If the earth of the circuit is TT, and the electrode is not far away from the greenhouse, then they will idle at more or less the same voltage. In effect if you bond you are adding another electrode in the form of the greenhouse frame, unless it is more or less built on insulators.

I'd not bother to bond if the green house is electrically floating, it can add neither to the safety nor to the danger if it is left alone. If it is actually a pretty good electrode then I'd probably bond to it rather than have two different TT grounds.

If the socket is plastic bodied and the things that get plugged in are double insulated it makes very little odds.

Less off the cuff answers will probably be along shortly.
Mike

What do you mean by this statement please?

"But then thinking about it. bonding would reduce the risk of shock inside the greenhouse in the case of the socket became live but increase the risk of shock outside the greenhouse if the socket and greenhouse became live."

The socket became live???????? The socket is best being a plastic weatherproof type to keep water out.

Z.

Sorry not very clear. I am thinking worst case scenario is case of metal clad socket becomes live along with cpc connections. Could happen if there is a short to earth and RCD fails to operate.

Then if the socket is bonded to the greenhouse there would be minimal Potential difference between the socket and green house frame but someone outside the greenhouse is at risk because there could be a significant potential difference between the green house frame and earth.

If the greenhouse has a very low resistance to earth, lets say 20 ohms then bonding makes sense but much above that, if the RCD fails there is a real potential for a fault to create a dangerous voltage.

First, I assume that it is a metal greenhouse. More importantly, what is the rest of the construction. A greenhouse could sit on paving slabs, a concrete base, or even on dwarf walls.

If it is extraneous, how does it introduce a potential: or in other words, how well is the metalwork connected to the general mass of the earth?

I have to admit I failed to do enough checks on the green house, at the time my initial reaction was that it probably needs bonding and didn't go much further with checks.

It's either straight on the ground or on one layer of bricks, I know a big difference. If it's well connected to ground with a low impedance bonding makes sense. If it's isolated no need to bond, but what if it's say 300 ohms.

But then thinking about it. bonding would reduce the risk of shock inside the greenhouse

Not necessarily - since the 'floor' is likely to remain somewhat closer to true earth potential even when the bonded frame is at some other (possibly hazardous) potential. And not just the floor itself either - but anything metallic placed on or on it - from metal plant supports to aluminium staging.

I wouldn't make any assumptions about the joints in the frame either - oxidized aluminium isn't the best of conductors and if the frame is factory painted/powder coated (as is increasingly popular these days) there's even less chance of a good metal-to-metal bond.

Personally I would try to deem that a greenhouse is not a proper building and refer to the new words in 411.3.1.2.

   - Andy.

I would not recommend a metalclad socket in a greenhouse. It will rust and deteriorate. Also it is not suitable for the humid and damp greenhouse conditions. It may get sprayed with water. A plastic outdoor socket is better suited to this location.

705.512.2

522.3

And no, I would not bond the greenhouse frame.

Z.

The R.C.D. will trip off with a 300 Ohm L to E short.

Table 41.5

Z.

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

Z.

"Sorry not very clear. I am thinking worst case scenario is case of metal clad socket becomes live along with cpc connections. Could happen if there is a short to earth and RCD fails to operate."

There are two R.C.D.s connected in series. The chance of both failing is very slim indeed.

Z.