Electrical Fuse is simple and cheapest device which are used for interrupting an electrical circuit under abnormal conditions (such as short circuit conditions, overload conditions etc). Fuses are used upto 66kv called high voltage fuse and low voltage fuse upto 400V. In this article, we discuss about definition of fuse, working principle and fuse, fuse element materials and types of fuse.
Definition of Fuse
What is fuse?
A fuse is small piece of wire, inserted in the circuit which melts when the value of current plows more than the permissible limit through it and thus break the circuit. Fuse is always connected in series with the circuit.
The fuse element is usually made of materials having low melting point, high conductivity etc. The action of a fuse is based on the heating effect of the electric current when flows through a fuse element. When the current flows through a fuse is in safe value, the heat developed in fuse element rapidly dissipated into the surrounding air and therefore fuse element remains at a temperature below its melting point. However when fault occurs in the electric circuit, the current exceeds the safe value or limiting value. The heat is generated due to this excessive current cannot be dissipated fast enough and fusible element gets heated, melts and breaks the circuit. If the value of current is large enough, the more rapidly the fuse will blow i.e. the fuse has inverse time current characteristics.
ADVANTAGES AND DISADVANTAGES OF FUSE
It is cheapest and simple form of protection.
It needs no maintenance.
It interrupts very large short circuit current without any noise, flame gas or smoke.
It operation is inherently completely. It means there is no required any more equipment for automatic action.
Its inverse time current characteristics enable. Its use for over load protection.
The fuse has following disadvantages: –
Considerable time is waste n rewiring or replacing a fuse after operation.
The current time characteristic of a fuse cannot always be correlated with that of the protected apparatus.
DESIRABLE CHARACTERISTICS OF FUSE ELEMENT
The fuse elements should have low melting point like tin and lead.
It should have high conductivity like silver and copper.
It should be free from deterioration due to oxidation. Silver material has such properties.
The fuse element should be cheap in cost like lead and tin.
FUSE ELEMENT MATERIALS
The materials used for fuse elements must be of low melting point, low ohmic loss, high conductivity, low cost and free from deterioration. A low melting point is available with a high specific resistance metal. The most commonly used materials for fuse element are lead, tin, copper, zinc and silver. For smaller value of currents tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For exceeding the current 15A this alloy is not used as the diameter of the wire will be lager so that beyond 15A rating circuits Copper wire fuses are employed. Zinc fuse is good if a fuse with considerable time-lag is required i.e. One which does not melt very quickly with a small overload.
The present trend is to use silver despite. Its high lost due to the following advantages.
It does not oxidized. It means it is free from oxidation.
It does not deteriorate when used in dry air. There is no effect of moist on the silver fuse element, when the silver surface is attacked and a layer of silver sulfide is formed at the surface of a fuse element which shield the silver fuse element from further attack.
The conductivity of silver fuse element is very high. Therefore, for a given rating of fuse element, the area of cross section of silver metal is required is smaller than that of other materials.
The conductivity of silver does not deteriorate with oxidation so that life of silver fuse element is long.
Owing to its high conductivity the mass of molten fuse element to be handled is minimum due to this property the operating speed is fast.
IMPORTANT TERMS OR DEFINITIONS
It is the r.m.s value of current which the fuse element can carry without overheating. It depends upon the temperature rise of contacts of the fuse holder, fuse material and the surrounding area of the fuse.
It is defined as the minimum value of current at which the fuse element melts. It means the value of fusing current is always more than the value of rated current of the fuse element.
Approximated value of fusing current is given by
I = kd3/2
I = current
K is constant
d is diameter of the fuse element
The fusing current is depends upon the following factors: –
The fusing current the fuse element depend upon the type of materials used for making fuse element.
Cross-section of area of the fuse element whether it is round or rectangular.
Length the shorter length of the fuse element the grater the current can conduct heat easily.
Diameter of fuse wire.
Type of enclosure used.
It is the ratio of minimum fusing current to the current rating of the fusing element is called fusing factor. It is always greater than unity.
For a semi-enclosed or rewirable fuse which employs copper wire as the fuse element, fusing factor is equal to 2.0 and for cartridge fuses fusing factor is equal to 1.45.
Breaking capacity of a fuse is the r.m.s value of the AC component of the maximum prospective current with at rated system voltage.
When a fault occurs, the fault current rises sharply and heat is generated in the fuse element. At the fault current attains it cut-off value the fuse melts and an arc is initiated.
This is the time between the instant of arc initiation and the instant of arc being extinguished.
TOTAL OPERATING TIME
The sum of Pre-arcing time and arcing time is called total operating time.
TYPES OF FUSES
A fuse unit usually consists of a metal fuse element, a set of contacts between which it is fixed and a body to support the fuse element and isolate them.
How does a Fuse works?
Fuse is the simplest current interrupting device for protection of the electrical circuit against short circuit or overloads which results excessive current flow through the electrical circuit. Many types of fuses also incorporate means for extinguishing the arc that appears when the fuse element melts.
Different Fuse Types are given below
Low voltages fuses
LOW VOLTAGE FUSES
Low voltage fuses are further divided into three types namely semi-closed or rewirable type and cartridge type.
Rewirable fuse is most commonly used fuse in house wiring and in small current circuits. It is simplest form of protection available. Semi-enclosed or Rewirable fuse is also sometimes called kit-kat type fuse. It consist a base and a fuse carrier. The fuse element is placed in fuse holder. Both are made of porcelain material.
When fault occurs, the value of current flowing through fuse element is increased which generated heat in the fuse element and fuse element melts.
In case of cartridge fuse the fuse element is totally enclosed in a enclosed container and is provided with metal contacts on both sides. The cartridge fuse divided into two types (i) D-type and (ii) Link type.