Fuses

                           Fuses

Fuse is a metal string which is enclosed by a cover. This is connect series with the circuit. At normal current level (current rating) metal rod conduct current without any problem. But when that current exceeds a certain value this string will be melted and hence the circuit will disconnect. The minimum current required to melt the string is known as the fusing current.

The operating time vs. current graph of a fuse is like below.

Types of fuses

Fuses can be categorized into two groups, low voltage fuses and high voltage fuses.

Low voltage fuses

Low voltage fuses again categorized into two groups. They are semi enclosed rewireable fuse and high rupturing capacity (H.R.C.) cartridge fuse.

• Semi enclosed rewireable fuse

[Original image source: http://www.jayesselectrotechnology.com/BS3036%20Ceramic.htm]

This type is used for low value of fault currents. As shown in above fuse consists of mainly two parts, base and fuse carrier. They are made with porcelain. Contacts are for incoming and outgoing lines. When the fuse blown fuse carrier can separate from the base and replace the fuse wire. So basically there are two advantages. First one is replacement cost is low. Other one is when fuse wire is melted fuse carrier can detach and replace wire. So there is no danger in electric shock in replacing the fuse wire.

Also there are disadvantages. When replacing if we don’t choose proper size fuse wire it will become useless. Here proper size means diameter and also the length. Another one is fuse wire may subjected to oxidization and operation may depend on the ambient conditions. Therefore it can be seen that breaking capacity of semi enclosed rewireable fuse is uncertain.

• High rupturing capacity (H.R.C.) cartridge fuse

In this type the weakness of above type is overcome. Here weakness means uncertainty of breaking capacity and low fault current level.

Fuse element is made with silver material and the cartridge is filled with a filling powder. When the fuse blown this silver fuse element is vaporized and react with filling material. Then it makes a high restance medium inside the fuse.

• HRC fuse with tripping device

 In this body is made by ceramic and it has two metallic caps at the ends. One side has a plunger and two end caps are coupling with silver fuse elements. Inside the chemical charge there is a weak link in series with the tungsten shunt. When a fault occurs at first silver fuse elements are blown. Then whole current flows through the tungsten element. It will cause to blown the weak link and hence the chemical charge will explode. This device is designed so that when the chemical charge is explode it will cause to pull the plunger to the outside and hence the circuit breaker will operate.   

High voltage fuses

• Cartridge type

Construction is similar with low voltage HRC type. But there may be bit differences. As examples, some types have two parallel fuse elements. One is made from silver and other is made from tungsten. Silver has low resistance and tungsten has high resistance. When there is a fault current at first silver one will blows and then tungsten one will blows by reducing the short circuit current.

• Liquid type

In this type glass tube is filled with carbon tetrachloride liquid. At each end there is a brass cap. At one end brass cap is attached to the fuse link. The other end attached to the spiral. When the current exceeds the rated value then the fuse blows. So when the fuse blows the spring comes backward in to the liquid. (Because the section liquid director provide space to move the spring forward and backward direction). It bring some part of melted material to the liquid and hence small amount of gas produce by effectively extinguishing the arc.  

• Metal clad fuse

This is a type of oil immersed fuse that can be used as a substitute for oil circuit breakers. Can be used for very high voltage circuits. 

References

Principles of power system by V.K. Mehtha and Rohith Mehtha.