There are various types of fuses are available in the market today. In terms of quantity, fuses more than any other overcurrent protection devices. They provide the economy in protection as well as flexibility and provide safety where they are used and time-current characteristics. They are used for overcurrent protection of transformers which makes them to easily separate, capacitors, and lateral taps in distribution systems.
Fuse can be classified into two types
1.Non-Current Limiting Fuses (Expulsion type)
The expulsion type fuse is used where expulsion gases do not cause any problem such as in the case of overhead circuits and equipment. These fuses also called current awaiting types and the working of interrupting medium is similar to that of an ac circuit breaker. The temperature range of the arc in between of 4000-5000K. At this high-temperature special material of fuse located in close proximity to the fuse element rapidly create gases. Mostly used gas generating materials are fiber, melamine, boric acid, and liquids such as oil or carbon tetrachloride.
Gases help to create a high-pressure turbulent medium surrounding the arc so as to suppresses it or when the current reach at zero and the arc channel reduces to a minimum the takeaway gases rapidly mix with remaining ionized gas and thereby deionize them as well as remove them from arc area. In turn, high-pressure turbulent increases to a rapid build-up of dielectric strength that can withstand the transient recovery voltage (TRV) and steady-state power system voltage.
TRV for expulsion fuse is shown in the above fig. In an inductive circuit, current zero occurs at lag 900 to voltage i.e. when voltage is at maximum value and current at minimum. The action of interrupting medium basis TRV to be seen in this region.
Vacuum fuse is generally a non-expulsive fuse but still a current zero awaiting type. The design, operation, and current-voltage-time relationship of this fuse are the same as expulsion fuse. The main difference between vacuum fuse and expulsion fuse is that it is a completely sealed unit and no expulsion action. Interruption occurs because of the rapid dielectric build-up in the fuse that occurs in a vacuum after the current zero holds out.
3.Current Limiting Fuse
Let’s assume an overcurrent protective element can insert a large resistance in series during fault current that reduces the value of high current. This improves the power factor in the fault circuit which otherwise is more or less inductive. Thus, the zero-crossing of the current and voltage may be in phase. This indicates that when the arc is extinguished temporarily at current zero, the applied voltage across be zero.
If at current zero, V(t) = Vm, then the presence of a large electric field does not support in quick deionization of arc. On the other hand when the current zero and voltage zero are in phase, then when the temporary arc is extinguished, the dielectric medium will be quickly de-ionized. This increases to speeding infuse action. The main question, however, is how to insert the high resistance in series in the fuse? Basically, the current limiting fuses achieve to compress the arc and it is cooled by sand.
A typical current-limiting fuse is shown in the above fig. In this case, the fusible element of the fuse is very long. The element is completely filled by a material, typically silica sand, to carry the arc as well as maintain a very high pressure to restrict the arc area caused by the practically simultaneous melting of the full length of the element. Due to the long length of the element, very high resistance can be easily produced in the circuit in a very short period of time (typically hundreds of μsec).