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Current flow and fuse ratings

  1. Mar 4, 2012 #1
    If I have a fuse with a rating if 5A and when I connect 10 bulbs in parallel, the current flowing will be 5A. When this happens will the fuse melt? Or does the current have to be more than 5A for it to melt?
  2. jcsd
  3. Mar 4, 2012 #2


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    You'll need to read the spec of the fuse. Fuses have time periods over which they'll sustain a given current. In the case of bulbs, there will be a higher current when you switch them on (until the resistance of the bulbs increases as they get hot), so you'll almost certainly likely burn out a 5A quick-blow fuse if it is 5A during normal operation.

    Fuses are coarse devices that help prevent (but do not prevent) further damage and safety issues in fault conditions. They help cut power under conditions of gross faults. For smaller ground faults that might not blow a fuse (e.g. you can't use fuses as a means to prevent electrocution risks), you need a RCD type circuit.

    Personally, I'd likely use a regular 13A slow-blow fuse for 5A worth of bulbs, unless there were some other sorts of high value circuitry attached that had some specific need for something more exotic to protect them.
  4. Mar 4, 2012 #3
    Oh, so for a 5A fuse, it will burn out in time depending on the specs, however it will blow if given enough time right? Thanks for the help!
  5. Mar 4, 2012 #4


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    In reality, it is impossible to specify at what current a fuse will blow. It could be slightly more than 5A; it could be slightly less.

    Likewise, it is impossible to specify that the 10 bulbs will draw exactly 5A of current. It could be slightly more or less.

    So the fuse may or may not blow; it is irrelevant to worry whether it blows for Icircuit>Irating or IcircuitIrating
  6. Mar 4, 2012 #5

    jim hardy

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    Your incandescent lamps are a deceptive load.
    Incandescent lamps draw ~10X rated current until the filament warms up.

    For that reason fuses are made with various time-to-melt parameters so you can start difficult loads. They do that by tweaking the thermal mass of the element inside.
    A slow fuse will have a massive element and a fast fuse a delicate one.

    Fuses have a "let-through" energy rating expressed as I2t, time to melt being inversely proportional to square of current. One always matches that rating to the load he is trying to protect, though in one application i had to match it to what the source could deliver... but that's another story.

    Anyhow your 5 amp fuse SHOULD carry 5 amps but you'd be well advised to leave a little extra margin. Being in a hot location reduces a fuse's current carrying ability.

    What you are protecting is usually not the light bulb but the wiring to it.
    Semiconductors on the other hand used to have the reputation of being "Good fuse protectors" but the manufacturers learned to make lightniing-fast fuses for that application.

    Littlefuse has a good reference on fusing, it's called "Fuseology"
    if this link doesn't work, a dsearch engine can find it for you.
  7. Mar 4, 2012 #6


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  8. Mar 5, 2012 #7
    So then if I have a question that has a 5A fuse. Then with 20 bulbs the current will be 5A. So they ask what's the maximum number of bulbs I can put without melting the fuse. So the answer will be 19 right? Cos the fuse might also melt at 5A?
  9. Mar 5, 2012 #8


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    Why are you posting this here and not the Homework & Coursework Questions area? Even if this is for independent study, textbook-style questions should be posted in the homework area.
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