Bonding in greenhouse

Question

On an eicr 
 I have a greenhouse connected to a sub DB in an outhouse, which is on a TT earth. Circuit is protected by upstream 30mA and 100mA RCD's. 
 
 I am debating in my mind if the frame of the greenhouse should be bonded to the single socket in the green house. 
 I would consider the frame as an extraneous part and therefore first reaction is to bond it. 
 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. 
 On balance I am tempted to go with not bonding, what are your thoughts?

gkenyon · Answer

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.

Bonding in greenhouse

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