Discussion in 'Electricians' Talk' started by unphased, Jun 19, 2014.
I'd say it would be more likely to fail in a dead short than in an overload?
In a short-circuit, either or both 3A and 6A would trip, if there was a slight overload such that the current was exceeding 1.45 times the trip current then the 3A should go first.
I would say the 3amp fuse would always blow first considering that a Type B circuit breakers trip current is between 3 and 5 times the rated current. So for a 6Amp breaker that is going to be at least 18Amp.
That is only for instantaneous operation. It must not operate below 1.13 x In but must operate within 1 hour at 1.45 x In.
A 3 Amp fuse will blow within 10 seconds of 200% of ln, I make that 6Amps.
A 6 Amp MCB won't trip at 6 Amp load.
That doesn't explain how a fan (which is a fixed load) could overload to 6A though.
I suppose we could say it's only a fixed load whilst it's running Risteard? A different matter when the rotor is stalled, just a thought!
Hi Sen. When I can accrue an old fan I will do a test. I will measure amperage on normal running and then stall the rotor and watch the amp meter. I suspect at first there will be a negligible amperage increase under stall conditions - however as the windings approach thermal runaway mode (gradual increase in temp with no cooling - just like a nuclear reactor going into meltdown I suppose) then I expect to see a rise in the amperage which directly corresponds to winding temp. At around 3 to 4 amps smoke and bols of molten copper spraying out the fan could manifest with a final puff - the fuse/mcb would more then likely still be closed - ie no rupture/trip evident as the windings themselves would act as a fuse in the end.
Whilst the rotor stalling may cause overheating I'm not at all sure that it would cause such an increase in current draw for a BS 1362 fuse to offer any protection whatsoever. To me it seems that the only justification is to adhere to manufacturers' instructions - I doubt that there is any credible evidence that a BS 1362 fuse could protect the fan.
It'll probably take a while, When I used to do alot with motors in the past the TE intermittent rated ones used to have a rating of 30mins of use before a cooling down period. Although I came across alot that had burned out, I never witnessed any burning out so can't say how long they took.
I remember a work mate of mine changed some brushes on a motor and didn't clip the leads back in properly, The lead rubbed on the commutator and probably jammed the motor by getting stuck on the wire hooks - That resulted in a burn out situation. The boss was not best pleased about that one
I woud say the fuse is more to protect failed electronics, rather than the titchy motor.
On a (sort of) related note, i was cutting down old concrete fence posts with a 9 inch angle grinder recently rated at 2350W and managed to stall it twice.
The plug protected by a 13A fuse and the socket supplied by a 16A 60898 MCB. On both occasions that the grinder stalled; it only took the breaker out. Nothing else on this circuit at the time.
Assuming the Zs was within limits this means that disconnected within 0.4 seconds - well before the fuse would have had time to see or rather feel any abnormal amount of current flowing through it.
Exactly Fats - just goes to prove things really. I might phone up Manrose technical dept and have a tête-à-tête with them about the 3 amp fuse versus the 6 Amp mcb..you only have to compare the curves really to see whats happening..
Let us know how the call goes mate!
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