I understand that I must have a 5KW double pole switch for a demand under-sink water heater, and that it must be within 2 meters of the device. But can the switch be in the same under-sink cupboard as the heater, or must it be visible and easily accessible?
Lastly, the consumer unit has an RCD, must the circuit be on a 30ma RCD?
First question on RCD. There is no need to have RCD protection on fixed appliance except for when fitted in a Bathroom. However the rules on cables buried in a wall mean not to have RCD protection often means using either surface cable runs or special cable like the Ali-tube.
Much will depend where the sink is. In a utility room surface cables may not be a problem. In a kitchen or bathroom one may take a different view specially since both those locations likely require Part P notification so any deviations from rules may not get LABC approval.
Switches and isolators are rated in max volts and max amps not watts so unlikely to find a 5kW switch. Don't know where you get the 2 meters from? The health and safety rules often refer to being within sight of the operator and also some times within reach is also referred to. But in the main that's for something with a motor not just a heating device. So I would consider why we fit an isolator. In your case if there was a burst you would want to isolate so you need the isolator where if there is a burst it is unlikely to get wet. Really a simple risk assessment. Also where some one working on the item can see it is off or on.
As to duel pole I would always try to use duel pole even with a TN supply specially where a RCD is used. But really it is only required with a TT supply.
Also consider the Part P regulations often mean in kitchens and other special locations it is just not worth DIY. It is cheaper to get a scheme member electrician to do the work. The instant heat under sink units give out just a dribble of water and in the main have been replaced by mini storage units with typically heat 7 litres of water and use a special linked tap and faucet so one gets full flow of water of around 8 litres as of course heating as it flows. These are normally within the 3kW limit for a 13A outlet. The insulation is that good they will stay hot for 24 hours. I used one in a caravan very successful. Try and google "Handyflow Undersink Point Of Use Water Heater 5ltr" look for any which give quantity of water in this case 5lts and you will then not need a huge supply.
There is a 100ma RCD on entire consumer unit, so I suppose I am "safe" in terms of burying wires.
Yes, I do not have a 5kw switch, but a 45 amp dual pole switch, so that is easy.
I do not remember where I saw the "2 meter rule", just that I thought it was on some official site. It does seem rather a long way to go for a switch. In my case, since the consumer unit is in the same room, I am not too concerned about the switch being accessible during "special disasters".
I take our point about undersink instantaneous heaters, but, well, I am rather far along to give up my plan. I've already run a 6mm cable into the consumer unit, so I can have a 7kw heater which should - I hope - provide enough hot water to wash artists paint brushes, and do the occasional small washing jobs along with hand washing. I do not expect it to be used every day, and sometimes not for longer periods, and the idea of only using power as needed appeals to me. However, I am not at all sure if 7kw is enough and I am uneasy about the reliability of these units. Still, the tank option does give me a "plan B".
There are three sizes of RCD listed in the regulations. The 500ma is a special so forget that. So two sizes 300ma and 30ma. The 300ma is to protect in case of fire. The 30ma protects personnel. As well using a RCD is required where a TT earthing system is used as with an earth fault not enough current will flow to trip a MCB or blow a fuse. The size in that case depends on the resistance measured on the earth rod. Two ways to measure both need expensive meters.
So the 100ma RCD is no good you need a 30ma RCD.
The problem with a 30ma RCD is they trip quite easy some times it seems without due cause so often the installation has to be divided into circuits, as necessary, to: (iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit. (iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation. To give two examples so it's not a simple replace the 100ma with a 30ma.
If you can get the load under 13A then you can use a RCD FCU (fused connection unit) but over 13A you need to use a mini consumer unit with a RCD or RCBO (that's a RCD and MCB combined) clearly over 13A means a lot more work which is why I recommend the mini storage type as under 13A in fact the one I had I changed the element for 1kW so could run it on 5A.
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