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  1. #1
    Bmell Guest

    Default current limiters vs. fuses

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    I was just wondering if anyone here would clarify the difference in operation between a fuse and a current limiter. I spoke to someone who mentioned something about upstream vs. downstream protection.

    Thanks

  2. #2

    Default Good Question Fuses 101

    Quote Originally Posted by Bmell View Post
    I was just wondering if anyone here would clarify the difference in operation between a fuse and a current limiter. I spoke to someone who mentioned something about upstream vs. downstream protection.

    Thanks
    a fuse (short for fusible link), is a type of overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which breaks the circuit in which it is connected, thus protecting the circuit's other components from damage due to excessive current.

    Large power fuses use fusible elements made of silver, copper or tin to provide stable and predictable performance.

    Expulsion Fuse: An expulsion fuse is a vented fuse in which the expulsion effect of the gases produced by internal arcing, either alone or aided by other mechanisms, results in current interruption. An expulsion fuse is not current limiting and as a result limits the duration of a fault on the electrical system, not the magnitude.

    Boric Acid: A vented expulsion fuse in which the expulsion effect of gases and water vapor produced by the arc coming in contact with the fuse's boric acid liner extinguishes the arc.

    Expulsion Power Fuses are divided into two types “Refillable” and “Replaceable”. Refillable fuses are constructed so that the inner components can be removed and reused when the assembly is recharged with a new refill. Since they reuse the spring and shunt assembly these components can be constructed with a heavy-duty design which allows the unit to have a higher Interrupting capability. Since the components are reused it is easy to change the fuse size by simply changing the refill. Replaceable fuses have a lower installed cost by providing a more cost-effective construction. This is generally at the expense of higher interrupting ratings.

    Fuse Class - Current Limiting
    Current-Limiting Fuse: A current limiting fuse is a fuse that, when its current responsive element is melted by a current within the fuse’s specified current limiting range, abruptly introduces a high resistance to reduce current magnitude and duration, resulting in subsequent current interruption.

    Backup, General Purpose and Full Range. An understanding of these definitions will help to ensure proper application of the fuse.

    Backup Fuses: A fuse capable of interrupting all currents from the maximum rated interrupting current down to the rated minimum interrupting current. Backup fuses are always used in a series with another interrupting device capable of interrupting currents below the fuse’s minimum interrupting current.

    General Purpose Fuses: A fuse capable of interrupting all currents from the rated interrupting current down to the current that causes melting of the fusible element in no less than one hour. General Purpose fuses are typically used to protect feeders and components such as transformers.

    Full Range Fuses: A fuse capable of interrupting all currents from the rated interrupting current down to the minimum continuous current that causes melting of the fusible element, with the fuse applied at the maximum ambient temperature specified by the manufacturer.

  3. #3

    Default

    Quote Originally Posted by CPOPE View Post
    Fuse Class - Current Limiting
    Current-Limiting Fuse: A current limiting fuse is a fuse that, when its current responsive element is melted by a current within the fuse’s specified current limiting range, abruptly introduces a high resistance to reduce current magnitude and duration, resulting in subsequent current interruption.
    I'll guess that's referring to Extra High Voltage fuses? To me that sounds more like a common HRC (High Rupturing Capacity) type of fuse where it rapidly quenches the arc with a loose chemical fill.

    My own understanding of current limiting protection is where even under a dead short the protective device will limit the current to a maximum level briefly while it breaks the circuit.

    I'm guessing that the limiting fuses must use a spiral element to introduce a level of induction and resistance to the protection.

    But then.... I could be wrong.
    Portable defibrillators were first invented to save the lives of linemen. Where's yours?

    www.bigclive.com

  4. #4
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    BC... What CPope was trying to say was that a limitor fuse has a built in snuffer... Though some of them have become a little more hightech now... the origional limitor was simple... A Fuse running through a tube of sand...
    When the fuse melts in two, the ARC begins... In a tube of sand, the heat of the arc immediatly turns the sand to glass... a glass insulator... effectivly snuffing the ARC limiting the wide open current durration. In the conventional fuse barrel, the arc continues untill the air is vaporized and there is nothing left to conduct the ARC. This small time frame of ARCing pulls a tremendeous current in its duration. When the air is vaporized, the air around the vacume comes back togather with a sort of clapping motion... resulting in a Boom as heard by the ear... Lighting does the same thing.

    Anyhow, the fault current time is cut short by the limitor fuse, resulting in less heat stress on the components... xfmr bushings etc.

    And now a bed time story for your marveling pleaure...

    One Christmas night it was more frigid than Hillary in the White House... They had the amps cranked way up at the Sub Station because everything was overloaded... A 35KV Circuit operated and though it came back on... some of the lights on the circuit were still out... I was sent to investigate... I marveled at what I eventualy found... Two spans out of the Sub, C phase had burned off at a splice... The 600 wire hit the sidewalk energized... The sand in the concrete was boiled out by the ARC and formed a glass insulator... The Phase was sitting on top of the insulator hotter than Monica Lewinski in the Oval Office... The glass was non opaque, a lime green color and the phase was sort of in the middle of it(not unlike Monica in her green dress)... I almost bumped into it walking down the sidewalk and I about sh@t my pants like a Muslim and Swamp at the second comming... Anyhow this is an example of natures very own current limiting fuse... After I got them to drop the circuit out, I cut the wire on both sides of the Insulator and chipped it out of the concrete...
    ....If you want to see it, it now resides in a the trophy case at the Sugarland Service Center next to the picture of the Company Co-Ed Softball Champions for what ever year it was... of which yours truly was is in...
    Last edited by CenterPointEX; 04-04-2008 at 08:47 AM.
    2Do you not know that the saints will judge the world? And if you are to judge the world, are you not competent to judge trivial matters? 3Do you not know that we will judge angels? How much more the things of this life! 1Corinthians 6

  5. #5

    Default

    Even a fast acting fuse will allow an astronomical peak of current unless it's limited. I'm pretty sure there are fuses (and circuit breakers) designed to limit the peak value of the current while they act. Obviously they don't do it for very long, because they have a very high dissipation in the element during that action.

    Keep in mind that at the currents we're talking about here the resistance of the safety device doesn't have to be very high.

    On one of my jobs I reckon I've got some current limiting HRC fuses. They're rated at 800A and because their load is almost entirely electronically controlled and has ridiculous levels of harmonics and phase chopping, the fuses themselves buzz alarmingly when the load is at it's nastiest. I'm guessing the most likely suspect here is an inductive component in the fuses electrical characteristics. This is on secondary by the way.
    Portable defibrillators were first invented to save the lives of linemen. Where's yours?

    www.bigclive.com

  6. #6

    Default now we really have him confused

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    The following link will provide a good explination overcurrent protection, expulsion fuse, current-limiting fuse choices in a padmount application.

    http://www.aedie.org/9CHLIE-paper-send/217-ZAMORA.pdf

    Good discussion obviously not a simple answer

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