- Materials & Industrial Technology
Rewirable fuses (Electric)
Fuse is the simplest and cheapest protective device. It is a self sacrificial device used in interrupting the circuit during over- current or short circuit conditions. Fuses are always connected before the load in series to the supply.
Rewirable fuse is also known as kitkat fuse. It is a simple reusable fuse used in the houses, offices and in the protection of outdoor distribution transformers. It is a fully enclosed type fuse and is available in a wide range of ratings. The fuse when blown out can be reused after replacing the fuse element and the circuit can be restored quickly and easily. Renewable fuses are simple in construction and operation but less reliable than HRC fuses.
Read more about fuses
Renewable fuse consists of a base and a separate fuse element carrier made of porcelain. The base consists of two terminals one for incoming supply and the other for the outgoing supply. Similarly fuse carrier also contains two terminals between which fuse element is connected. Usually one or more strands of fuse wires are used as fuse element. The fuse base and the carrier are designed such that each terminals of base coincides with a terminal of the fuse element. The circuit will be closes only if the fuse carrier is inserted into the base and the fuse element is continuous.
Reliability of rewirable fuses
The working principle of rewirable fuses is very simple. Whenever the current exceeds the predefined value, the flow of current heats the coil and melts it and the fuse is blown out. Thus the circuit is interrupted.
Once the fuse is blown out, it can be reused by replacing the fuse element with a new one. Fuse carrier is pulled out, blown out fuse element is replaced by a new one and the supply is restored by replacing the fuse carrier in the base. The specialty of renewable fuses is that the fuse carrier can be taken out or reinserted back into the base without risk, even without opening the main switch.
Have you ever rewired a blown out fuse?
1. Switch of the main switch. (Safety precaution)
2. Take out the fuse element carrier.
3. Loosen the terminal screws and remove the blown out fuse wire.
4. Replace it with a new fuse wire as per the requirement (refer the table below), wrap it around the terminals and tighten the screws.
5. Reinsert the fuse carrier and switch on the main.
The normally used fuse wires are lead and aluminum. Tinned copper and an alloy of tin-lead can also be used. The actual fusing current must be twice the rated current. When two or more fuse elements are used, the wires should be kept apart and a derating factor of 0.7 to 0.8 should be employed to arrive the total fuse rating.
Selection of fuse wire
Size of Fuse Wire
2.0 Mm X 2 Nos.
Standard ratings are 6, 16, 32, 63, 100 and 200 amperes. A fuse wire can have a rating not exceed the rating of the fuse, for example, in a fuse have a rating of 100 A, a fuse wire of any rating not greater than 100 A can be used.
Disadvantages of rewirable fuse
1. Time lag: Due to small time lag, large transient currents which are encountered during the starting of motors and switching on operation of transformers, capacitors, fluorescent lights etc can blow the fuse unless sufficient large rating are used.
2. Low rupturing capacity: Rewirable fuses have limited breaking or rupturing capacity.
3. Slow operation: No feature is employed for extinction of arc that blows after the fuse melts.
4. Risk of flame and fire
5. Single phasing of three phase induction motor is a common occurrence where renewable fuses are employed in motor circuits.
Comparison of rewirable fuses with HRC fuses
Apart from reusability and less cost, HRC fuses are far better in operation and reliability than the rewirable fuse. Because of its simplicity and cheapness rewirable fuses are employed in the protection against short circuit and overload on low and medium voltage circuits only.
Read more on Electric fuse.
- Electric Fuse
Electrical fuse is a simple, less resistive, self sacrificial and cheapest device used to interrupt a circuit under short circuit, excessive overload or over current conditions.