Shower circuit design.

I think you may have missed the point, Andy, in some ways both Graham and I may have been encouraging you to think! Your C2 coding is simply wrong because the circuit is not in any way dangerous in your example. It might be a nuisance but I doubt that the breaker would operate with the normal use of both rings and oven unless one turned the whole lot on together. As I said above, no one would design these circuits this way, but they have ended up in a slightly curious state, where tripping could occur without a fault being present. However, this is still no different from a standard ring or radial circuit with sockets.

You should be aware that you would certainly fail your 2391 practical with that C2 and a couple of other mistakes, C1 or C2 codings are for danger, not nuisance problems.  A C3 suggests improvement, which is the outcome you would like to see, but you cannot force someone in this way, they may enjoy resetting the MCB.

As far as the suggestion that this breaker may get too hot in the OP, this is unlikely and even if it did it would trip at a lower current/shorter time from the data. I could show you many places with fairly hot switchgear, certainly too hot to touch for long, the dissipation in a large board can be a couple of hundred Watts, much more than a domestic CU, and the casing is not ventilated, and there may be a lot of bunched hot cables, whereas domestics rarely get even noticeably warm. These are the same BS 60898 breakers in many cases, and they generally do not burn up at all (which they should not from the spec!). It may be that they trip at a slightly lower current, but this rarely shows up.

I am NOT advocating using breakers in the way described in the op, by the way, just analysing the problem fully. It would be useful if the full range of currents were available easily, 6,10,16,20,25,32,40,50,63A, often there are inconvenient gaps in ranges even from manufacturers, particularly if one wants a C or D type.

I think you may have missed the point, Andy, in some ways both Graham and I may have been encouraging you to think! Your C2 coding is simply wrong because the circuit is not in any way dangerous in your example. It might be a nuisance but I doubt that the breaker would operate with the normal use of both rings and oven unless one turned the whole lot on together. As I said above, no one would design these circuits this way, but they have ended up in a slightly curious state, where tripping could occur without a fault being present. However, this is still no different from a standard ring or radial circuit with sockets.

You should be aware that you would certainly fail your 2391 practical with that C2 and a couple of other mistakes, C1 or C2 codings are for danger, not nuisance problems.  A C3 suggests improvement, which is the outcome you would like to see, but you cannot force someone in this way, they may enjoy resetting the MCB.

As far as the suggestion that this breaker may get too hot in the OP, this is unlikely and even if it did it would trip at a lower current/shorter time from the data. I could show you many places with fairly hot switchgear, certainly too hot to touch for long, the dissipation in a large board can be a couple of hundred Watts, much more than a domestic CU, and the casing is not ventilated, and there may be a lot of bunched hot cables, whereas domestics rarely get even noticeably warm. These are the same BS 60898 breakers in many cases, and they generally do not burn up at all (which they should not from the spec!). It may be that they trip at a slightly lower current, but this rarely shows up.

I am NOT advocating using breakers in the way described in the op, by the way, just analysing the problem fully. It would be useful if the full range of currents were available easily, 6,10,16,20,25,32,40,50,63A, often there are inconvenient gaps in ranges even from manufacturers, particularly if one wants a C or D type.