Shower circuit design.

The design current of a circuit is not defined by the CPD Andy, as you know. There is also a great difference between “ambient temperature” and the MCB internally generated heat from the load current as must be fairly obvious. In a domestic CU, the ambient temperature is probably between 20 and 25 C in most houses. The dissipation from all the MCBs is a few Watts maximum, and in the case we are discussing, there is by definition very little other load. If you look at some large industrial boards with many fully loaded circuits the conditions may be somewhat different, but in a domestic the CU never feels hot does it?

The point of the thermal temperature rise applies to all MCBs and all sizes dissipate approximately the same power at the trip point, simply to achieve the same temperature. A bigger value one will get slightly less hot at 30ish amps, but this makes no important difference.

I don't really see why you are arguing so hard to justify your feeling that this is in some way dangerous, it is just a CPD operating in exactly the way designed into it. If the breaker gets warm, the operating current gets less, which is ideal for “safety” isn't it? It is possible that this CPD may trip, given enough time, but that is just a nuisance. My garage RCD trips sometimes due to the inrush to my compressor, I put up with it because the RCD is a good idea, but it is not “dangerous”. It is also not ideal, it is an inherent defect of RCDs that large currents sometimes cause spurious trips.

Following your reasoning the CPD for a ring circuit should not be 32A, it should be the maximum current that could be drawn from all the sockets, with cable size to match!  The point is that 32A protects the CABLE sufficiently, even if I plug in 50A of load. This is exactly the same as these showers.

The design current of a circuit is not defined by the CPD Andy, as you know. There is also a great difference between “ambient temperature” and the MCB internally generated heat from the load current as must be fairly obvious. In a domestic CU, the ambient temperature is probably between 20 and 25 C in most houses. The dissipation from all the MCBs is a few Watts maximum, and in the case we are discussing, there is by definition very little other load. If you look at some large industrial boards with many fully loaded circuits the conditions may be somewhat different, but in a domestic the CU never feels hot does it?

The point of the thermal temperature rise applies to all MCBs and all sizes dissipate approximately the same power at the trip point, simply to achieve the same temperature. A bigger value one will get slightly less hot at 30ish amps, but this makes no important difference.

I don't really see why you are arguing so hard to justify your feeling that this is in some way dangerous, it is just a CPD operating in exactly the way designed into it. If the breaker gets warm, the operating current gets less, which is ideal for “safety” isn't it? It is possible that this CPD may trip, given enough time, but that is just a nuisance. My garage RCD trips sometimes due to the inrush to my compressor, I put up with it because the RCD is a good idea, but it is not “dangerous”. It is also not ideal, it is an inherent defect of RCDs that large currents sometimes cause spurious trips.

Following your reasoning the CPD for a ring circuit should not be 32A, it should be the maximum current that could be drawn from all the sockets, with cable size to match!  The point is that 32A protects the CABLE sufficiently, even if I plug in 50A of load. This is exactly the same as these showers.