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Fuses

Discussion in 'Electrical Engineering' started by Den, Nov 3, 2004.

  1. Den

    Den Guest

    Group:

    I'm going to embarrass myself now.

    I've been thinking about a problem too much, and now can't see the wood for
    the trees.

    Can you just check this for me - I've been thinking about this too much, and
    got myself confused.

    Let's ignore, for the moment, that many fuses will allow more than the rated
    current to flow through them.

    I think that a fuse limits the amount of *current* through it. It does this
    buy popping when the power (I^2.R) exceeds a limiting factor - since the
    resistance of the wire in the fuse is constant, the factor is the square of
    the current. This means that the fuse will pop at a given current,
    regardless of the voltage. Despite this, fuses are rated in terms of
    voltage and current. I assume that the voltage rating is a safety thing -
    correct?. So, a fuse rated at 250V/2A will pop whenever >2A crosses it,
    regardless of the voltage - correct?

    Where I got myself confused (in two different ways!) was here:
    1. A fuse (say) 3A rated at 230V will allow 690W of power to be drawn before
    it pops. But 690W @ 115V is 6A. Therefore a 3A / 230V fuse is the same as
    a 6A / 115V fuse; and
    2. A fuse (say) 3A rated at 230V, implies the fuse has a resistance of
    76.66Ohm. But 76.66Ohm @ 115V is 1.5A. Therefore a 3A / 230V fuse is the
    same as a 1.5A / 115V fuse.

    I sort of know (err hope) that 1 and 2 are wrong but am having difficulty
    explaining to myself why! Can someone (gently) put me straight.

    Cheers

    Den
     
  2. GKN

    GKN Guest

    Den. My own personal opinion is that a fuse will rupture when the stated
    current is exceeded "regardless" of the voltage that is applied. The speed
    of rupture is based on how much current tries to pass. ie. if a short
    circuit occurs downstream then the maximum current would flow rupturing the
    fuse almost instantaneously. However should the current be just of an
    overload on the fuses rating then this could cause the fuse to rupture after
    a period of time dependant on the magnitude of the overload.

    Regards. GKN.
     
  3. Nope, not equal, the 3A will blow at 3A using either voltage. you are
    calculating power and using V=IR, which dosent apply to fuses, as they are
    non linear with heating and braking. Cannot model it as a resistor.
    No! No! Bad!

    3 * 76 = 228 volts, the FUSE drops ALL the voltage??
    No way. Only drop about 10% at most.

    You can use a 3A/230 in a 115 slot and also use a 3A/115 in the 115 slot and
    it will work at 3 amps forever.
    But if you put 6 amps through it, it will pop in 100 ms or so.
    If you put a 3A/115 into a 3A/230 slot it may ark over. ZAP!!
     
  4. John G

    John G Guest

    1 and 2 are wrong.

    The fuse does not have any significant resistance.
    The 230 volts or 115 volts is almost all dropped across the load device
    when the current is within the limits.
    If the current rises high enough to blow the fuse then NO current flows
    any more and the full line voltage appears across the fuse holder.
    It is only at this time that the voltage rating is important. (Apart
    from the insulation that is used to mount the fuse.)

    If a 12 volt rated fuse holder was used in a HIGH voltage circuit there
    is a chance that the insulation will break down most particularly if the
    fuse is blown. Or an arc could devlop across the fuseable part and not
    stop the flow.
     
  5. Den

    Den Guest

    Folks:

    Thank you for your cogent responses. Of course, it is all blindingly
    obvious when it's explained!

    Cheers

    Den
     
  6. Rowbotth

    Rowbotth Guest

    This one gets my vote.

    (The voltage determines the length of the fuse, too, I think - so when
    it fails the voltage doesn't jump across the holders...)

    HR.
     
  7. Den

    Den Guest

    Colin

    Thanks for the input. Although I accept that a 3A fuse is a 3A fuse is a 3A
    fuse, and that the voltage rating is safety thing, my problem was that I
    couldn't articulate (to myself or others) what was wrong with my reasoning
    in the two erroneous calculations that I made i.e. I know that the following
    are wrong, but can't see why ... can you explain.

