Two high-power appliances on a single 40A RCD

Oh dear we are all becoming far too agitated. The Ib is not greater than In for this circuit is it, there is no intention to use both cooker and shower at the same time. I have pointed out that this is no different to any domestic final power circuit. The cable is protected fully against overload. Nothing bad is going to happen anyway, and the circuit is fine as far as BS7671 is concerned.


Then we get to the bit where the moral attitudes of "I know best" come in. The MCB may wear out if the customer is careless etc. It may but would then fail to contact rather than weld up (as you should know). This criticism may be applied to any circuit breaker, presumably in your installations of those on ring or radial power circuits, or lighting with filament lamps. Then we get to the "we must prevent both being used at once even by accident". Why is this, everything is under control and the only outcome is a possible MCB trip, so what? Then I am attacked for "unprofessional" something or the other just because you have not quite understood what BS7671 says. I give examples and no one likes them. This circuit is designed to power either the shower or the cooker, not both! I do know BS7671 rather well, I teach electricians to pass the wee 18th test very successfully. I am however often surprised by the lack of knowledge of what it really says, rather than even looking up the correct answers. The regulations operate in the same way as UK law, the law saying what you cannot do rather than what you can, just like BS7671. It may give some hints of what you can do but these are in the appendixes at the back or in the notes to various regulations.


I pointed out that changing a CPD to a different greater value is notifiable under part P, You are changing the circuit to a new one by definition, you have changed its design and rating. You ask what code this should be given, and I would say none, in that it is compliant with BS7671 as you have all found out by now, although it may not be something you have seen before. Saying that is dangerous or likely to fail in some way fails to understand its usage, and is not accurate or true as I have pointed out. Long term slight overloads are a risk for circuit designs but you need to understand exactly what that means, and the interpretation being given by me for this circuit explains what you need to examine. These are the circuit loss, thermal time constant, installation method and real terms loading etc. so not just a yes / no question. The slight overload is not 1% overcurrent, it is enough to cause significant extra cable heating, and needs to take account of the fusing factor, so perhaps 30 - 40% or more. The diversity on a circuit includes all loads, and the times they are likely to be operational, which were explained in the OP.


I think that everything has been explained several times now by me, and useful comments from some others. Use this as a thinking exercise, it is useful because you have probably never considered this kind of thing before, and it is the process necessary to design a more complex installation.

Oh dear we are all becoming far too agitated. The Ib is not greater than In for this circuit is it, there is no intention to use both cooker and shower at the same time. I have pointed out that this is no different to any domestic final power circuit. The cable is protected fully against overload. Nothing bad is going to happen anyway, and the circuit is fine as far as BS7671 is concerned.


Then we get to the bit where the moral attitudes of "I know best" come in. The MCB may wear out if the customer is careless etc. It may but would then fail to contact rather than weld up (as you should know). This criticism may be applied to any circuit breaker, presumably in your installations of those on ring or radial power circuits, or lighting with filament lamps. Then we get to the "we must prevent both being used at once even by accident". Why is this, everything is under control and the only outcome is a possible MCB trip, so what? Then I am attacked for "unprofessional" something or the other just because you have not quite understood what BS7671 says. I give examples and no one likes them. This circuit is designed to power either the shower or the cooker, not both! I do know BS7671 rather well, I teach electricians to pass the wee 18th test very successfully. I am however often surprised by the lack of knowledge of what it really says, rather than even looking up the correct answers. The regulations operate in the same way as UK law, the law saying what you cannot do rather than what you can, just like BS7671. It may give some hints of what you can do but these are in the appendixes at the back or in the notes to various regulations.


I pointed out that changing a CPD to a different greater value is notifiable under part P, You are changing the circuit to a new one by definition, you have changed its design and rating. You ask what code this should be given, and I would say none, in that it is compliant with BS7671 as you have all found out by now, although it may not be something you have seen before. Saying that is dangerous or likely to fail in some way fails to understand its usage, and is not accurate or true as I have pointed out. Long term slight overloads are a risk for circuit designs but you need to understand exactly what that means, and the interpretation being given by me for this circuit explains what you need to examine. These are the circuit loss, thermal time constant, installation method and real terms loading etc. so not just a yes / no question. The slight overload is not 1% overcurrent, it is enough to cause significant extra cable heating, and needs to take account of the fusing factor, so perhaps 30 - 40% or more. The diversity on a circuit includes all loads, and the times they are likely to be operational, which were explained in the OP.


I think that everything has been explained several times now by me, and useful comments from some others. Use this as a thinking exercise, it is useful because you have probably never considered this kind of thing before, and it is the process necessary to design a more complex installation.