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Leave a 15mm gap between plasterboard and insulation and fix 25mm boxes direct to insulation. I take it they're 95x45 studs? I can't see there being a great deal of benefit in having insulation hanging out the back of the studs, if there's any water ingress there it'll soak your boards and...

Depends how much of a stickler your electrician is but he wouldn't be able to dead test the circuits unless the relays have a manual override (doesn't look like they do). You could feed the coils with a separate supply but that'd mean your lights go out with the sockets (for example).

Are you wanting independent light control or group control of all lights currently switched in each room?

What are the room controllers out of interest?

I'm no housebasher but imagine 25mm is the standard because it fits in nicely with dot and dab.

Cheers for the update. Maybe worth going for a low pressure model next time, electric showers are never that great though!

Yeah, apart from drowning I'm yet to hear a good reason for not falling asleep in the bath.

You need a multimeter, set to continuity and check red-red and black-black. If it beeps you're in business.

R1+R2 = 0.5 and Ze = 0.3? I don't know where you got this number from. A parallel path reduces the resistance it doesn't increase it, regardless it's negligible.

Not sure your maths is adding up there. To get the prospective fault current you need to know the Ze and R1 R2, the Zs. To work out the touch voltage at that fault you would need to know R1 and R2 independently e.g. if they were both 0.4 ohms the touch voltage would be 230/2 = 115V. I'm not...

I stand corrected

Yes you'd bond from towel rail to cast iron bath for instance.

Why would you bond a class 2 fitting?

It's only required for parts that can be simultaneously touched if that answers your question. Suppose it depends on if you're wiring the BFG's house or not.

I'm using resistances to demonstrate the theory and why supplementary bonding is used. You may be right about making a lower resistance path but don't forget you're shortening the path to the MET, and all that really matters is the ratio between sup bond : path back to earth - theoretically this...

Feel like a stuck record here. Heres a picture. If you were to swap the resistances in the below picture you would have 80 volts between A and C rather than the 20 illustrated.

No, B and C will be the same but by changing the bonding point you're changing the relationship of A-C and C-MET. You want the resistance of A-C to be much lower than C-MET, if it's higher you'll drop significant voltage by the time it gets to C.

Your statement was that Which is wrong. Without supplementary bonding, you can consider all cpc/bonding resistances to be negligible - the only load that matters is you (because your resistance will be exponentially higher than any of them). So if there's 100V between A and the MET, that 100...

Yeah, if there's evidence of burning of the conductors they should be cut back to good copper

Let's say you have 100V on your enclosure, and the resistance between your water pipe and earth is 2 ohms. If it's 10 metres long and you touched them together, the voltage at 5 metres will be 50V. Now if you add a 1000 ohms for instance (like, a person) between the 100V and your water pipe...