Aluminium Greenhouse on Steel Base

[taycast]

My aluminium greenhouse is set on a steel base in the ground. It has a mk masterseal socket supplied by 2.5mm 3 core swa taken from a 13 amp fused spur from the kitchen ring final in the house a few meters away. It is used for one propagator.

Reading other posts, and excuse my lack of knowledge of the terms, I see I may need to earth the greenhouse frame as it is an extraneous conductive part , in contact with the ground? Furthermore, I may also need a similar arrangement for my aluminium staging, which is portable? Is this incase it accidentally becomes live?

If so, would this be done by a local earth rod, or need to go back to the CU? If so, could the wire armour be used for this, or I see from other posts a 10mm2 earth wire may be needed?

I am DIY and have no intention of doing this type of work but just wanted to get an understanding before getting a spark in.

Supply is TN-C-S.

 

[ricicle]

Is the frame actually in contact with the steel frame bedded into the ground ?

 

[JACKC]

There is a figure of 22k ohm resistance for a lump of metal to be just a lump of metal and not an extraneuos conductive part. A simple insulation test to earth would confirm if this framework is a lump of metal or indeed an extraneous conductive part which requires bonding.

 

[taycast]

Is the frame actually in contact with the steel frame bedded into the ground ?

Yes

 

[plugwash]

I think it's important to first explain the basic principles

Earthing systems primerally exist to provide a path (either through the general mass of earth) for fault currents to make thier way back. This path is tied to earth via large electrodes at the transformer or generator because if it wasn't it could rise above earth in nasty ways.

In a perfect world that is all we would need to do, in the real world however nothing has zero resistance. So different "earths" can end up at different potentials which is rather bad if someone touches them at the same time. This is why when there are multiple earthed items (mains earth, utility services, burried metalwork) entering a property they need to be bonded together.

TN-C-S complicates things further. With TN-C-S the electricity supplier combines neutral and earth in thier wiring. This saves them money but creates a couple of issues. Firstly there can be currents circulating in the bonding during normal (non-fault conditions). Secondly a lost neutral on the suppliers side can send all neutral current down the bonding. This is why TN-C-S systems require extra attention to bonding (and particulaly sizing of bonding conductors) and why they are banned from supplying certain high-risk users (caravans and boats are banned by law and petrol stations usually don't use them either though I don't know if thats actually a requirement).

If the greenhouse is a reasonable distance from the house and there are no piped servies such that touching stuff connected to the house earth at the same time as the greenhouse is unlikely the easiest option is probablly to make it an independent TT system (earthing through a local electrode). If it's close to earthed metalwork on the house system or shares piped services I see little option but to run in an appropriate sized bonding cable back to the house earth.

As for the staging you will have to consider local site conditions where you set it up before making a descision on that.