Fuses, Fuse Holders

In electronics and electrical engineering a fuse, short for 'fusible link', is a type of overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows. more...

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When the metal strip melts, it opens the circuit of which it is a part, and so protects the circuit from excessive current.

A practical fuse was one of the essential features of Edison's electrical power distribution system. An early fuse was said to have successfully protected an Edison installation from tampering by a rival gas-lighting concern.

Fuses (and other overcurrent devices) are an essential part of a power distribution system to prevent fire or damage. When too much current flows through a wire, it may overheat and be damaged, or even start a fire. Wiring regulations give the maximum rating of a fuse for protection of a particular circuit. Local authorities will incorporate national wiring regulations as part of law. Fuses are selected to allow passage of normal currents, but to quickly interrupt a short circuit or overload condition.

Fuse characteristics

The speed at which a fuse operates depends on how much current flows through it. Manufacturers of fuses plot a time-current characteristic curve, which shows the time required to melt the fuse and the time required to clear the circuit for any given level of overload current.

Where several fuses are connected in series at the various levels of a power distribution system, it is very desirable to clear only the fuse (or other overcurrent device) electrically closest to the fault. This process is called "coordination" and may require the time-current characteristics of two fuses to be plotted on a common current basis. Fuses are then selected so that the minor, branch, fuse clears its circuit well before the supplying, major, fuse starts to melt. In this way only the faulty circuits are interrupted and minimal disturbance occurs to other circuits fed by the supplying fuse.

Where the fuses in a system are of similar types, simple rule-of-thumb ratios between ratings of the fuse closest to the load and the next fuse towards the source can be used.

Fuses are often characterized as "fast-blow" or "slow-blow" or "time-delay", according to the time they take to respond to an overcurrent condition. The selection of the characteristic depends on what equipment is being protected. Semiconductor devices may need a fast or ultrafast fuse for protection since semiconductors may have little capacity to withstand even a momentary overload. Fuses applied on motor circuits may have a time-delay characteristic, since the surge of current required at motor start soon decreases and is harmless to wiring and the motor.

Interrupting rating

A fuse also has a rated interrupting capacity, also called breaking capacity, which is the maximum current the fuse can safely interrupt. Generally this should be higher than the maximum prospective short circuit current. Miniature fuses may have an interrupting rating only 10 times their rated current. Fuses for small low-voltage wiring systems are commonly rated to interrupt 10,000 milli amperes. Fuses for larger power systems must have higher interrupting ratings, with some low-voltage current-limiting "high rupturing capacity" (HRC) fuses rated for 300,000 amperes. Fuses for high-voltage equipment, up to 115,000 volts, are rated by the total apparent power (megavoltamperes, MVA) of the fault level on the circuit.

Read more at Wikipedia.org