Fuses

A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit.

• Made of materials having low melting point,high conductivity, least oxidation like silver copper etc.
• Inserted in series with circuit
• Under normal condition fuse element is at temperature below melting point.
• In case of short circuit or overload condition fuse melts and blows away, therby disconnecting circuit protected by it.

Advantages

1. It is the cheapest form of protection available.
2. It requires no maintenance.
3. Its operation is inherently completely automatic unlike a circuit breaker which requires elaborate equipment for automatic action.
4. It can break heavy short-circuit currents without noise or smoke.
5. The smaller sizes of fuse element impose a current limiting effect under short-circuit conditions.
6. The inverse time-current characteristic of a fuse makes it suitable for overcurrent protection.
7. The minimum time of operation can be made much shorter than with the circuit breakers.

Disadvantages

1. Considerable time is lost in rewiring or replacing a fuse after operation.
2. On heavy short-circuits, *discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned.
3. The current-time characteristic of a fuse cannot always be co-related with that of the protected apparatus.

Desirable characterstics of fuse element

1. low melting point e.g., tin, lead.
2. high conductivity e.g., silver, copper.
3. free from deterioration due to oxidation e.g., silver.
4. low cost e.g., lead, tin, copper.

Fuse element material

The most commonly used materials for fuse element are lead, tin, copper, zinc and silver. For small currents upto 10 A, tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For larger currents, copper or silver is employed. It is a usual practice to tin the copper to protect it from oxidation. Zinc (in strip form only) is good if a fuse with considerable time-lag is required i.e., one which does not melt very quickly with a small overload.

Why silver is used as fuse these days??

1. It is comparatively free from oxidation.
2. It does not deteriorate when used in dry air.
3. The coefficient of expansion of silver is so small that no critical fatigue occurs. Therefore, the fuse element can carry the rated current continuously for a long time.
4. The conductivity of silver is very high. Therefore, for a given rating of fuse element, the mass of silver metal required is smaller than that of other materials. This minimises the problem of clearing the mass of vapourised material set free on fusion and thus permits fast operating speed.
5. Due to comparatively low specific heat, silver fusible elements can be raised from normal temperature to vapourisation quicker than other fusible elements. Moreover, the resistance of silver increases abruptly as the melting temperature is reached, thus making the transition from melting to vapourisation almost instantaneous. Consequently, operation becomes very much faster at higher currents.
6. Silver vapourises at a temperature much lower than the one at which its vapour will readily ionise. Therefore, when an arc is formed through the vapourised portion of the element, the arc path has high resistance. As a result, short-circuit current is quickly interrupted.

Types of fuses

1. Low voltages fuses
2. High voltage fuses

Difference between fuse and circuit breaker

Fuse	Circuit Breaker
It performs both detection and interruption functions.	It performs interruption function only. The detection of fault is made by relay system.
Operation Inherently completely automatic.	Requires elaborate equipment (i.e. relays) for automatic action.
Small breaking capacity	Very large breaking capacity
Operating time very small like .001 sec or so	Operating time comparatively large like .1 or .2 second
Requires replacement after every operation	Does not require replacement after every operation