yes.. the FCU should be placed in the permanent live before it reaches the switch, thus protecting the permanent live and the switched live.1) If the fan was on a lighting circuit in traditional style, and the MI to fuse it at 3A was done with 2 FCUs, one in the permanent live and one in the switched, would you object?
I've an incling that 431.1.1 applies.Which regulation would be broken?
It mentions 3 phase motors as ONE example where danger can arise from the disconnection of just one line conductor, but I put it to you that if one fuse or breaker operates on one conductor ( or is switched off ) then it's dangerous in as much as a general DIYer might think that it's safe to start fiddling in there without realising that the other line may still be live.
simplest solution since we're talking breakers is to use a 2 pole breaker.
2 line conductors exit 2 different OCPD's and return on only one neutral..2) It's not a shared neutral.
that is the very definition of a shared neutral..
I'm sure that is correct - all line conductors must disconnect at the same time (on a 3 phase supply) - that is why although a motor could be protected by BS88s or 1361s or similar, it must have an overload relay which allows this to happen.
Of course in a single phase installation, only the live conductors must be disconnected - both if there are 2, all three if there are 3 etc.
So a double pole MCB would suffice in this case. I wouldn't go down the route of 2x fCUs as that is what a 3pole fan isolator replaces from years ago!