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Over-current Protection by Fuse

written by: Chief Engineer Mohit Sanguri • edited by: Lamar Stonecypher • updated: 8/31/2011

Several types of over current protection devices are available in the market, and the simplest of them is the fuse. Though we consider it an ancient and low technology solution, the truth is that a lot of engineering and research is behind it.

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    Fuses

    Fuses are the earliest known over-current protection devised and have been used from the time electricity was first harnessed by man for easing his work and for giving illumination. Though they are very simple and are also referred as low technology products, they are popular today even when cutting edge technology overload relays and MCB’s are in use because of their low cost.

    The fuses of original design had a ceramic holder and a fuse wire wound between two points. The wire of different capacities is available and according to the application they are chosen and used. The common rating of the fuse wires is 5 ampere and 15 amperes for domestic uses.

    fuse 

    The original design of the fuses are still being used in many countries where open wiring is still used. The sealed cartridge type design of fuse is also available and must be used as it has a faster arc quenching properties. Fuses are basically self- sacrificing and if the current flowing through them exceeds their rating they melt, thus breaking the circuit and protecting the machines and appliances.

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    Properties of Fuses

    Fuses have been around for a long time and found in all homes and work places. Though they are very old invention and have been used without any change in principle for a long time, yet they are as indispensable today as they were a hundred years before.

    The current in any circuit can increase due to some fault in the appliance, such as a short circuit, or through overloading. Though overloading can also burn a motor or appliance, it is the short circuit current that is the highest, and fuses are very suitable for these types of over-current. They are the most common type of protection against short circuits taken in domestic circuits.

    The old rewireable fuses are reusable, but can be a fire hazard as the arc does not go off quickly. In the sealed ceramic body or glass encased fuses, some compounds are filled inside the capsule that cater to fast arc quenching.

    Fuses are of a very simple construction, are reliable and cheap, and therefore are inherently popular. However there is a disadvantage of fuses in that they are insensitive to small amounts of over-current. They will burn out, but only after a time long enough for the appliance to become damaged, and for this purpose additional protection like miniature circuit breakers have to be used.

    The plus point in the favor of fuses is that they have a very fast speed of operation when high currents are involved and this is highly beneficial in case of short circuit over currents as they operate faster than circuit breakers.

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    Fuse Construction

    There are many types of fuses. While the old rewireable fuses were of ceramic construction and had a fuse wire wound between two contacts, more modern ones are of a different construction. Current automotive fuses have a plastic body and the fuse wire is embedded in between. The high rupturing capacity fuses have a silver fusible element at the center which is held between the metal end caps. The fuse element is sealed in a glass capsule and special material like boric powder of quartz sand is filled inside. The purpose of these materials is that they help in fast quenching of the arc, thus reducing the fire risk.fuse construction 

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    Precautions in Changing Fuses

    Though the changing of fuses seems to be a simple job there is always a risk of electrocution. The handling of the fuses must be done by wearing rubber insulation gloves and by using fuse toggles or tongs. The circuit must be isolated or made dead before the work. The other precautions that must be taken are as follows:

    1. The cause of the fault that led to the fuse blowing must be found out and repaired before changing the fuse. If you do not follow this another fuse will blow but your appliance also comes to risk.
    2. The rating of the replacement fuse must be exactly same as the old one.
    3. When one fuse has broken in one phase of a three phase circuit then you must replace all the fuses as the rest also may have been damaged.
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    Image Credits

    1. Fuse Construction: Wikipedia Commons
    2. Fuse: Wikipedia Commons

References

  • Practical Marine Electrical Knowledge by Dennis T. Hall
  • Marine Electrical equipment and Practice by H.D. McGeorge