3A fuses for extract fans

Discussion in 'Electricians' Talk' started by unphased, Jun 19, 2014.

  1. seneca

    seneca Screwfix Select

    I'd say it would be more likely to fail in a dead short than in an overload?
  2. unphased

    unphased Screwfix Select

    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.
  3. Cheburashka

    Cheburashka Member

    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.
  4. Risteard

    Risteard Member

    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.
  5. Cheburashka

    Cheburashka Member

    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.
  6. Risteard

    Risteard Member

    That doesn't explain how a fan (which is a fixed load) could overload to 6A though.
  7. seneca

    seneca Screwfix Select

    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!
  8. JP.

    JP. Screwfix Select

    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.
  9. Risteard

    Risteard Member

    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.
  10. Cheburashka

    Cheburashka Member

    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 :D
  11. Lectrician

    Lectrician Screwfix Select

    I woud say the fuse is more to protect failed electronics, rather than the titchy motor.
  12. FatHands

    FatHands Active Member

    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.
    Last edited: Jun 25, 2014
  13. JP.

    JP. Screwfix Select

    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..:D
    FatHands likes this.
  14. FatHands

    FatHands Active Member

    Hey JP,
    Let us know how the call goes mate!

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