Then the cable to that lamp should be capable of tripping the breaker without burning out in event of a short.
That's all the fuse does. If the appliance requires additional protection for safety reasons, it will have internal fusing of some sort. Quite likely non replaceable. A thermal fuse in the motor winding, etc.
whereas the internal wiring that I could see was probably only 0.75mm^2.
But does not indicate if it's the *right* sort of connection.
True but not particularly relevant.
This is why you need to keep the concepts of fault and overload protection separate in your mind. Usually they are both provided by the same protective device, but not always - the responsibilities can be separated.
Fault protection must always be present at the origin of the circuit. Overload protection can be elsewhere, or possibly not present at all and provided simply by design options limiting or eliminating the possibility of overload.
A common example would be the 3A drop flex to a pendent fitting on a lighting circuit. That circuit may be protected by a B6 MCB, which will not provide overload protection to the drop flex. However the single lamp holder on the end, and the choice of lamps available does impose a limit. So additional protection for sustained overload is not needed. However it would be possible to cause a short circuit at the end of the drop flex, and that must be accounted for. The way you work out whether a wire has adequate fault protection is to use the adiabatic calculation:
You start by working out the likely fault current - this will be inversely proportional to the the total circuit impedance at the point of the fault. The impedance simply being the sum of the resistances of the cable runs, and the supply and possibly earth impedances. Once you have this and the k factor for the cable in question you can work out the minimum cross sectional area of cable required to survive long enough to clear the fault.
So for example, take your nominal 0.75mm^2 cables in the fan. Say the loop impedance was 1 ohm at the point of fitting. That gives a potential fault current of 230A, which will open the B32 MCB in 0.1 secs or less (i.e. on the magnetic part of its response). If your wires are PVC insulated we can take K at 115. So that gives us a minimum wire size of:
sqrt( 230^2 x 0.1 ) / 115 = 0.63mm^2
So even if someone sticks a 6" nail through the fan, and manages to short a couple of those wires, we are still ok ;-)
Now for the avoidance of doubt:
I am not suggesting here that directly connecting fans to 32A ring circuits is a good idea. In most cases I would expect the manufacturers instructions (which take precedence) to impose lower limit protection anyway. Even if they don't it would (IMHO) still be good practice to include a switched FCU for applications like this.
Thank goodness for that.
As always, I am pleased to be educated about the wider picture.
When our bathroom was re-done, pre Part P, the fitters insisted in fitting an unswitched FCU feeding the over mirror lights/isolated shaver point. This was fed from a 6A lighting MCB.
I couldn't see the point, especially as he left the 13A fuse in it as supplied.
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