In this article we will discuss about:- 1. Meaning of Fuse 2. Metals and Alloys Used for Fuse 3. Types 4. Merits 5. Demerits.
Meaning of Fuse:
An electric circuit is designed to carry a definite amount of current. If a much greater current passes through the circuit, there may be serious damages. To check it, a length of a conductor of a comparatively fusible material is introduced in a part of the circuit. This conductor, culled fuse breaks the connection and melts when current in the circuit exceeds the maximum current for which the circuit is designed.
The fusing current depends on the following factors:
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1. Length of the fuse wire.
2. Material surrounding the fuse wire.
3. Cooling effects of contacts to which fuse wire is attached.
Fuse is placed in series with the circuit or subcircuit to be protected from overload and short circuits. For carrying out closing and opening of the circuits some type of switching devices is connected in series with the fuses, for example a knife switch. In general, the regulations regarding fuses require that fuses shall be accessible. The time for blowing out of the fuse depends upon the magnitude of the excess current. Larger the current, the more rapidly the fuse will blow, i.e., fuse has inverse time characteristics.
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Preece’s Law:
This is called Preece’s law which gives the relationship between fusing current and diameter of fuse wire. This law is based on the assumption that all heat generated is dissipated by radiation.
Metals and Alloys Used for Fuses:
The following metals and alloys are used for fuses:
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Its chief advantage besides a low melting point is that it will carry two thirds of its fusing current continuously without deterioration. Its disadvantage is that being a relatively poor conductor a comparatively stout wire is required for a given current, so that when fuse blows, a correspondingly large amount of molten vapour metal is released. It is not recommended except for the 5 ampere size.
It is most commonly used material. It is mechanically strong and the mass required to carry a given current is comparatively small. Its chief disadvantage is its tendency to oxidise and deteriorate at moderate temperatures. This tendency is reduced by tinning the wire, and tinned copper will satisfy the requirement of carrying 53% of its fusing current without damage.
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It is technically the best material of all fuse purposes. The resistivity is slightly less than that of copper and owing to freedom from oxidation it can carry continuously without deterioration a current a very little short of its fusing current. It is used for protecting delicate instruments on high voltage lines, but its cost precludes it from general application.
(i) It contains 2% Bi; 1% Pb, 1% Sn.
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(ii) Its melting point is 94°C.
(i) It contains 4% Bi; 2% Pb; 1% Sn; 1% Cd,
(ii) Its melting point is 71°C.
There are also other fusible alloys which are made from bismuth, cadmium, lead and in different proportions haying melting point in the range of 60 to 120°C.
Types of Fuses:
The most commonly used fuse units are:
1. Rewireable or kit kat fuses
2. Cartridge type fuses
3. High Ruptering Capacity (H.R.C.) fuses.
1. Rewirable (or Kit Kat) Type Fuses:
It consists of a procelain base carrying the fixed contacts to which the incoming and outgoing live or phase wires are connected and a procelain fuse carrier holds the fuse element.
The fuse carrier is a separate part and can be taken out or inserted in the base without risk, even without opening the main switch. If fuse holder or carrier gets damaged during use, it may be replaced without replacing the complete unit.
Merits:
(i) Negligible replacement cost.
(ii) Easy removal for replacement without any damage of coming into contact with a live part.
Demerits:
(i) The fuse wire deteriorates over a period, due to oxidation through the continuous heating up of the element.
(ii) There is a possibility of renewal by the fuse wire of wrong size.
(iii) Breaking capacity is low.
(iv) Accurate calibration of fuse wire is impossible, as a longer fuse operates earlier than one of shorter length.
Uses:
Employed in domestic installation and in motor installation of voltage rating upto 400 volts and of current ratings upto 300 A.
2. Cartridge Types Fuses:
This fuse unit is totally enclosed type. It consists of an insulating container of bulb or tube shape and sealed at its ends with metallic cap known as cartridge enclosing the fuse element and filled up with powder or granular material known as fillers. There are various types of materials used as filler like sand, calcium carbonate, quartz etc.
There is sometimes a blowout device in the side of the tube to indicate when the fuse is blown. On overloads or short circuits the fusible element is heated to a high temperature causing it to vapourise. The powder in the fuse cartridge cools and condenses the vapour and quenches the arc thereby interrupting the flow of current.
3. High Rupturing Capacity (H.R.C.) Fuses:
H.R.C. cartridge fuse (cartridge type) in its simplest form consists of a heat resisting ceramic body having metal end caps to which are welded fusible silver (a bimetallic) current carrying elements. The complete space within the body surrounding the elements is filled with a powder which acts as arc extinguishing agent. The current increases due to fault and the fuse operates. This type of fuse is reliable and has inverse time characteristics.
High voltage H.R.C. cartridge type of fuses, similar as construction to 1.v. designs are used upto 33kV. High voltage H.R.C. liquid type fuses have the widest range of application of h.v. system. They may be used for voltage transformer protection or for circuits upto about 400 A rated current, on systems upto 132 kV or higher. The liquid which surrounds the fuse elements helps extinguish the arc when the fuse blows.
Merits:
(i) No maintenance is required.
(ii) They are cheap as compared with other types of circuit interrupter of same breaking capacity.
(iii) The operation is quick and sure.
(iv) They have inverse time characteristics.
(v) They do not deteriorate with time.
(vi) They are capable of clearing high as well as low currents.
(vii) They are quite reliable and can be checked for proper discrimination.
(i) Interlocking is not possible.
(ii) They lack relays in complete discrimination.
(iii) They are required to be replaced after each operation.
Merits of Fuse:
1. It requires no maintenance.
2. It is cheapest type of protection.
3. It interrupts enormous short circuit current without noise, flame, gas or smoke.
4. The minimum time of operation can be made much smaller than that with circuit breakers.
5. It has current limiting effect.
Demerits of Fuse:
1. Time is lost in rewiring or replacing fuses after operation.
2. Discrimination between fuses in series cannot be obtained unless there is considerable difference in the relative sizes of the fuses concerned.