Overcurrents can be roughly classified into two groups; overloads and short-circuits. In all cases, when a fuse interrupts a fault there are two stages:
The term "overload" is used for excess current flowing in a circuit which is electrically sound. Overload currents are usually not much greater than the normal full-load current of the system, but if allowed to persist will eventually cause damage. Damage to the system, especially to insulating materials in contact with the circuit conductors, can result due to the heating effect of the current. The heating time is relatively long (from seconds up to several hours), and the overload can therefore be characterized by the r.m.s. value of the overload current. For overload protection, the requirement for a protective device is that it should limit the duration of the overload current. Most semiconductor fuses are not designed to provide protection against long duration overloads. Electronic or other means must be used to switch the circuit off when overloads occur.
Short-circuits are usually due to a catastrophic electrical failure, such as insulation breakdown or accidental conditions, and the resulting r.m.s. value of the prospective (available) short-circuit current is high, typically more than 10 times the normal full load current of the system. The heating effect is so rapid that damage to the system can occur within milliseconds, which is of the same order as the duration of an AC half-cycle. The heating effect cannot be characterized by the r.m.s. value of the prospective (available) current, as in the case of overloads, because it depends upon the waveform of the current.
In this case the requirement for the protective device is to limit the energy associated with the fault, which depends upon the integral
where i is the instantaneous current (i.e. i = i(t) ). This integral is usually called the "I²t" and is a measure of the thermal energy delivered to each ohm of the circuit by the short-circuit current during the time t. An additional requirement for a short-circuit protective device is that it should also limit the peak value of current permitted to flow in the circuit. Short-circuit current may cause mechanical damage to equipment. If shortcircuit currents are allowed to flow unchecked, after mechanical damage is caused to the components of the circuit, melting of circuit conductors can occur and be followed by arcing between the molten fragments, possibly causing fires and hazards to personnel as well as the further destruction of the electrical system. High-speed semiconductor fuses open very rapidly under short circuit conditions thus providing excellent protection in case of short-circuit faults.
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