitted with ip rated switchgear.
The greenhouse is made of aluminium.

Since this is an extraneous conductive part am i right in thinking that the aluminium frame would require a 10mm bond at least? (tncs supply)

Garage is fed with a 4mm armour (3core)

Should i still take the supply from the garage but run in a seperate 10mm earth from the house MET?

 

[Strima]

Why not TT it?

 

[Neon Vagabond]

i dont know, ive always had a bad feeling about TT

 

[Neon Vagabond]

Im also not really sure whats down there

 

[Dave OCD]

It isn't necessarily extraneous if mounted on a concrete plinth, I'd test it.

 

[Wilko]

What's being put in the greenhouse? Maybe an IP rated outlet just outside it would allow use of garden tools etc.

 

[Neon Vagabond]

the guys wanting a light, lightswitch and 2 sockets. He says he plans on putting a 1kw heater in there as well (plugged in)

 

[Neon Vagabond]

It isn't necessarily extraneous if mounted on a concrete plinth, I'd test it.

not heard about this...

 

[Dave OCD]

I'm surprised you're not aware, but if the metalwork tests at >22kOhms to earth then it isn't extraneous.

 

[buzzlightyear]

not heard about this...

Concrete slab - Wikipedia - https://en.wikipedia.org/wiki/Concrete_slab

 

[Neon Vagabond]

Concrete slab - Wikipedia - https://en.wikipedia.org/wiki/Concrete_slab

just checked the pictures, theres no slab, its right on the dirt.

Ive looked into to this >22k ohm scenario but what about if the guy is goin to be watering his plants in there, will that not increase the chances of it being under the 22

 

[buzzlightyear]

the guy wants a search lights, and 2 shot guns . He says he plans on putting a 11kw electric chair in there as well (plugged i

bright eyes ..

 

[Baddegg]

Concrete slab - Wikipedia - https://en.wikipedia.org/wiki/Concrete_slab

H aha ha aha you kill me buzz! Ha ha

 

[buzzlightyear]

just checked the pictures, theres no slab, its right on the dirt.

what ,now I'm now on the floor p**ing my self laughing .

 

[Dave OCD]

I'm sorry I don't know what else can be said.

 

[Neon Vagabond]

I'm sorry I don't know what else can be said.

would you like to elaborate on what yous are talking about?

you've mentioned this concrete plinth idea. (which ive never heard of)
Why would an extraneous conductive part not need earthed if sited on a concrete plinth? Wheres the logic?

 

[sinewove]

would you like to elaborate on what yous are talking about?

you've mentioned this concrete plinth idea. (which ive never heard of)
Why would an extraneous conductive part not need earthed if sited on a concrete plinth? Wheres the logic?

Because,combined with the human body,a res of 23k would only allow a current of 10ma to flow which is considered safe.

 

[Neon Vagabond]

Right that makes perfect sense, thanks.

But, as ive already said, (to much hilarity) the greenhouse is not on a plinth, so why would this apply?
Am i missing something here? (apart from a brain)

 

[davesparks]

It's a metal framed building with the frame exposed, in my opinion to follow the rules strictly your only option is to set up a seperate TT earthing system.

 

[Intoelectrics]

Right that makes perfect sense, thanks.

But, as ive already said, (to much hilarity) the greenhouse is not on a plinth, so why would this apply?
Am i missing something here? (apart from a brain)

Because the concrete plynth may offer more insulation resistance against the path to earth, than if the metal frame was in direct contact to the earth. Just test the frame as already been mentioned if its greater than 25kohms (a safe bet value) then it would not be considered extraneous. But if you find that the tested value is very close to the minimum accepted value (22kohms) and environmental conditions might reduce this value further then its your call.

 

[Neon Vagabond]

so what about, i take the tncs supply from the garage (2.5mm 3 core armour). This would supply earth to all of the circuits only. And then seperatley i connect a 10mm earth onto the greenhouse frame and then into a earth rod?

 

[Wilko]

No good I'm afraid.
By earthing the frame with a local rod you ensure it is an extraneous conductive part and so needs connection back to main earth terminal. Or TT'ing. See @davesparks post #19 .

 

[Neon Vagabond]

basically im trying to get out of digging a trench back to the house (many reasons).

But my other problem is that inexperienced with "tt ing" something.
So im just supposed to run a live neutral from the garage (unearthed) to the greenhouse, then stick a 30ma board/earth rod.

or do i earth the armour at the garage side but leave the greenhouse side unconnected to its respective earth terminal.

 

[telectrix]

or do i earth the armour at the garage side but leave the greenhouse side unconnected to its respective earth terminal.

yes. earth armoured at supply end. TT the greenhouse. bond the metal frame if required.

 

[Neon Vagabond]

or do i earth the armour at the garage side but leave the greenhouse side unconnected to its respective earth terminal.

yes. earth armoured at supply end. TT the greenhouse. bond the metal frame if required.

top man, cheers