Page 6 - Application Guide Semiconductor Fuse Link

Protection against overcurrents

Overcurrents can be roughly classifi ed into two groups; overloads and short-circuits.

Overloads

The term «overload» is used for excess current fl owing 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. However new
style gR semiconductor fuses can provide overload protection as low as 160% of fuse rated current.
Consult the Ferraz Shawmut datasheets for precise values.

Short-circuits

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 20 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 a.c. 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

t

i dt
2

0
2
where i is the instantaneous current [i.e. i = i(t)]. This integral is usually called the «I t» 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 fl ow in the circuit. Electromagnetic forces are dependent on the
square of the instantaneous current and may produce mechanical damage to equipment. If short-
circuit currents are allowed to fl ow unchecked, after the mechanical damage to the components of
the circuit, melting of circuit conductors can occur and be followed by arcing between the molten
fragments, possibly causing fi res 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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