A fuse is the small piece of wire connected between the two terminals of insulated mounted base. Fuse is always connected in the series of the circuit of low voltage equipment. It is the simplest and cheapest form of protection from overload and short circuit. The Fuse is expected to carry the normal current without heating and during overload / short circuit; Fuse gets overheated up to its melting point rapidly and thus breaking the circuit.
The materials used for the Fuse are Tin, Lead, Silver, Zinc, Copper etc. For small value of current an alloy of Lead and Tin, in the ration of 37 & 63% are used. But for current more than 15A, this alloy is not used as the diameter of the wire will be large and after fusing the metal release will be excessive.
Important Terminologies Related to Fuse:
Following are some of the important and frequently used terminologies used in Fuse.
Minimum Fusing Current:
It is the value of current flowing through the Fuse wire at which the Fuse wire will melt.
Fuse rating is given in Ampere. It is basically that value of current at which the Fuse is expected to operate safely without melting. This value of current will definitely will be less than the Minimum Fusing Current.
Fusing Factor is defined as the ratio of minimum fusing current to the fuse rating.
Fusing Factor = Minimum Fusing Current / Fuse Rating
The value of Fusing Factor is always greater than 1.
Prospective Current of Fuse is the value of current which will flow through it just before the melting of the fuse wire under Short Circuit condition.
Melting Time / Pre-arcing Time:
This is the time taken by a fuse wire to be broken by melting. It is counted from the instant; the over current starts to flow through fuse, to the instant when fuse wire is just broken by melting.
After breaking of fuse wire there will be an arcing between both melted tips of the wire which will be extinguished at the current zero. The time from the instant of arc initiated to the instant of arc being extinguished is known as Arcing Time of fuse.
Total Operating Time:
Total Operating Time of Fuse is the sum of Pre-arcing and arcing time.
Types of Fuses:
There are basically two types of Fuses:
- AC Fuse
- DC Fuse
This classification of Fuse arises because of arcing. In DC it is quite time taking to extinguish the arc. Therefore DC Fuses are made with longer wire so as to avoid arc. Hence DC Fuses are bigger in size. But in AC fuse as the current reduces to zero in every haft cycle (10 ms assuming 50 Hz frequency), arc is extinguished.
Other Types of Fuses:
Different types of Fuses with their brief description are listed below.
Cartridge Type Fuses:
Cartridge fuses are used to protect electrical appliances such as motors air-conditions, refrigerator, pumps etc, where high voltage rating and currents required. They are available up to 600 A and 600 V AC and widely used in industries, commercial as well as home distribution panels.
Blade Type Fuses:
This type of fuses, also called as spade or plug-in fuses comes in plastic body and two metal caps to fit in the socket. They are used in automobiles for wiring and short circuit protection.
Other Types of Fuses are SMD Fuses, Axial Fuses, Thermal Fuses, HRC (High Rupturing Capacity) fuse and High Voltage fuses.