Training
WHY IS CURRENT LIMITATION SO
IMPORTANT?
If a protective device cuts off a short circuit current in less
than one half cycle, before it reaches its total available (and
highly destructive) peak value, the device is "current limiting".
It restricts fault currents to such low values that a high degree
of protection is given to circuit components against even very high
short circuit currents. This minimises the needs of other components
to have high short circuit current "withstand" ratings.
If not limited, short circuit currents in industrial
applications can reach tens of thousands of amperes in the first
half cycle after the start of a fault. The heat that can be produced
in circuit components by the immense energy of short circuit currents
can cause severe insulation damage or even explosion. At the same
time, huge magnetic forces developed between conductors can crack
insulators and distort and destroy bracing structures. It is therefore
important that a protective device limits fault currents before
they reach their full potential level. A fuse interrupts a short circuit current very quickly.
Consequently the energy let-through is very low. An example:- A fuse 100A, 20kA rms, 400V reduces an initial, half
cycle, fault current of 4,000,000A2s to 46,000 A2s.
This ratio is indicated in the picture below.
The volume illustrates the impact of the limitation
feature of a fuse. The benefit is that smaller components can be
used downstream of the fuse resulting in:
COMPACT INSTALLATIONS
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