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Sahithyan's S1
Sahithyan's S1 -- Electrical Fundamentals

Fuses

A device for opening a circuit by means of a conductor designed to melt when an excessive current flows along it. Simple. Relatively cheap.

When overcurrent flows through a fuse, the fuse element melts, vaporizes. A gap is formed in the circuit. Electrical current tries to jump over the gap causing a high-temperature plasma path, which is seen as an arc.

Fuse element

Part of a fuse. Designed to melt and open the circuit when overcurrent flows.

Part of a fuse, which comprises a fuse element and a cartridge (or other container) and is capable of being attached to the fuse contacts.

Current rating

Maximum current the fuse can carry without breaking.

Fusing current

Minimum current that will cause the fuse element to heat up melt or blow.

Fusing factor

Always greater than . Usually in the range of .

Rupturing capacity

Product of maximum current and supply voltage.

Types of fuses

Semi-enclosed fuse

Consists of a fuse holder, which is made up of a fuse base and a fuse carrier. The fuse carrier contains the fuse element usually in wire form. “Rewireable” because the elements are directly replaceable. Cheap. Low rupturing capacity.

Disadvantages

But not recommended nowadays because of these disadvantages:

  • Deterioration with time - due to oxidation. cross sectional area might get reduced with time. resistance will increase. may operate at lower currents than expected.
  • Very easy for an inexperienced person to replace a blown fuse-element with a wire of incorrect size or type
  • Slow: time taken for the fuse to blow may be as long as several seconds during which time considerable electrical and physical damage may result to the circuit conductors and the equipment being protected.
  • Not accurate: calibration of re-wirable fuse can never be accurate
  • Unsuitable for circuits which require discriminative protection. i.e. it is possible in certain circuit conditions for the 15 A rated fuse element to start melting before the 10 A rated element completes fusing
  • Not capable of differentiating between a transient high current and a continuous fault current
  • Has an associated fire risk
  • When the fault current is particularly high, though the fuse works, an arc may still be maintained by the circuit voltage (through air and metallic vapour)

Fully enclosed (catridge) fuse

Developed to overcome the disadvantages of the re-wirable type of fuse. Fuse wire is enclosed in a evacuated glass tube with metal end chips. Expensive compared to semi-enclosed fuses. Low rupturing capacity.

High-rupturing capacity (HRC) fuse

Used for high current applications. Expensive.