Extraneous Parts Earth sizing

[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