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Over-current protection

Former Community Member
Former Community Member
Why would you use a fusing factor when calculating the size of a circuit breaker?
  • 0
    You may find this previoius discussion on the old BB answers your query, but in summary although we call it a "fusing" factor, it is really just a de-rating to handle the fact that neither fuses nor breakers have an instant transition from tripping to not tripping at the rated current.

    Rather there is a soft transition between some lower limit value of 'never trip' and a higher limit current of 'always trip' with a fair amount of uncertainty about  what will happen and how fast, for any current of intermediate value.

    So a 13A fuse will carry 13A all day at room temperarture without operating, (the never trip condition) and will fire in a some seconds to  minutes at 20A (the always trip condition) but at anything over 13A but  less than 20A may trip  or may not, depending on local factors, and the manufactureing tolerances on the fuse wire, the duration of the overload and so on. The fusing factor is just a measure of the ratio of the two limits. A very similar consideration applies to breakers with a thermal mechanism, though the exact factor may vary a bit.


    regards,

    Mike.

  • Former Community Member
    0 Former Community Member
    No that won't help, that was just many pages of the usual nonsense; and I was banned from the forum therefore unable to answer.


    Are you aware that Dr. J Duncan Glover, the American electrical expert who diagnosed the cause of the fire at Grenfell Tower does not know that there is a fuse in a 13amp plug?
  • Former Community Member
    0 Former Community Member
    If a fuse is wrapped in silver paper and then the rate of rise in over-current (within the faulty appliance) is as much as 100mA per hour, even if your over-current protective device is as little as half an amp too large, it is still going to take 5 hours with your cable exceeding its' current carrying capacity before the device even notices that there is a problem!


    Small overloads of a long duration!
  • 0
    Well, if the fuse is replaced by foil as you suggest, then you need to re-calculate the ratings. However, that is not a responsible design.


    Of course if the overload is a few times 100mA, or even  a few times 1000mA above the correct load, there will be very few cases where we fear for the integrity of the fixed wiring, so there  is no need to disconnect that. A dangerous fault, say one that draws twice or ten times the normal load current is the sort of thing that might cause damage and fuses and MCBS are intended to be sized to detect that.


    Much as the fuse does not suddenly blow at 1% over its nominal rating, neither does the cable, it just goes from being a bit warm to being a bit warmer.

    Even if we could make devices that had a knife edge tripping behaviour, they would be no use to us, as they would then also need to include delays to allow switch on surges and other transients to be pass without false tripping.
  • 0
    Often the greatest trip hazards are from the ones that we don't see coming. Too much port, for instance, in a short period can cause an attack of guout. Similarly, pushing a circuit beyond its safe limits by changing the characteristics of its protection is likely to cause damage.

    Legh
  • 0
    I feel there's a rant on its way, but someone is trying to sidle around to it via circuitous route, so we don't know what it's going to be about until it happens.
  • Former Community Member
    0 Former Community Member
    mapj1:

    Well, if the fuse is replaced by foil as you suggest, then you need to re-calculate the ratings. However, that is not a responsible design.


    Of course if the overload is a few times 100mA, or even  a few times 1000mA above the correct load, there will be very few cases where we fear for the integrity of the fixed wiring, so there  is no need to disconnect that. A dangerous fault, say one that draws twice or ten times the normal load current is the sort of thing that might cause damage and fuses and MCBS are intended to be sized to detect that.


    Much as the fuse does not suddenly blow at 1% over its nominal rating, neither does the cable, it just goes from being a bit warm to being a bit warmer.

    Even if we could make devices that had a knife edge tripping behaviour, they would be no use to us, as they would then also need to include delays to allow switch on surges and other transients to be pass without false tripping.


    Why would you need to fear for the integrity of the fixed wiring?

    That is what over-current protection is in place for, to remove the fear for the integrity of the fixed wiring!


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