In article , Funzbo
wrote:

I've got an overrun timer fan in my bathroom controlled from the
lighting pull-switch.

The builders who installed it (before my time here) put the (non
switched) Fused Unit in the (stud) wall above the bath (ca. 50 cm from
the shower head).

Nasty and dangerous.

At present if the fuse is removed, the lights work, but the fan doesn't.

Even worse. An overrun fan needs 2 live feeds, one via the light switch
to trigger it and another permanent live to power the overrun time. The
FCU will only control one of these, if it's in the switched live the fan
will continue to run for the overrun time if you pull the fuse when the
fan is running with the light on. If it's in the permanent live I would
expect the fan to stop immediately you pull the fuse, or continue to run
until you turn the light off when it would stop immediately without any
overrun. Either way parts of the fan will still be live when the fuse is
removed. In view of the proximity of the bath and shower, take
precautions that the FCU is dry and you are not exposed to any water if
you try this.

I would like to move the fused unit to outside the bathroom, as
recommended in the regs (required?) and replace it with a switched fuse
unit.

Assuming the fan was correctly wired to take it's power from just the
lighting circuit you shouldn't need a FCU, just a 3 pole isolator switch
as others have suggested. This will ensure that the fan can be
completely isolated for maintenance.

I want to be able to override the fan using the switch, but still be
able to use the lights.

A conventional circuit with a 3 pole isolator will enable you to disable
the fan and just use the lights, you won't be able run the fan without
the lights (apart from the overrun time after the lights go out) without
adding a bit more complication.

All the diagrams I have seen for overrun fan wiring show the FSU in the
supply BEFORE the light and fan.

You should only need a FCU if you're powering the fan (and therefore the
light) from a higher rated circuit like the ring circuit.

It doesn't seem logical to have a double-pole FSU going to the fan if
there's a switched live that may still be live even though the FSU is
open circuit.

Neither logical nor safe.

--
Mike Clarke