Fuses

Fuse is a simplest current interrupting devise for protection against excessive currents.Several developments and changes have been made in today's fuse and variety of fuses are available. The first was developed by Edison. Some types of fuses are used for arc Extinguishing that appears when fuse elements melts. Generally fuse elements are classified into two types.

1)Low Voltage fuse
2)High voltage fuse

LOW VOLTAGE FUSE

1)Semi-enclosed rewireable fuse:
This type of rewireable fuse is used where low values of fault current are encountered. It consists of base and fuse carrier. The base of porcelain and carries the fixed contacts to which the incoming and outgoing phase wire are connected. The fuse carrier is also of porcelain and holds the fuse element between its terminals. The fuse carrier can be inserted in or taken out of base when desired. When Fault occurs, the fuse element is blown out and the circuit is interrupted. the fuse carrier is taken out and the blown out fuse is replace by new one. the fuse carrier is then reinserted in the base to restore the supply. This type of fuse has two advantages.
1)It allows us to replace the blown out fuse by a new one
2)It is very cheap

Disadvantages
1)It cannot be used for high amount of fault current
2)It generally operated at lower than its real rating
3)It has not good amount of protection from external factors.

2)High rupturing capacity cartridge fuse:
The drawback of rewireable fuse having capacity to carry low fault current is overcome by this fuse. This type of fuse consists of heating resisting ceramic body having metal end caps to which they are wielded. The space within the body surrounding the element is completely packed with a filling powder. The filling material may be chalk, Plaster of Paris, quartz or marble dust and acts as an arc blowing and cooling medium. So this fuse carries normal current without overheating. When fault current passes, the fuse element  melts and stops current reaching its maximum value. The heat produced during melting of fuse element is used to  vapourise the melted silver element, which reacts with the filling powder and forms a high resistance substance which is used for arc quenching.

Disadvantages
1)Heat produced may sometimes affect the switches and other devices
2)Replacement is to be done after each operation.

Advantages
1)They are reliable and high speed of operation
2)They are cheap and require less maintenance
3)they are capable of clearing low and high fault current , thus they have consistent working.

3)H.R.C. fuse with tripping device
There are some type of H.R.C. cartridge fuse provided with ripping device. the reason for using a tripping devise is when fuse is blown out under fault conditions, the tripping device causes the circuit breaker to operate. The body of fuse is made up off ceramic material with a metallic cap rigidly fixed at each end. These are connected by a number of silver silver fuse elements. At one end is plunger which under fault conditions hits the tripping mechanism of the circuit breaker and causes it to operate. The plunger is electrically connected through a feasible link, chemical charge and a tungsten wire to the other end of cap. When a fault occurs, the silver fuse elements are first blown out and then current is transferred to tungsten wire. The weak link in series with the tungsten wire gets fused and causes the chemical charge to be detonated. This forces the plunger outward to operate circuit breaker.The travel of plunger is so set that it is not ejected from fuse body under fault conditions.

Advantages
1)It is generally capable of dealing with small small fault current, which avoids changing of fuse after getting used
2)If there is fault in any one phase of 3 phase system, the plunger will break open all he contacts of all the three phases.

HIGH VOLTAGE FUSE

Low voltage fuse fail to operate where we are dealing with high current rating. So to overcome this, High voltage fuse is used. They are of following types:

1)Cartridge type

2)Liquid type

3)Metal Clad fuses

1)Cartridge type fuse
This is similar in construction to low voltage cartridge type fuse and some additional features are added. Some design to use fuse elements wound in helical form to avid corona effects at higher voltages. On some of them there are two fuse elements in parallel; one of low resistance silver wire and other of high resistance tungsten wire. Under normal load condition, the low resistance silver wire carries current. When fault occurs, the low resistance silver wire burns and blows out and high resistance tungsten wire reduces the short circuit current and finally opens the circuit.

2)Liquid type
This fuses are filled with carbon tetrachloride and have more number of applications in high voltage system.It consists of glass tube filled with carbon tetrachloride solution and sealed at both the ends with brass caps. The fuse wire is sealed at one end and other end is held by strong phosphorous bronze spiral spring fixed at the other end of glass tube. When the current exceeds the rating of the fuse, the fuse is blown out. As the fuse melts, the spring again attracts part of it through a liquid detector and draws it well into liquid. The small gas generated at the point of  fusion forces some of liquid into the passage through liquid detector and at that point it extinguishes the arc.

3)Metal clad fuses
They are used in order to provide protection to oil circuit breaker. They are used in high voltage system and operate under short circuit conditions approaching their capacity.