the garden building will be a TT system utilising an earth rod

:

The garden building CU will probably be metal

Be aware that great care is needed on a TT system to avoid earth faults before the first RCD - get it wrong and a single fault can result in the entire TT earthing system being permanently at mains voltage. Usually you need to ensure that the incoming live conductors are protected by double or reinforced insulation all the way up to the first RCD terminals. Also exposed conductive-parts of one system aren't simultaneously accessible with those of the other.

In this particular case, an RCD upstream (in the house) would physically provide reasonable protection although there is some debate whether relying on such an arrangement is entirely conforms to the letter of the wiring regs (specifically the TT requirement that all exposed-conductive-parts protected by a single device be connected to a common earth).

   - Andy.

Andy, I was presuming an RCD in a spare slot in the house CU and another in the garden building.

Just working my way through the On Site Guide. I have 3 months to figure this all out ...

Mike

Thanks for all the advice, just one more question for now please. Can I run the SWA up from the trench in the cavity between the SIP wall and the external timber cladding? Any physical protection needed (e.g. steel conduit?).

Mike

Thanks for all the advice, just one more question for now please. Can I run the SWA up from the trench in the cavity between the SIP wall and the external timber cladding? Any physical protection needed (e.g. steel conduit?).

Mike

so long as the SWA is connected in such a way that penetration by screws, nails, garden implements etc would operate some automatic disconnection - in practice this means earthing the armour at the supply end and verifying the resistance is low enough to pretty much instant trip the MCB or fuse supplying, no further precaution is required by regs.

Non-armoured cables do however require protection except in a route that is obvious by the nature of being a line of least surprise to a fitting, or on the surface.

Probably freedom of route is another reason to run the SWA indoors as far as possible.

Mike

Thanks Mike.

Mike

After reading the advice here and the On Site Guide I have a better idea of what's required, and two more questions please!

The SWA will enter the building through a SIP panel, which is a sandwich of OSB and polyurethane insulation. Do I need to run the cable through the wall in a conduit?

How should the cable between the consumer unit and earth rod be protected where it leaves the building? Is flexible conduit adequate? It will be above ground and very short.

Mike

The SWA will be fine in contact with the polyurethane. There have been problems in the past with some polystyrene plasticiser formulations  and PVC reacting, but modern manufacturing methods mean it is not an issue.

If the wire to the electrode is thin (say 4mm2) then it will need mechanical protection against strimmers etc. In such a case a conduit may be a good idea.

But given it is sold by the metre  example then for a short run it is probably cheaper to buy something chunky enough not to need protection, so 10mm2 or 16mm2, and leave it in plain sight. What is your ground like ? Gravelly/stony ground is not so good with earth rods, and there are alternative approaches if that is the case.

Mike.

Thanks Mike, that's very helpful. I already have both 6mm2 and 10mm2 cable which I used for my antenna projects. The ground is soft soil. We live by a lake and the water table is quite high.

Mike