    Incorrect #1. A fuse (say) 3A rated at 230V will allow 690W of power to be
    drawn before it pops. But 690W @ 115V is 6A. Therefore a 3A / 230V fuse is
    the same as a 6A / 115V fuse; and

    Incorrect #2. A fuse (say) 3A rated at 230V, implies the fuse has a
    resistance of 76.66Ohm. But 76.66Ohm @ 115V is 1.5A. Therefore a 3A / 230V
    fuse is the same as a 1.5A / 115V fuse.

    Cheers

    Den
     
  8. daestrom

    daestrom Guest

    Den, your confusion is about the voltage rating of fuses. The voltage
    rating of a fuse is *not* the voltage drop through a fuse. It is the
    maximum voltage that the fuse can have impressed acrossed it when blown and
    not allow continued arcing.

    Imagine for a moment if you put a 3A/12V fuse in a 4160V circuit. As long
    as the current stays below 3A, things would seem normal. But 3A at 4160V is
    12 480 watts. Yet the fuse doesn't blow at 3A. The problem with such an
    arrangement will come when the current exceeds 3A. The fusible link will
    melt and break the connection. But the ends of the link are so close
    together, the high voltage will just arc across the gap and current will
    continue to flow. The arc will probably generate more heat than the fusible
    link did, and the whole thing will start to melt/explode. But a high
    voltage like that will just keep arcing across the fuse-holder's mounting
    clips. Doesn't stop the current flow so whatever you're trying to protect
    with the fuse isn't protected and you've got a charred mess where the fuse
    was. A fuse rated at 3A and the proper voltage will open at almost the same
    current and the ends of the link will *not* have a continuous arc across.

    A 3A/230V fuse may have a voltage drop across it (when passing 3A through
    it) of only 1/4 V. Under this operation, it will dissipate only 0.75 watts,
    while the rest of the circuit dissipates 3A* <circuit voltage>. If the
    circuit it is in has 200 V supply, then this fuse will allow 3A * 200V =>
    600W of power. If the circuit is only 12V, the power it will allow is only
    3A*12V = 36W. But at these voltages, if the link does melt from
    overcurrent, the voltage is not high enough to create a continuous arc
    across the gap between the bits of melted link. If a 3A fuse has 1/4V
    voltage drop across it, it has an internal resistance of 0.25 / 3 = 0.08333
    ohms. Regardless of the voltage of the circuit it is in.

    Generally, a fuse of a given voltage rating can always be used in lower
    voltage circuits if the physical size is the same. But not the other way
    around (never put a low voltage fuse in a high voltage circuit). Using a
    higher voltage rating sometimes leads to confusion later when someone else
    comes along and sees a 230V fuse in a 120V circuit and reads a
    label/schematic that says it is supposed to be a 120V fuse.

    daestrom
    P.S. And in power applications, fuses have a *maximum* current interrupting
    rating, but lets not go there.
     
  9. Bob Peterson

    Bob Peterson Guest


    No. Fuses are not power limiting devices. They limit current. As long as
    the volatge applied is within the capability of the fuse to operate, the
    fuse will open above the specifed rating. Does not matter at all if the
    supply voltage is 120V, 220V, or 25 volts (within the voltage limits of the
    fuse).
    The fuse has close to zero resistance. The load is where the resistance is
    and what determines how much current flows through the fuse.

    <snip>
     
  10. Den

    Den Guest

    Wow, thank you so much for taking the time to explain so cogently and
    thoroughly. Now I can see exactly where I was going wrong in my reasoning.

    Many thanks

    Den
     
  11. Remember of course that there are many different types of fuses, anti-surge,
    slow blow, quick-blow, and motor control fuses which area different thing
    altogether,
    fuses are rated at a maximum current over a set period of time.

    gb
     
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