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Thread: 5A Fuse Blows

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    Re: 5A Fuse Blows

    Thank you for the replies, I do trust you but it's just a total contradiction from what I have read in other forums about similar power problems. They all say never to use a 13A fuse since that would be 2290W and my operational maximum for my psu is aroung 1000W been drawn from the socket and it also wouldnt be safe. I agree that is if i get an earth leakage or catastrophic short circuit the fuse will blow almost instantly, but will it blow fast enough to protect the circuitry in the psu been a 13A fuse. I will work with a 13A fuse but ill have to make sure the cable is rated higher than the fuse, but that wont be a problem.

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    Re: 5A Fuse Blows

    If the short is in the cable, there will be no risk to the PSU - and that is what the plug top fuse is for. If the fault is in the PSU - the internal PSU fuse should fail (if fitted) but if it is a catastrophic failure, the plug top fuse will blow anyway - and speed of the fuse blowing is unlikely to be an issue.

    As I explained in the previous posts, you need to take into account power factor, efficiency and inrush current - which is why a 5A fuse is inadequate. If it makes you feel happier, try a 7 amp (not easy to find - try rswww.com - packets of 10) or a 10 amp (same source).

    http://uk.rs-online.com/web/search/s...=7A#breadCrumb

    http://uk.rs-online.com/web/search/s...10A#breadCrumb

    They are about £1.60 per packet plus VAT and P&P.

    You have correctly identified that the fuse rating should not exceed the current rating of the cable it is protecting, but that is unlikely to be an issue. However you could replace the entire cord set to be certain.
    Last edited by peterb; 31-10-2008 at 10:49 AM.
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    Re: 5A Fuse Blows

    Im still quite sceptical about using a higher rated fuse other than a 5A.

    It is probably the inrush current causing it, but still to blow a 5A fuse from inrush current is abit high isn't it?

    A 5A should be sufficiant enough even with the power factor in the equation.
    The box states it has 80% efficiency (so 0.8 power factor), and using 230V for the supply we have the following equations.

    I = P/V

    So lets work out maximum drawn power if its an 800W psu with 80% efficiency, supplying maximum load.
    800 x 1.2 = 960W power drawn at max load with 80% efficiency.
    I = 960/230 = 4.17A
    even if the supply voltage is 195V, a 5A fuse will still be sufficient

    I don't think when a pc starts it will be using maximum load which will mean the current it draws at the start wont be very high. But then I dont know how PSU's work.

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    Re: 5A Fuse Blows

    Quote Originally Posted by GarethG View Post
    Thank you for the replies, I do trust you but it's just a total contradiction from what I have read in other forums about similar power problems. They all say never to use a 13A fuse since that would be 2290W and my operational maximum for my psu is aroung 1000W been drawn from the socket and it also wouldnt be safe......

    .
    In that case they don't understand the principles of electrical protection - what justification do they give?


    Quote Originally Posted by GarethG View Post
    Im still quite sceptical about using a higher rated fuse other than a 5A.

    It is probably the inrush current causing it, but still to blow a 5A fuse from inrush current is abit high isn't it?

    A 5A should be sufficiant enough even with the power factor in the equation.
    The box states it has 80% efficiency (so 0.8 power factor), and using 230V for the supply we have the following equations.

    I = P/V

    So lets work out maximum drawn power if its an 800W psu with 80% efficiency, supplying maximum load.
    800 x 1.2 = 960W power drawn at max load with 80% efficiency.
    I = 960/230 = 4.17A
    even if the supply voltage is 195V, a 5A fuse will still be sufficient

    I don't think when a pc starts it will be using maximum load which will mean the current it draws at the start wont be very high. But then I dont know how PSU's work.

    You are confusing power factor and efficiency. P=IV for unity power facor. The power factor for a SMPS will typically be .8 to point 9. In my earlier post I tool the worst case as 0.8 and efficiency as 80%. so power =V*I*0.8

    Efficiency is typically calculated as output power/input power *100 - so if oupout power is 800 watts input power will be 1000Watts

    (Your calculatiuon is only correct if you assume the efficiency is as a percentage of the output power)

    So 1000=I*230*0.8

    I=1000/(230*.8)

    = 1000/184

    = 5.4 amps.

    Now I accept these are worst case conditions - so the steady state current will be less than that (however there will still be an inrush current which can be more than double the steady state current for a short time - milliseconds) but that is imaterial because the purpose of the plug top fuse is to protect against faults in the connecting lead - not in the appliance. (There are exceptions to that - electric kettles for example where the load is just a low resistance) but that is the principle. so provided the power cord set is rated at 13 Amps or more, you can use a 13Amp fuse. If it is rated at 10 amps, you use a 10 amp fuse.

    Using your figures and a power factor of .9, the steadty stae current at full load will still be 4.6 amps - not much margin for the inrush current - and plug top fuses are designed to be fast blow. The data sheet for a BS1362 fuse shows that it will blow in 100ms at twice the rated current - oir 5 cycles of mains power .

    However if you are still unhappy, I have given you a source for slightly higher rated fuses.

    Who fitted the 5 amp fuse to the plug?
    Last edited by peterb; 31-10-2008 at 01:41 PM.
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    Re: 5A Fuse Blows

    superb reply peterb thank you very much

    Well I used two kettle leads both with 5A fuses, one came with the new BFG ES-800W PSU which had been fitted with a 5A fuse out of the box and the other was from my older Antec 430W? PSU which was also prefitted with a 5A fuse.

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