Bonding in greenhouse

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.

the thing that makes a greenhouse unusual compared to a normal building, and more like outside, is the intimate connection of the floor to the terra-firma earth. A few other types of farm buildings, maybe stables, certainly barns, can be like this. As such the distinction between inside and out is lost, and all pretence of creating an equipotential zone at any other voltage than that of the bare earth of the greenhouse is also lost. So, ideally in the no expense spared world you would TT,and the electrode would be in the centre of the floor, or perhaps better a perimeter tape with rods at corners. At this point it would make sense to bond the thing to that TT earth as it would all go up and down together. The question has to be how closely does it approximate that ideal - because as soon as the local CPC is not at local ground potential you have an issue.

Mike.

mapj1 said:

the thing that makes a greenhouse unusual compared to a normal building, and more like outside, is the intimate connection of the floor to the terra-firma earth.

There are other situations like this also. Sports club showers is a good example. Grid in the floor is the usual remedy where the likelihood is increased (e.g. PME).

mapj1 said:

A few other types of farm buildings, maybe stables, certainly barns, can be like this.

Note 2 to Regulation 705.415.2.1 is a good practical example.

Zoom has a good point regarding RCDs, but that's not the only issue if the voltage arises from phenomena outside the installation.

"Note 2 to Regulation 705.415.2.1 is a good practical example."                                                                                              An earth grid in a farm building under the animals' feet may equalise any stray Voltages present. The biggest problems will involve acidic animal waste that will be conductive and corrosive to electrical connections and grid material, and mechanical damage by animals. Typically the grid may be useful in a milking parlour.

BUT, there will be a point where the four footed animals like cattle will have two feet on the grid area and two off as they approach or leave the area with the grid.

Will there be any potential problems there from the point of practical compliance with the regulation regarding installation and maintainability? (705.415.2.1)

I do not think that our greenhouse problem involves any animals.

Z.

"I do not think that our greenhouse problem involves any animals."  Humans ?

Yes I know, other animals might have a longer "wheelbase"