Had a run on timer fan installed in the loft ducted into bathroom then out of the wall.Bathroom has two windows,looks like its taken off the bathroom light circuit,the live switched live and neutral run through a fused spur the switched live is actually chocolate blocked inside the spur,it then goes through a triple pole isolator and then to fan.Does the switched live not need fuse protection as well ?
inline bathroom fan,mounted in loft wiring requirements
- Thread starter donetello
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In theory yes
In practice no.
He has been good to actually fit one fuse, many dont.
In practice no.
He has been good to actually fit one fuse, many dont.
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This has been debated many times.
The lights are normally fused to 6A because the ceiling roses are only rated at 5 or 6 amp in the main one can't use a larger size fuse or MCB so it will protect the wiring to fan.
Many fans stipulate a 3A fuse but likely this is purely because the preferred sizes are 3 and 13 amp so of the two 3 is more appropriate but fusing to 5 or 6 would likely offer no less protection. The windings in many fans would fail even with 1 amp if stalled so 3 amp is really only to protect cable and since 3A normally fuse and 6A normally MCB putting in a 3 amp fuse will not ensure it will blow before main lights MCB.
The triple pole isolator does allow one to remove fan load should it go faulty allowing one to still use the lights.
The other method is to use a double pole light switch so although same mechanical device it is electrically isolated and the fan is powered from the socket supply through a FCU.
With out working out the electronics in the fan run on timer can't really say if the activation pulse needs a fuse. It could be to an optical isolator or it may not so only the manufacturer can really answer that question.
Where the problem lies is when some one has swapped the standard 6A MCB for a 10A MCB not only for the fan but for the lighting. In theroy every bulb should have a built in 2A fuse so when it blows should ionisation happen inside the bulb the fuse will stop the MCB tripping. In practice these are often missing so with a 10A MCB when a bulb blows the 50A to open the MCB with magnetic part of trip (B type) is also enough to weld the bulb into the holder so instead of changing a bulb you have to change the holder as well. This has also been an argument with RCBO where often a C6 instead of B6 is used again can result in bulb welding onto holder. (60A instead of 30A to trip magnetic part)
So personally as long as a B6 MCB/RCBO is used I would not bother with the fuse. However strictly if manufacturer say use 3A fuse you should.
The lights are normally fused to 6A because the ceiling roses are only rated at 5 or 6 amp in the main one can't use a larger size fuse or MCB so it will protect the wiring to fan.
Many fans stipulate a 3A fuse but likely this is purely because the preferred sizes are 3 and 13 amp so of the two 3 is more appropriate but fusing to 5 or 6 would likely offer no less protection. The windings in many fans would fail even with 1 amp if stalled so 3 amp is really only to protect cable and since 3A normally fuse and 6A normally MCB putting in a 3 amp fuse will not ensure it will blow before main lights MCB.
The triple pole isolator does allow one to remove fan load should it go faulty allowing one to still use the lights.
The other method is to use a double pole light switch so although same mechanical device it is electrically isolated and the fan is powered from the socket supply through a FCU.
With out working out the electronics in the fan run on timer can't really say if the activation pulse needs a fuse. It could be to an optical isolator or it may not so only the manufacturer can really answer that question.
Where the problem lies is when some one has swapped the standard 6A MCB for a 10A MCB not only for the fan but for the lighting. In theroy every bulb should have a built in 2A fuse so when it blows should ionisation happen inside the bulb the fuse will stop the MCB tripping. In practice these are often missing so with a 10A MCB when a bulb blows the 50A to open the MCB with magnetic part of trip (B type) is also enough to weld the bulb into the holder so instead of changing a bulb you have to change the holder as well. This has also been an argument with RCBO where often a C6 instead of B6 is used again can result in bulb welding onto holder. (60A instead of 30A to trip magnetic part)
So personally as long as a B6 MCB/RCBO is used I would not bother with the fuse. However strictly if manufacturer say use 3A fuse you should.
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Acceptable, but I very much doubt that many people would bother. As eric has said, the need/point of any fusing for the fan is very debatable, and the usual arrangement of the electronics is such that the argument for fusing the S/L would be extremely weak.
An alternative is to modify the wiring to the light in question (the one which is activating the fan), such that it (the light and associated switch) is fed via a 3A FCU and thus to take both L and S/L from the output of the FCU. However, again, I think that very few people would bother.
Kind Regards, John
I've studied a good few fan timer modules in my time, and have never seen one with an optical isolator. However the trigger input ('switched live') invariably goes through a very high value resistor (typically 150-300 kΩ
to either the base of a transistor or input of a CMOS gate. That being the case, and given the extremely low likelihood that the resistor would ever fail (let alone fail into a low-resistance mode), I think that any electrical argument for needing to fuse it would be exceptionally weak.Kind Regards, John
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