Bonding in greenhouse

Good to see my question has created a healthy debate.

Actually from my experience at least most people that own greenhouses also have pets.

I could easily see a dog being in contact with the ground and the greenhouse at the same time. Less likely to have the same issues with corrosion, but presumably still with 4 legs more susceptible to electric shock? 

Pets. An even greater reason then just to leave the greenhouse "floating" and not connected in any way with the electrical installation.

This applies to humans as well.

And, if the internal greenhouse electrical installations is that dangerous, lock the door and stay outside.

Z.

THE DANGERS OF THE GREENHOUSE, AND NOT ALL OF THEM ARE ELECTRICAL. By Singed Moustache.

https://ggs-greenhouse.com/blog/top-10-causes-injuries-greenhouse

Z.

Would you bond my greenhouse here illustrated at the bottom of my garden?

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

Planning permission has nothing to do with the definition of a building - you can need planning permission for a garden wall or impermeably surfaced parking space - neither are buildings. As BS 7671 has no definition of a building I presume we fall back on the normal meaning of the word - my dictionary says a building is "something built with a roof and walls, such as a house or factory" - to my mind a greenhouse is nothing like a house or a factory, especially when considering the internal environment that would be relevant to considerations for shock protection.

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.

But what about other metallic parts within the greenhouse - anything from staging (aluminium tables) from spiral plant supports (e.g. https://www.twowests.co.uk/products/two-tier-standard-up-staging and https://www.spgreenhouse.com/plant-support/55076102.html) If the frame is extraneous things like these certainly will be as well. Are we to return to 15th Ed interpretations and bond these as well?

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.

Greenhouse frames usually aren't anchored very deeply into the ground - mine's just bolted down onto a couple of courses of brick that finish at about ground level. I suspect the effect on local ground potential will be minimal - and the risk is likely to be for those not just outside but inside too.

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

Could be up to 1s for TT systems if the fault is on a distribution circuit or large final circuit (larger installations may have RCDs with substantial time delays to ensure discrimination) - and anything above 40ms starts to look dodgy from a shock protection point of view where there's contact with true earth. Also TT earthing systems can be held at anything up to 50V with no requirement for any RCD to trip - usually the product of standing leakage currents and electrode resistance, or even import hazardous voltages from outside the installation via bonding to metallic water or gas services; so you can't rely entirely on ADS to keep touch voltages down. You might even want to think about the reliability of RCDs...

If I get a bit of time outside later, I'll do some experiments with my greenhouse and a meter and see if I can get some numbers to play with.

   - Andy.

I partly agree. The showers at the local Rugby club do seem to support plant life, at least in an algeal form. However the building has a damp course, (well a membrane really) and no body seems to dig the floor up to plant anything, I suspect the contact of bare feet is to an internal potential only loosely coupled to the over-site cement, and even more loosely to the ground beneath. In a green house you may well be on the earth, but unless its a hippy commune, less likely to be barefoot, let alone naked and wet.

Mike.

In a green house you may well be on the earth, but unless its a hippy commune, less likely to be barefoot, let alone naked and wet.

But on the other hand, you're quite likely to have hand contact with metallic objects in good contact with the soil (e.g. plant supports, digging tools, staging) - often with damp/dirty hands, and if it's in an awkward corner, quite likely to be steadying yourself with the other hand on the greenhouse frame.

   - Andy.

AJJewsbury said:

Planning permission has nothing to do with the definition of a building - you can need planning permission for a garden wall or impermeably surfaced parking space - neither are buildings. As BS 7671 has no definition of a building I presume we fall back on the normal meaning of the word - my dictionary says a building is "something built with a roof and walls, such as a house or factory" - to my mind a greenhouse is nothing like a house or a factory, especially when considering the internal environment that would be relevant to considerations for shock protection.

I think the Building Act or other legal definition would be used?

Alan B said:

I could easily see a dog being in contact with the ground and the greenhouse at the same time. Less likely to have the same issues with corrosion, but presumably still with 4 legs more susceptible to electric shock? 

Might be a concern if the frame is NOT REALLY an extraneous-conductive-part. But if it is, it also has earth electrode properties in terms of voltage equalization.

Technically, BS 7671 is really only concerned with protecting pets that are classed as livestock.

Zoomup said:

Would you bond my greenhouse here illustrated at the bottom of my garden?

What's the earthing arrangement and means of protection against electric shock?

If it were TT, would it need a separate earth electrode (depending on construction of the metal structure)?