Over-run extractor to come on same time as shower

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?
yes.. the FCU should be placed in the permanent live before it reaches the switch, thus protecting the permanent live and the switched live.

Which regulation would be broken?
I've an incling that 431.1.1 applies.
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) It's not a shared neutral.
2 line conductors exit 2 different OCPD's and return on only one 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!
 
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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?.

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!

Where would you locate this DP MCB?
 
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?.

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!

Where would you locate this DP MCB?

Could it not be incorporated into the DB? Then run a new feed from there to the fan or light or whatever this thread was about now!!
Otherwise, a 2way DB outside the bathroom could be used.
 
Could it not be incorporated into the DB? Then run a new feed from there to the fan or light or whatever this thread was about now!!
Otherwise, a 2way DB outside the bathroom could be used.

In this instance, no it couldn't be installed within the consumer unit and a 2 way CU outside the bathroom is going to look like a dogs dinner and totally overkill for a simple 4" extract fan.
I think the job should be carried out properly so a single overcurrent protective device is responsible for isolating both the permanant and switched lines to a fan.
 
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never said there was a need..
it's the MI instructions that do, and it's nothing to do with keeping the lights on..
they don't know if you're feeding the fan from the lights or a spur of a ringmain.. so they want it fused suitably to protect the fan.. 6A should do fine..

That is why I said "There is absolutely no need whatsoever to fuse down a switched line from a lighting circuit."
 
It's still not compliant with BS7671 by way of not complying with the MI as they don't seem to differentiate between differing circuit arrangements with differing sizes of OPD's.
A brief note in the departures box may make the EIC look a little more credible.
 

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