A rewirable fuse is one of the most commonly used protective devices in domestic wiring and low-voltage electrical installations. It protects electrical circuits from overload current and short circuit faults by melting the fuse wire whenever excessive current flows through the circuit.
Rewirable fuses are simple in construction, economical, easy to replace, and widely used in homes, shops, and small industries. These fuses are also known as Kit-Kat Fuses because of their removable fuse carrier design.
What is a Rewirable Fuse?
A rewirable fuse is a semi-enclosed type fuse in which the fuse element can be replaced manually after the fuse operates. The fuse wire melts due to excessive current and disconnects the faulty circuit from the supply.
After fault removal, a new fuse wire of proper rating is rewired into the fuse carrier, making the fuse reusable.
Definition of Rewirable Fuse
A rewirable fuse is a protective device used in electrical circuits in which a replaceable fuse wire melts under fault conditions to protect the circuit from damage caused by overcurrent or short circuit current.
Construction of Rewirable Fuse
A rewirable fuse mainly consists of the following parts:
- Base
- Fuse carrier
- Fuse wire (fuse element)
- Terminals
1. Fuse Base
The base of the fuse is made of porcelain or ceramic material because these materials can withstand high temperatures and provide good insulation.
The base contains:
- Incoming and outgoing terminals
- Contacts for the fuse carrier
It is fixed permanently to the switchboard or distribution board.
2. Fuse Carrier
The fuse carrier is a removable part that holds the fuse wire. It can be easily inserted or removed from the fuse base.
The carrier is also made of insulating material such as porcelain.
3. Fuse Wire (Fuse Element)
The fuse wire is the most important part of the rewirable fuse. It is connected between the two terminals of the fuse carrier.
The fuse wire is made of materials having:
- Low melting point
- High conductivity
- Low cost
Common Fuse Wire Materials
- Lead
- Tin
- Copper
- Silver
- Lead-tin alloy
The fuse wire melts whenever current exceeds the safe limit.
4. Terminals
Terminals are provided for electrical connections. One terminal is connected to the supply side and the other to the load side.
Construction Diagram of Rewirable Fuse
The rewirable fuse consists of a porcelain base and a removable fuse carrier containing the fuse wire connected between two terminals.
Working Principle of Rewirable Fuse
The working of a rewirable fuse is based on the heating effect of electric current.
When normal current flows through the circuit, the fuse wire carries the current safely without overheating.
During overload or short circuit conditions:
- Current increases excessively
- Heat generated increases according to I²R losses
- Fuse wire temperature rises rapidly
- Fuse wire melts
- Circuit gets disconnected
Thus, the faulty circuit is isolated from the supply and electrical equipment is protected from damage.
Operation of Rewirable Fuse
- Under normal conditions, the fuse wire carries rated current safely.
- When overload occurs, excessive heat is produced.
- The fuse element melts due to overheating.
- The electrical circuit opens automatically.
- After removing the fault, the damaged fuse wire is replaced with a new one.
Fuse Materials Used in Rewirable Fuse
| Material | Properties |
|---|---|
| Silver | Excellent conductivity and reliable operation |
| Copper | High conductivity and strength |
| Lead-Tin Alloy | Low melting point and economical |
| Tin | Good corrosion resistance |
Characteristics of Rewirable Fuse
The characteristics of a fuse define its operating behavior under fault conditions.
1. Current Rating
The current rating is the maximum current the fuse can carry continuously without melting.
2. Fusing Current
Fusing current is the minimum current at which the fuse element melts.
3. Fusing Factor
The fusing factor is defined as:
Fusing Factor = Minimum Fusing Current / Rated Current
The value of fusing factor is always greater than 1.
Typical value:
Fusing Factor = 1.25 to 2
4. Cut-Off Current
Cut-off current is the maximum current allowed to flow before the fuse melts completely.
5. Breaking Capacity
Breaking capacity is the maximum fault current that the fuse can interrupt safely without damage.
6. Time-Current Characteristics
The operating time of a fuse decreases as fault current increases.
This means:
- Small overload → longer melting time
- Large short circuit current → faster operation
Advantages of Rewirable Fuse
- Simple construction
- Low cost
- Easy installation
- Easy replacement of fuse wire
- Reliable operation for low voltage circuits
- Requires very little maintenance
- Suitable for domestic wiring
Disadvantages of Rewirable Fuse
- Low accuracy in operation
- Slow operating speed compared to modern protection devices
- Fuse wire may be replaced with wrong rating
- Low breaking capacity
- Not suitable for high fault currents
- Frequent replacement required after operation
- Possibility of overheating due to loose contacts
Applications of Rewirable Fuse
Rewirable fuses are mainly used in low-voltage applications.
Common Applications
- Domestic wiring installations
- Lighting circuits
- Small industrial loads
- Distribution boards
- Protection of small motors
- Residential electrical systems
- Low-power electrical appliances
Difference Between Rewirable Fuse and Cartridge Fuse
| Parameter | Rewirable Fuse | Cartridge Fuse |
|---|---|---|
| Construction | Open type fuse wire | Fully enclosed cartridge |
| Cost | Low | High |
| Breaking Capacity | Low | High |
| Replacement | Fuse wire replaced manually | Entire cartridge replaced |
| Reliability | Moderate | High |
Precautions While Using Rewirable Fuse
- Use correct fuse wire rating
- Never use copper wire in place of fuse wire
- Ensure tight terminal connections
- Replace damaged fuse carriers immediately
- Avoid overloading of circuits
Why is Rewirable Fuse Called Kit-Kat Fuse?
The rewirable fuse is commonly called a Kit-Kat fuse because its removable fuse carrier resembles the shape of a Kit-Kat chocolate bar.
Conclusion
A rewirable fuse is a simple and economical protective device used for protecting electrical circuits against overload and short circuit faults. Its operation is based on the heating effect of current, where the fuse wire melts whenever current exceeds the safe limit.
Although modern protective devices such as MCBs and MCCBs are replacing rewirable fuses in many applications, they are still widely used in domestic and low-voltage electrical installations because of their low cost and simple construction.
FAQs on Rewirable Fuse
What is a rewirable fuse?
A rewirable fuse is a protective device in which the fuse wire can be replaced manually after melting during fault conditions.
Why is a rewirable fuse called a Kit-Kat fuse?
It is called a Kit-Kat fuse because its removable carrier resembles the shape of a Kit-Kat chocolate bar.
What is the working principle of a rewirable fuse?
It works on the heating effect of electric current. Excessive current produces heat that melts the fuse wire.
What is the fusing factor of a fuse?
Fusing factor is the ratio of minimum fusing current to rated current of the fuse.
What are the advantages of a rewirable fuse?
Rewirable fuses are simple, economical, easy to install, and suitable for domestic applications.
What are the disadvantages of rewirable fuse?
They have low breaking capacity, slower operation, and require replacement of fuse wire after operation.
Where are rewirable fuses used?
They are used in domestic wiring, lighting circuits, small industrial loads, and low-voltage electrical systems.