Can Screw In Fuses be Safe If Sized Correctly?

Discussion in 'Electrical Forum discussion & Blog' started by molo, Aug 25, 2013.

  1. molo

    molo New Member

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    Can Screw In Fuses be Safe If Sized Correctly?

    If no, why not?
  2. jwelectric

    jwelectric Electrical Contractor/Instructor

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    The trip curve for a fuse is a lot less than for a breaker. If sized correctly they are the safest overcurrent devices on the market today for residential wiring
  3. DonL

    DonL Out of the Trades

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    The only time I have found them unsafe is when you do not have a proper spare and put in a bigger size or do the penny trick.

    You should have plenty of the proper size spares on hand.

    They seem to be getting harder to find.
  4. hj

    hj Moderator & Master Plumber Staff Member

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    The "sized correctly" is the critical thing, because all the normal fuses use the same size socket, which means you can replace a 15 amp with a 30 amp thus creating the "unsafe condition". Fusetrons which use an adapter to limit the interchange capabilities make a fuse socket safer.
  5. DonL

    DonL Out of the Trades

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    I really prefer fuses over breakers. I use them in my sub panels.

    Sometimes by the time a breaker trips, more Electronics Fry.

    Most Fuses will also Pop from a Lightning strike, A breaker can get blown apart in the box and never trip, Or I guess that is called tripping big time. Even if a fuse blows when Hit, The spark gap is really not enough to protect from a direct Lightning strike. But it can help from Everything in the house getting smoked.



    Be Careful when Playing with Electricity, And have Fun doing it in a safe manner.
    Last edited: Aug 26, 2013
  6. Wet_Boots

    Wet_Boots Sprinkler Guy

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    The one valid knock I would assign to thread-in fuses is the possibility of overheating due to a combination of poor contact and high currents.

    One my electronics teachers introduced the basic concepts if voltage and current and power by way of recounting a barn fire that was blamed on a fusebox that had gotten hot enough to set the wooden wall afire, from which we learned of current and resistance becoming power (and heat)

    Odd thing, is that a few years later, I hear a faint 'sizzling' sound in the basement of a house I was renting, and it's coming from a auxiliary fusebox/switch that feeds an electric water heater. Sure enough, the contacts for the screw-in fuses are burnt beyond redemption, and the little switch box was mighty warm.
  7. DonL

    DonL Out of the Trades

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    That is one reason why Aluminum wire is not good to use in a house, even if code allows it.
    Last edited: Aug 26, 2013
  8. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Something to think about, when sizing non-time delay fuses which is what an Edison base fuse is, for motors we use 300% but when sizing a time delay, the ones that use an adapter we use 175% the full load amps of the motor.

    Does that mean that the old Edison base fuses will blow before the fuse stat blows under the same load?

    If the Edison base fuse will blow faster, then, which do you think is the safest? For breakers a general rule of thumb is at 135% it can hold for two hours and meet the UL Standard, 300% for two minutes or 600% for .003 seconds and meet the UL Standard.
  9. DonL

    DonL Out of the Trades

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    I don't think there is any doubt that a Fast Blow of the same rating is safer.

    When you start changing % ratings because of a "Expected" load then it becomes unsafer, unless it is under 100%.

    When you compensate for surge current every device is different. Some devices are close to a direct short until a complete AC cycle or 60 has occurred. Transformers are a example. Motors take until the motor comes up to speed, before the current levels out. This is also why you can not use a VOM to measure a inductive load, The meter looks at the load with a DC prospective.

    This is one reason we built "Soft Start" devices in our Higher Power Equipment.

    The First thing many people do is install a bigger fuse if it keeps blowing on occasion. A slow blow is safer than installing a bigger fuse. It is important to use the proper fuse type and rating. The smaller the better, for the "True" load.


    Did I get all of that right ?

    I think it is past my bedtime.
    Last edited: Aug 26, 2013
  10. Stuff

    Stuff Member

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    For 240v circuits a standard two pole common trip breaker would be safer for a direct short condition.

    I'm thinking how people used to put pennies in their fuseboxes. Now that pennies are mostly zinc it might be an even more dangerous practice as they would heat up.
  11. jwelectric

    jwelectric Electrical Contractor/Instructor

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    I don’t understand how you come to this conclusion as a breaker will let through current in excess the rating of the breaker but an Edison base fuse will blow at the point of its rating.

    I am speaking of compliant installation without any of the old tricks of defeating the overcurrent device. In my years of experience I have found as many 20 amp breakers on 14 gauge wire as I have found 20 amp fuses on the same.
  12. DonL

    DonL Out of the Trades

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    The Fuse or Breaker rating is the maximum current that the device can continuously conduct without interrupting the circuit.

    I was talking in terms of the speed a fast blow fuse takes to open. For short duration a breaker can pass well above its rating.

    Instantaneous tripping current depends on the type being used.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    FWIW, there are slow-blow fuses, too.
  14. DonL

    DonL Out of the Trades

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    Very True.

    They are needed for many loads where a FB would Pop.

    But many loads will work with a FB, even if not required.
  15. DonL

    DonL Out of the Trades

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    Was this just a test ?
  16. molo

    molo New Member

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    DonL, No, this is a subject I've wondered about, and I thought the experts here could help!
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    If close control of the maximum current in a circuit is required, a fuse will typically protect things quicker than a circuit breaker, and often, be more reliable at that task. But, it is certainly easier to throw the CB back on than to go find the proper fuse should it eventually die. Both fuses AND CB can fail over time from corrosion, or thermal stress, vibration, and other things. So a failed one isn't necessarily an indication of overload. My guess is that a failed CB is more likely to still pass current than a failed fuse, but failure isn't all that common on either.
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