OK, following my earlier posting, I’ve been thinking about the greenhouse electrics business, and also looking around the forums and seeing an awful lot of differences of opinion!
As far as I can see, the only really ‘safe’ option is to isolate the greenhouse’s electrical installation from the house’s CPC/earth, have a local TT electrode as the earth for the greenhouse’s electrical installation and bond the metal frame etc. of the greenhouse to that. I presume that if one does that, it’s still OK to rely upon upstream RCD protection (i.e. in the house), since the L-N imbalance that would cause the RCD to operate does not care what path to earth the ‘lost current’ has taken.
If, as I suspect is the approach most likely to be taken by a DIYer, one simply exports the house’s earth to the greenhouse (and bonds the greenhouse metal to that earth), then I see theoretical problems. Whether by virtue of the CPC or even if one ran an adequate bonding cable from house’s MET to the greenhouse frame etc.(which I suspect very few people would do), there would, under some fault conditions, be a dangerous potential difference between the greenhouse metal and the soil below. With an L-E fault (whether TT or TN at the house), a significant pd should only exist for a fraction of a second before the fault was cleared by a protective device, so ‘not too unsafe’, but with the much-discussed ‘lost neutral’ fault with a TN-C-S/PME supply, the pd between greenhouse metal and soil below would presumably be persistent, and hence very dangerous.
So, it seems to me that if someone chose not to have a local, isolated, TT system in the greenhouse, it could be argued to be ‘not unreasonable’ to bond the greenhouse metal to the exported earth if it were TT, but not acceptable if it were TN-C-S. In fact (and many of those here may not wish to comment on this!), if one did decide not to have a local TT system for the greenhouse, the ‘least of the evils’ may actually be to leave the greenhouse metalwork floating.
As for ‘outside sockets’, I really can’t think of anything one can do apart from ensure there is adequate RCD protection and encourage/hope that only Class II equipment will be plugged into the sockets – since I don’t see what else can realistically be done; one obviously can’t ‘bond’ a whole garden!
Is what I say reasonable? Any comments?
Kind Regards, John
As far as I can see, the only really ‘safe’ option is to isolate the greenhouse’s electrical installation from the house’s CPC/earth, have a local TT electrode as the earth for the greenhouse’s electrical installation and bond the metal frame etc. of the greenhouse to that. I presume that if one does that, it’s still OK to rely upon upstream RCD protection (i.e. in the house), since the L-N imbalance that would cause the RCD to operate does not care what path to earth the ‘lost current’ has taken.
If, as I suspect is the approach most likely to be taken by a DIYer, one simply exports the house’s earth to the greenhouse (and bonds the greenhouse metal to that earth), then I see theoretical problems. Whether by virtue of the CPC or even if one ran an adequate bonding cable from house’s MET to the greenhouse frame etc.(which I suspect very few people would do), there would, under some fault conditions, be a dangerous potential difference between the greenhouse metal and the soil below. With an L-E fault (whether TT or TN at the house), a significant pd should only exist for a fraction of a second before the fault was cleared by a protective device, so ‘not too unsafe’, but with the much-discussed ‘lost neutral’ fault with a TN-C-S/PME supply, the pd between greenhouse metal and soil below would presumably be persistent, and hence very dangerous.
So, it seems to me that if someone chose not to have a local, isolated, TT system in the greenhouse, it could be argued to be ‘not unreasonable’ to bond the greenhouse metal to the exported earth if it were TT, but not acceptable if it were TN-C-S. In fact (and many of those here may not wish to comment on this!), if one did decide not to have a local TT system for the greenhouse, the ‘least of the evils’ may actually be to leave the greenhouse metalwork floating.
As for ‘outside sockets’, I really can’t think of anything one can do apart from ensure there is adequate RCD protection and encourage/hope that only Class II equipment will be plugged into the sockets – since I don’t see what else can realistically be done; one obviously can’t ‘bond’ a whole garden!
Is what I say reasonable? Any comments?
Kind Regards, John