411.3.1.2 (v) might apply though. "Exposed metallic structural parts."

Only if it's part of a building (see 1st phrase of 411.3.1.2 and my earlier comment).

   - Andy.

Isn't a greenhouse a BUILDING then? Often called an outbuilding by insurance companies. An outbuilding like a garden shed or detached garage or garden privvy. If the greenhouse is not a building then how would you define it Andy.

Z.

If the greenhouse is not a building then how would you define it

Perhaps a "garden structure" (at least in a domestic setting). BS 7671 gives us no definition to work with but my thinking is that it's rather unlike most buildings where it would be reasonable to try to create the equipotential zone that 411.3.1.2 is all about (reasonably dry, or non-conductive construction) . In most respects the inside of a greenhouse is much more like outdoors than indoors of most buildings.

There's a whole range from structures from tiny lean-to greenhouses (not much more than a tall cloche) to car ports to dolls houses that might well fall under some people's dictionary definition of a building, but nevertheless wouldn't be sensible to treat as such from a wiring regs point of view. We have to fall back on our engineering judgement.

    - Andy.

A greenhouse is a building, and although most come under permitted development classification, some require planning permission.

BS 7671 requires protective bonding to be applied within each building where ADS is used, and that includes structural parts of the building if they are  extraneous-conductive-parts.

Note 1: the protective bonding applies to each building; however, if a PME earthing arrangement is exported to a building or outbuilding, the PE conductor to the outbuilding should meet or exceed the minimum cross-sectional area requirements for the PME service of the intake of the premises.

Note 2: Depending on the DNO's local network characteristics, they may recommend against exporting PME earthing arrangements to certain metal-clad or metal-framed buildings or outbuildings. See G12/4 and guidance from your local DNO.

The only thing to decide is whether (depending on the structure itself) the metal frame is actually an extraneous-conductive-part. There is guidance for this. As has already been said, if there is a wooden footing frame or brick footing wall extending above ground level, effectively separating the frame from the ground, then the metal frame may not be an extraneous-conductive-part.

The key risk to avoid is for persons within the greenhouse having contact with both exposed-conductive-parts and extraneous-conductive-parts. Again, as has already been said:

(a) If the metal frame is not an extraneous-conductive-part, then bonding it may increase risks for those touching the greenhouse frame from outdoors if there is a fault. If it is not an extraneous-conductive-part, it may not be wise to bond it.

(b) If the metal frame is an extraneous-conductive-part, it should be bonded, but note that it will cause a slight ground potential rise around the building during a fault if it is main bonded, so the risk to someone touching the frame from outside is reduced.

The greenhouse supply is TT. And two R.C.D.s connected in series protect the installation. Any earth leakage will disconnect the supply in a very short time indeed.

Why bond the greenhouse?

P.S. on a dry sunny day the greenhouse may just be electrically  "floating", On a wet day it may be an extraneous-conductive-part. Oh for the wisdom of Solomon.

Z.

Well you could have a box with a relay in it that bonds it or not if the effective electrode resistance falls low enough to warrant it (20k ohms or so I reckon),  I suppose. I cannot see anyone bothering that much.

A more practical point is how much of an installation is there, and how is it installed - if it is a rag tag of hundreds of metres of flexes draped over sharp edged angle iron supplying grow lamps, or there is a lot of class 1 kit, the argument to earth and bond, so it is clearly not floating, is higher, as there is a credible path to a live chassis fault. If there are a couple of sockets that supply double insulated tools occasionally, then the off- earth condition is more likely.

Mike

Why not just connect the meatal frame to an earth electrode like the countryside oil heating pipes? Then the earth is earthed, the green house floor is earthed, and the outside soil is earthed, and the greenhouse frame is earthed and safely watched over by Mr. R.C.D. (X 2).

Z.

Zoomup said:

The greenhouse supply is TT. And two R.C.D.s connected in series protect the installation. Any earth leakage will disconnect the supply in a very short time indeed.

Why bond the greenhouse?

Because the greenhouse is a building, and, if the frame of the greenhouse is actually an extraneous-conductive-part, it is a requirement of Regulation Group 411.3, specifically Regulation 411.3.1.2, which is common to both TN and TT systems.

Quite simply, if the frame of the greenhouse were an extraneous-conductive-part, and the greenhouse contains fixed wiring, omitting the bonding means the installation in the greenhouse might not comply with BS 7671.

For clarity, if the frame is NOT an extraneous-conductive-part, then it does not need to be (and perhaps as discussed above should not be) bonded.

It also doesn't have to be connected to the rest of the TT system, or to the PME supply (if originally supplied from PME) - see below. However, the bonding between cpc's in the greenhouse, and the greenhouse itself (if an extraneous-conductive-part) is required by BS 7671.

Zoomup said:

Why not just connect the meatal frame to an earth electrode like the countryside oil heating pipes? Then the earth is earthed, the green house floor is earthed, and the outside soil is earthed, and the greenhouse frame is earthed and safely watched over by Mr. R.C.D. (X 2).

You can ...  provided it's connected to the cpc in the wiring (if it's an extraneous-conductive-part).

I'm not saying bond it to the PME.

THe greenhouse can have its own TT earth, but this must be protective bonded to the cpc's in the outlets in the greenhouse.

It does NOT have to be bonded to the rest of the TT system, just the cpc's in the greenhouse. It's still protective bonding to 411.3.1.2.

Do bear in mind, though, that in small curtilage properties, having too many earthing arrangements can cause an issue with Reg 411.3.1.1 para 2.

If the greenhouse contains a Dimplex Class 1 frost heater for use in the winter, and due to a fault the case becomes live, what then happens? And is the metal greenhouse frame involved?

Z.