Maybe but as I said the fuse is to protect the cable. Any protection for the fan motor must be provided internally by the manufacturer. It is quite likely these are sold in other countries where FCUs are non compliment and unavailable and where lighting is protected by a 16 amp MCB. Manufacturers MUST provide internal protection, probably a heat detector.
New bathroom extractor fan and light switch
- Thread starter gdunbar
- Start date
Maybe but as I said the fuse is to protect the cable. Any protection for the fan motor must be provided internally by the manufacturer. It is quite likely these are sold in other countries where FCUs are non compliment and unavailable and where lighting is protected by a 16 amp MCB. Manufacturers MUST provide internal protection, probably a heat detector.
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I agree - but, in any event, as I have described above, my experiment appears to have confirmed my gut-feeling that even a stalled fan motor (of a standard kitchen/bathroom extractor fan) draws so little current that any fuse (or other OPD), whether within the fan or external to it, would be both totally unnecessary and totally useless for addressing that possible scenario.
Kind Regards, John
Yes, the isolator is only there for when the fan requires maintenance such as cleaning.
... or to enable it to be isolated (so that some, maybe half, the house doesn't have to remain without power until it is mended) in the (very unusual) event that something happens to it which causes an RCD or MCB to trip (make sure that you have a torch to hand if the isolator is in the loft
).Kind Regards, John
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When stalled there is no air flow to cool the fan and the 20 watts now confined in the motor will result in the coils and cores heating up. This could easily lead to breakdown of insulation in the windings, shorted turns, reduced inpedance and increasing current to the point that the coil eventually burns out, short circuits and takes out t the smallest fuse in the supply.
Will a 3A BS 1362 operate before a B6? What do the time-current curves, discrimination guidance, etc say?
And they'll keep on doing it until seriously challenged over it.
I'd suggest the OP writes to them asking why they have made a product so flaky that it needs to rely on the external circuit protection for it to be safe to use, and to ask if he can obtain the version which they sell in countries where FCUs do not exist and it simply is not possible to provide 3A fuse protection.
Not impossible, but I would suspect fairly unlikely. Don't forget that, despite what I wrote, we're actually talking about ~20 VA, so the amount dissipated in the winding could be a lot less than 20W ... which itself is not all that much.
If the process you described occurred, it would presumably result in a true 'fault' (of 'negligible impedance') suddenly appearing at some point (not a gradual increase in current). If that happened, it is pot luck as to whether the 6A MCB or 3A fuse would operate first - and, even if there were not a 3A fuse, the MCB would (if the circuit were properly designed) operate 'immediately'.
Kind Regards, John
As I've just written, the series of theoretically possible events that Bernard described would presumably eventually result in a 'fault of negligible impedance' suddenly appearing (not a gradual increase in current). In that situation, both the fuse and MCB should (given satisfactory Zs) operate 'immediately' - so it's pot luck as to which (or both) would operate first. Even without the 3A fuse, the B6 should operate 'immediately' in that situation.
Kind Regards, John
Just the opposite. as the motor windings dis-integrate the current ramps up over a period of time. At first with one or two shorted turns the increase is small but as more and more insulation fails the number of shorted turns increases and the current increases. At the end the current increases rapidly but still slow enough for a 3 amp device to trip before a 6 amp device trips
So I wonder how such an appliance is made safe to use in countries where a 3A fuse simply cannot be provided?
Maybe with an inline fuse, or the appliance burns, or the fuse to the lighting circuit trips and there is darkness.
#2 is not what I would term "safe".
For a start, as I've said, I'm far from convinced that any windings would "disintegrate" with whatever power dissipation that ~20 VA represents (you may even entice me to 'do the experiment' if I can find a fan that I wouldn't mind destroying!). However, in any event ....
IF one ever did get to the stage of that 'rapid increase' in current, then I doubt that the difference in time taken for a 3A fuse and a B6 to operate would be particularly significant! Don't forget that a current less than about 5.8A will not blow a 3A BS1362 ever.
Kind Regards, John
I thought about suggesting that.
You'd need to simulate the environment of a fan mounted in a wall (leaving it in free air would increase heat dissipation), and you'd have to make it a safe area in which it could catch fire.
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