I am about to lay some 10mm2 CSA SWA from one end of the garden to the other.
I need to decide whether to export the (Protective multiple earthed) CPC from the garage via the SWA cable (where a supplementary CU is being installed) or use an earth rod at the shed & greenhouse at the bottom of the garden to form an independent earth.
Total seperation distance laterally is 13m but length of SWA cable is nearer 20m.
And why is all SWA cable all 3 phase? I can't seem to get any 1 phase cable so I am forced to shove some brown, blue, Green& yellow PVC tape on both ends of the SWA cable?
You can if you try really hard, but to be honest, it's a good to have a bit more CPC via a core as well as the armour - saves a lot of wibbling trying to decide if the armour is sufficient to supply an earth for the scenario concerned.
The main question I would ask:
Is there ready access to "local ground earth" in the location? You said greenhouse, so that would be a yes. Even if it was a wooden affair, you have open ground inside. More so if it has a metal frame and you bond that (because you are mounting lights or sockets on it).
In the event of various conditions/faults on a PME (TN-C-S) supply, you can get the supply earth going up to some potential above true earth - which would mean a voltage between your imported earth and the likely wet ground you are standing on. This is where the danger lies.
In this scenario, I would consider a local TT (earth rod) for safety, obviously RCD on all circuits as per regs and do NOT connect the incoming earth to the earth rod - separate it in a plastic enclosure that terminates the SWA.
If your house supply is TN-S, it's less of an issue, but bear in mind that TN-S is going our of fashion and the electricy co could convert you to TN-C-S at any time.
Of course, you'll need to test your earth rod - and that's beyond me - never worked with them...
In my scenario, my garden supply goes to a wooden shed with a wooden floor, so no extraneous earths in there and very little risk even if the house earth did rise to some voltage above local ground.
I'd have thought with the current state of electronics, it could be arranged that the earth was mainly from the house and a circuit between the local ground and the main earth could be monitored and the whole lot cut off if the potential rose to outside tolerances.
In the murky past I've actually just bonded the incoming earth both to any local mass and a ground rod, and nothing ever went bang or caught fire! I can see however that there could be circumstances where a earth is effectively from live to earth and it would be hoped that before the whole lot failed or caught fire, the local fuse for that circuit be it breaker or whatever would trip and isolate it all. Of course there are other issues as has been mentioned before to do with nasty noise on the mains etc, but that would probably only affect somebody looking for electronically quiet supplies.
I do sometimes wonder as complexity goes up in this apparently simple wiring, if we are not making things so awkward nobody will understand it and just ignore it, which is also a danger. Most folk around here I notice still just string some mains cable down their fence and fit a 13 a socket in the shed. Brian
THe SWA cable has 3 equally sized conductors plus steel wire armour so I can use one of these as a CPC.
My house has a PME supply supplied so the earth is supplied to me by the electricity supplier. So there is no secondary earth provisioned within the main house.
So which is better: should I then export this supplied earth to the outside CU or provide my own local independent earth to the outbuildings?
The seperation distance laterally is 13m.
All of my current and new consumer units all or will use RCBOs.
The ones in the house are 30mA RCBOs. If I use a local independent earth for the outbuildings, won't that mean I then have to consider using time delayed or 100mA RCBOs to avoid nuisance trips?
I do agree with Brian. Rightly or wrongly, the percentage of people who really know about this sort of thing must be minuscule, and that includes those who know there is more to it than just running a length of cable down the garden, even if they don't understand what really needs doing.
SOP is a length of cable hopefully at least tacked to the fence, socket on the shed end, job done.
Is there power to the greenhouse or just the shed? I might be more tempted to TT it if the greenhouse is getting power.
No. This would only apply if you had a TT supply to your house OR you want to run RCDs in series. Normally with a PME supply you would use a MCB at the head end and have the RCD protection down at the shed (SWA does not need RCD protection).
(places like TLC Direct (and most other wholesalers) will do two core though if you want)
If one of those is a greenhouse, it can be near impossible to export the PME equipotential zone into it, so TT would become a much better option.
Have a look at:
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and the following bit about exporting equipotential zones.
You can use a normal MCB for the submain feed rather than a RCBO. You isolate the PME earth at the point it connects to the outbuilding CU. So you have overload and fault protection for the SWA provided by the MCB. Earth fault protection for the outbuilding is then provided by the local RCD/RCBO.
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