Chaps,

I have a requirement to fit a bathroom extract fan. (Inline above ceiling).

Customer requires timer control so three core supply etc.

Sadly there is no access to the lighting connections. All I have is the three core (L/N/Sw L).

In order to get around the 3A fuse connundrum could I wire as follows?

L/N into FCU (3A).
Load side of FCU to fan (via 3pole isolator)
Commoned off the load side live of the FCU I take a feed to the contact of a relay.
Load side of the relay contact to the fan timer terminal (via remaining pole of isolator).
Relay coil is controled by the lighting SW/L. (this loop fused independantly via a second FCU).

Will this comply? My first FCU will isolate the fan completely regardless of the state of the second FCU. The three pole isolator is belt and braces.

S.

is it a must that the fan comes on with the light ?

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese

Sounds plausible - although I haven't worked out yet why you need a second FCU (unless the relay manufacturers also specify a 3A fuse).

I might be tempted to look for a better fan myself though... (one that doesn't need fusing down). Someone posted a list a while back.

- Andy.

I wouldn't bother with the fcu's at all. You're planning to have an enclosure with a relay in it; instead have an enclosure with a couple of DIN rail fuseholders in it.

The Xpelair DX100s used to have a pair of 5x20 fuses under the cover on the live poles...

How would you wire it? Just fusing L & SL separately is prohibited by BS 7671 (you'd be creating two circuits with a shared N).
- Andy.

From the definition of a circuit in the Big Red, the L and SL are one circuit anyway. And these fused lines are at the END of the circuit, immediately before the appliance they're feeding; they're not being distributed away through the building.

I would probably install a celing mounted PIR sensor just for the fan, job done

A fan isolator just before the sensor would be installed aswell

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baldelectrician.com

Only up to the point where you're introducing new overcurrent protective devices.

The definition doesn't have any limitations on the length of a circuit - you're creating new circuits, even if they're only a few mm long.
- Andy.

When does one circuit become two or more? Surely a three phase circuit could have a fuse on each phase and the same neutral. As to the fan just buy one that doesn't need a 3amp fuse or just ignor the instructions like the other million plus fans already fitted.

As I read the definition - when you have overcurrent devices in series - conductors downstream of the 2nd device aren't protected in by the same device(s) as upstream of it, so they constitute two different circuits. One circuit can be protected by several devices, not necessarily just one, but all conductors need to be protected by the same devices if they are to be the same circuit.
- Andy.

Simple way is to install a fan By a manufacturer that does not specify a 3a fuse if the supply is via a 6A mcb. Greenwood and Vortice both say this is OK for their products so you can forget the relay