A fuse is a device that protects a circuit from an over current condition only.
It has a fusible link directly heated and destroyed by the current passing through it. A fuse contains a current-carrying element so that the heat generated by the flow of normal current through it doesnot cause it to melt the element; however, when an over current or short-circuit current flowsthrough the fuse, the fusible link will melt and open the circuit.
The Underwriter Laboratories (UL) classifies fuses by letters e.g. class CC, T, K, G, J, L, R, and so forth. The class letter may designate interrupting rating, physical dimensions, and degree of current limitation.
Construction of Fuse:
The typical fuse consists of an element which is surrounded by filler and enclosed by the fuse body. The element is welded or soldered to the fuse contacts (blades or ferrules).
The Element:The element provides the current path through the fuse. It generates heat at a rate dependent on its resistance and the load current.
The heat generated by the element is absorbed by the filler and passed through the fuse body to the surrounding air. The filler material, such as quartz sand, provides effective heat transfer and allows for the small element cross-section typical in modern fuses. The effective heat transfer allows the fuse to carry harmless overloads. The small element cross section melts quickly under short-circuit conditions. The filler also aids fuse performance by absorbing arc energy when the fuse clears an overload or short circuit.
Inverse Time Characteristic of Fuse:
When a sustained overload occurs, the element will generate heat at a faster rate than the heat can be passed to the filler. If the overload persists, the element will reach its melting point and open. Increasing the applied current will heat the element faster and cause the fuse to open sooner. Thus, fuses have an inverse time current characteristic: that is the greater the over current, the less time required for the fuse to open the circuit.
This characteristic is desirable because it parallels the characteristics of conductors, motors, transformers, and other electrical apparatus. These components can carry low-level overloads for relatively long periods without damage. However, under high-current conditions, damage can occur quickly. Because of its inverse time current characteristic, a properly applied fuse can provide effective protection over a broad current range, from low-level overloads to high-level short circuits.
Types of Fuse:
A fuse unit essentially consists of a metal fuse element or link, a set of contacts between which it is fixed and a body to support and isolate them. Many types of fuses also have some means for extinguishing the arc which appears when the fuse element melts. In general, there are two categories of fuses.
(1) Low voltage fuses.
(2) High voltage fuses.
Usually isolating switches are provided in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety.
In absence of such isolation means, the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse is being inserted or removed.
(1) Low Voltage Fuse:
Low voltage fuses can be further divided into two classes namely
a) Semi enclosed (Rewireable Type Fuse / Kit Kat Fuse):
The most commonly used fuse in ‘house wiring’ and small current circuit is the semi-enclosed or rewire able fuse. (Sometime known as KIT-KAT type fuse). It consist of a porcelain base carrying the fixed contacts to which the incoming and outgoing live or phase wires are connected and a porcelain fuse carrier holding the fuse element, consisting of one or more strands of fuse wire, stretched between its terminals.
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.
The actual fusing current will be about twice the rated current. When two or more fuse wire are used, the wires should be kept apart and a derating factor of 0.7 to 0.8 should be employed to arrive at the total fuse rating.
The specification for re wire able fuses are covered by IS: 2086-1963. Standard ratings are 6, 16, 32, 63, and 100A.
A fuse wire of any rating does not exceeding the rating of the fuse. We use 80 A fuse wire in a 100 A fuse, but not in the 63 A fuse. On occurrence of a fault, the fuse element blows off and the circuit is interrupted. The fuse carrier is pulled out, the blown out fuse element is replaced by new one and the supply can is resorted by re-inserting the fuse carrier in the base.
Easily removal or replacement without any danger of coming into the contact with a lie part.
Negligible replacement cost
Lack of Discrimination.
Small time lag.
Low rupturing capacity.
No current limiting feature.
Slow speed of operations.
b) Totally Enclosed(Cartridge Type Fuse)
The fuse element is enclosed in a totally enclosed container and is provided with metal contacts on both sides. These fuses are further classified as
I) D- Type Cartridges Fuses
It is a non interchangeable fuse comprising fuse base, adapter ring, cartridge and a fuse cap. The cartridge is pushed in the fuse cap and the cap is screwed on the fuse base. On complete screwing the cartridge tip touches the conductor and circuit between the two terminals is completed through the fuse link. The standard ratings are 6, 16, 32, and 63 amperes.
Breaking or rupturing capacity: 4k A for 2 and 4 ampere fuses the 16k A for 63 A fuses.
Ratings of D Type Cartridge fuses: 2, 4, 6, 10, 16, 25, 30, 50, 63
D-type cartridge fuse have none of the drawbacks of the re wire able fuses. Their operation is reliable. Coordination and discrimination to a reasonable extent and achieved with them.
II) Link type Cartridge or High Rupturing Capacity (HRC)
Where large numbers of concentrations of powers are concerned, as in the modern distribution system, it is essential that fuses should have a definite known breaking capacity and also this breaking capacity should have a high value.
High rupturing capacity cartridge fuse, commonly called HRC cartridge fuses, have been designed and developed after intensive research by manufactures and supply engineers in his direction.
The usual fusing factor for the link fuses is 1.45. The fuses for special applications may have as low as a fusing factor as 1.2.
Knife Blade Type HRC Fuse:
It can be replaced on a live circuit at no load with the help of a special insulated fuse puller.
Bolted Type HRC Link Fuse:
It has two conducting plates on either ends. These are bolted on the plates of the fuse base. Such a fuse needs an additional switch so that the fuse can be taken out without getting a shock.
Ratings of HRC fuses: 2, 4, 6, 10, 16, 25, 30, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400, 500, 630,800, 1000 and 1,250 amperes.