Ever since I went to my sisters place and did some tests and found her 30 ma rcd welded shut I have always had doubts
about relying solely on 1 rcd, especially in a garden, at least with my sister she had a low enough EFLI to trip an mcb in the event of an earth fault, in a garden
that is TT'd you would not have that luxury, the circuit would simply not switch off in the event of an earth fault and would remain live until someone touched something metal.
This garden already has an existing underground cable but somewhere along the line someone has buried an underground joint to extend this cable and the readings I get from testing are very erratic.
The Line to neutral IR resistance jumps from being at 0.2 M Ohms up to about 2.5 M Ohms at 500 V and sort of flitters about between these two extremes and won't settle.
The existing underground cable is a two core roughly 6mm but in reality is an older multistrand cable with the armour used as the cpc.
The armour has broken down and I get an end to end resistance reading of 12 Ohms on the armour, the cable is only 15 metres long so it really does look like this underground joint has got water contamination and the cable should be abandoned IMHO.
There are three small buildings in the garden, one greenhouse, one shed and a sort of chalet, he wants power to all three.
The greenhouse will be damp with a floor that is basically real earth IE the garden soil.
The shed will also be damp but has a wooden floor leading out onto decking.
The chalet seems quite dry with a wooden floor also.
I'm thinking the best way to do this would be to run a new cable which is going to be really hard but I never thought of using trunking to conceal this cable from the fuse board to the outside of the house. This is a good idea as long as I can get some nice skirting trunking maybe, I will have a look today so thanks for the suggestion.
Once the cable is outside the house I can then surface clip or bury it around the perimeter of the garden down to the end.
At the moment I think the best method would be to run a new supply in trunking (not easy) protected by a 100 ma S type.
This would go to a central distribution point in the garden somewhere.
From there I could run a supply to the greenhouse and rod it there.
From the central point I could also run a supply to the shed and rod there also.
Also run a supply to the chalet and again rod it.
This would give me three rods in the garden at different places which should give me a better overall Ra reading.
The main house supply cable is under ground TN-S with a lead sheath that is terminated at the MET in the house so this should also help lower my Ra readings (the lead sheath of the supply cable runs underground through the garden)
Each garden building would have its own small CU and each small CU would be protected by a 30 ma RCD.
The other way I was thinking would be to bond everything and export the main bonding from the house but I would have to put a ground mat down in the greenhouse
to bond the floor to the frame but if I managed to extend the bonding from the house at least I could use the low EFLI of the supply circuit IE not TT anything so that if an rcd did fail in the event of an earth fault I would have the added protection of an MCB tripping at the main fuse board in the house.
Bonding everything in this damp garden enviroment seems a bit hit and miss to me especially with the damp and possibly kids running around in bare feet so this idea gives me the willies a bit, I mean how can I make the whole garden an extension of the house equipotential zone especially when he wants a couple of outside sockets installed?
I am going to do a bit more testing today.
As far as part P goes I would like to wait until after my assessment but if the guy really wants me to get a shift on I will have to notify LABC before I start and pay the fees.
Any thoughts on this one?
Thanks for taking the time to read this lengthy post.
