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.

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.

411.3.1.2. How can the aluminium green house frame introduce "a dangerous potential difference?" I can't see that it can.

It is either floating or earthed. In both cases a harmless situation exists.

Z.

The reason for main protective bonding, and a look at the touch voltages involved, are discussed in Section 13 of Guidance Note 5.

Just walk it though - in TT systems, it's possible for the case of the equipment to reach Uo with Earth, until the protective device operates. If the frame is actually an extraneous-conductive-part, connected to Earth, it plays a part if someone is inside touching the heater and the frame, as the resistance is probably less than the person's footwear to 'Earth' ... same situation class I product just outside.

If the frame is actually an extraneous-conductive-part, it will act as an earth electrode in any case, helping equalize potential in the ground around it, helping reduce potential differences between the ground (from someone standing outside).

If not, then by bonding it, you introduce a shock risk to anyone standing outside, due to the fact it won't be in proper contact with the ground ... although that would be less if the person is not in contact with an exposed-conductive-part supplied from the greenhouse.

"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."

Some years ago B.S. 7671 had us bonding stainless steel sinks/draining boards in domestic kitchens. The makers even provided a small tag with a hole in it for the bonding cables as stainless steel is not easy to drill. That requirement is not now required. So, B.S. 7671 is not always right and its requirements not set in stone.

Are non bonded kitchen draining boards more dangerous or less?

Z.

Zoomup said:

Some years ago B.S. 7671 had us bonding stainless steel sinks/draining boards in domestic kitchens. The makers even provided a small tag with a hole in it for the bonding cables as stainless steel is not easy to drill. That requirement is not now required. So, B.S. 7671 is not always right and its requirements not set in stone.

Yes, that went out more than 30 years ago I seem to remember.

We are, I hope, discussing BS 7671:2018+A2:2022. The current requirements may well change: standards do change and evolve as people learn.

Until then, not providing the bonding if the frame is an extraneous-conductive-part might be considered a departure, and that is up to the designer to determine and justify, of course. There may be cases this is advantageous ... I don't think we could comment further on that for a "general case" discussion.

Guidance Note 5 provides further information on this topic.

Zoomup said:

Are non bonded kitchen draining boards more dangerous or less?

Good question - perhaps another lenghty discussion on the different use-cases and installation arrangement examples.

"just walk it though - in TT systems, it's possible for the case of the equipment to reach Uo with Earth, until the protective device operates."

Yes, but that only occurs for a fraction of a second before the R.C.D. disconnects the supply.

Z.

Zoomup said:

Yes, but that only occurs for a fraction of a second before the R.C.D. disconnects the supply.

Agreed ... but Regulation 411.3.1.2 still applies.

There are other reasons, though, not necessarily a fault within the installation, but local ground potential rise for other reasons. What if the greenhouse is in an area subject to ground potential rise from some buried metalwork connected to PME ... or  HV faults? The frame rises in potential with respect to the earthing arrangement.

Now, if this is local to the greenhouse, as you proposed earlier, risk is less, but if this is at a different location (earth electrode serves all outbuildings), then there is a potential difference with no protection in the installation.

Zoomup said:

Yes, but that only occurs for a fraction of a second before the R.C.D. disconnects the supply.

Agreed ... but Regulation 411.3.1.2 still applies.

There are other reasons, though, not necessarily a fault within the installation, but local ground potential rise for other reasons. What if the greenhouse is in an area subject to ground potential rise from some buried metalwork connected to PME ... or  HV faults? The frame rises in potential with respect to the earthing arrangement.

Now, if this is local to the greenhouse, as you proposed earlier, risk is less, but if this is at a different location (earth electrode serves all outbuildings), then there is a potential difference with no protection in the installation.