Definition of Electrical Fuse:
It is a short metal piece, which is used in circuits and it melts down when excessive current flows through it which also breaks the circuit. Electrical Fuse is although an old protective device but still in use due to its high operating time.
Construction of Electrical Fuse:
The element is made up of those materials whose melting point is low and has high conductivity. The material used for its bi-metallic wire is silver or copper. It is in series with the circuit which is to be protected. During normal conditions, the fuse element is at a temperature below its melting point as soon as the temperature becomes due to the flow of fault current, the element melts down and disconnects a circuit.
the Operating time of fuse depends upon the magnitude of excessive current.
Advantages of Electrical Fuse:
- It is a less expensive form of Protection
- It can break heavy short-circuit currents without noise or smoke.
- Over-current protection can be done with it
- It has a high operating time than a circuit breaker. i.e it can trip a circuit faster than circuit breakers.
Disadvantages of Electrical Fuse:
- Much time is lost in rewiring or replacing a fuse after the operation.
- On heavy short-circuits, the Circuit breaker is usually preferable.
- Due to the simple operation of Circuit breakers, Fuse is not used extensively.
Characteristics of Electrical Fuse Element:
Following are the desirable characteristics
- Electrical Fuse low melting point.
- It has high Conductivity e.g silver and copper
- It has usually inexpensive
Fuse Wire Rating:
It is the current which wire can carry without overheating or melting. It depends upon the temperature rise of contacts of the fuse holder, fuse material and surrounding of the fuse.
It is the minimum current at which fuse elements melts and trips the circuit
k is constantly called fuse constant.
The fusing current depends upon the following factors
- Size and location of the terminal.
- Enclosure type.
- The diameter of the wire.
- Length of wire. Smaller the length of wire, the greater the current.
It is the ratio of minimum fusing current to the current rating of fuse material.
The value of the fusing factor is always more than one. Greater the fusing factor, smaller is the difficulty in avoiding deterioration due to overheating and oxidation at rated current.
Types of Electrical Fuse:
- Low Voltage
- High Voltage
It is further divided into
- Semi-Enclosed rewireable fuse
- High Rupturing Capacity (HRC) cartridge type fuse
Semi-Enclosed rewireable fuse: Such type is used where low values of fault current occur in any power system. It consists of a base and a carrier. The base is made up of porcelain and carries incoming and outgoing phase wires. The carrier can be inserted in or out of the base, When a fault occurs, the fuse element melts out and the circuit is disconnected.
High Rupturing (HRC) Cartridge type fuse:
It consists of a heat resistive ceramic body which has metal ends and is welded silver current carrying element. The inner body is completely filled with powder, the filling material may be chalk, plaster of Paris, quartz and marble dust and acts as a cooling medium
Under normal condition, the material is below its melting point and carries a normal current, when there is a short circuit or fault, the material is melt and heat is produced, the chemical reaction between silver fuse material and powder helps to quench the arc
Advantages of HRC:
- They are used for clearing high as well as low fault currents.
- They do not require any maintenance.
- Electrical Fuse has a high-speed operation
Disadvantages of HRC:
- Usually, they have to replaced after its operation
- A large amount of heat is produced
- Cartridge type:
This type is similar to the HRC, the difference is between in their design, there are two elements i.e one of low resistance silver wires and other is of high resistance, Under normal operating conditions the low resistance element carries a normal current and when there is a fault, it melts down and high resistance element reduces the fault current and finally disconnects the circuit.
High voltage cartridge type is indifferent rating up to 33kV with a breaking capacity of 9000 A at that voltage.
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