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Can't find a 20A 250V glass fuse

Discussion in 'Electronic Basics' started by RJ, Jul 1, 2004.

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  1. RJ

    RJ Guest

    My son's scooter uses a 20A 250V glass fuse. Boy has this been
    impossible to find. I can however find the fuse in a ceramic
    configuration. Is is okay to replace a glass fuse with a ceramic one
    of the same amps etc.?
     
  2. Yes, usually. The ceramic versions often have a higher interruption
    current rating than the glass ones.

    Are you sure you have any use for the 250 volt rating?

    Would the fuse ever have to interrupt more than 32 volts DC?
    These can interrupt 1000 amps at 32 volts DC:
    http://www.bussmann.com/library/bifs/2004.pdf
    Is there any chance that the scooter can exceed this?
    It is the slow blow (high inrush version).

    Here is the fast acting version:
    http://www.bussmann.com/library/bifs/2001.pdf
    It has the same interruption rating.
     
  3. DaveC

    DaveC Guest

    If this is the common 1-1/4 inch long, 1/4-inch diameter form factor (3AG?),
    it must be the most common fuse on the road. Look in automotive stores. All
    the pre-90's models (except German) cars use these fuses.

    The 250 volt rating is a maximum, of course, and you can use any 20-amp fuse
    with a voltage rating that encompasses your scooter's voltage.

    Good luck,
     
  4. DaveC

    DaveC Guest

    I should add...

    AND is the same type (ie, fast-blow) as the original fuse.
     
  5. RJ

    RJ Guest


    Thanks for the information. All I know is that the scooter manual
    lists the motor at 24V 250W and the fuse had 20A 250V on it. Why I
    need the 250V fuse is beyond my comprehension. Requesting your advise
    please.
     
  6. Rich Grise

    Rich Grise Guest

    You don't. A 32-volt automotive fuse will be fine. They put in a 250V
    fuse at the factory because that's probably what they had on hand, or
    a batch of 20A 250V fuses was cheaper than a batch of 20A 32V fuses.
    It's a perfectly safe replacement, but not necessary.

    Years ago, I and some other people were wondering, "what the heck is
    a voltage rating on a fuse?" It turns out that it's the maximum open-
    circuit voltage at which the fuse can safely interrupt its rated
    current.

    I've seen 32V fuses in a 240V circuit. They work fine, until one
    blows - they blow, literally! I've seen one shoot that little
    bayonet cap half-way across a factory floor. The 32-volt fuse
    makes a nice arc at 240, you see.

    I saw a 20A, 15,000V fuse once, that somebody had made a typo
    when they ordered it. It was a USAF shop, so we couldn't send it
    back. It was shaped just like a 3AG, but about 3" diameter and
    about a foot long. The glass tube was about 3/8" thick, and it
    was full of sand, with a ceramic core with 5 or 6 fine wires
    in parallel, wound around it at about 3" per turn.

    Hope This Helps!
    Rich
     
  7. I have not been able to get back for a couple days, but it looks like
    others have given you the thumbs up on the low voltage replacement,
    and I concur.

    As long as the fuse is rated for at least the supply voltage it may
    have to interrupt and the supply is not capable of a fault (short
    circuit) current greater than the interruption rating of the fuse, it
    should function safely.

    There are 4 important ratings for any fuse.

    long term carrying current
    overload time curve
    interrupting current limit
    interrupting voltage rating
     
  8. RJ

    RJ Guest


    Thanks Rich and John. I got the 32V fuse today and it is working
    fine. This fuse was easy to find.
     
  9. Rich Grise wrote...
    It would have been even more interesting if the contract had
    required that components be 100% tested for all functionality
    before leaving the factory.

    Thanks,
    - Win

    (email: use hill_at_rowland-dot-org for now)
     
  10. Don Kelly

    Don Kelly Guest

    Ah but it is a wonder that someone in military procurement didn't specify
    the sure fire test of a fuses functionality - if it operates satisfactorily,
    it was good and passes the test :).
     
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