Page 18 - Application Guide Semiconductor Fuse Link

Current-limiting operation

prospective (available) current (threshold current), melting occurs within the fi rst half-cycle, and
current-limiting action occurs. At high prospective (available) currents the peak current is much
lower than the peak prospective (available) value. For the 30A fuse shown, the peak current is
limited to only 3.25kA with an available current of 100kA r.m.s.

For a given r.m.s. prospective (available) current, the peak let-through current varies, depending
upon (the angle on the source voltage wave at which the short-circuit occurs). At the 100kA
level the highest value is obtained with a symmetrical short-circuit wave, while in the region
just above the threshold current the asymmetrical wave gives the highest value. Published data
always shows the highest possible (i.e. worst-case) value.

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Melting I t
For high short-circuit currents, which cause melting in times of the same order as the a.c. cycle,
the melting time varies greatly, depending upon whether the short-circuit current waveshape
is symmetrical or asymmetrical. In these cases the time-current characteristic is of limited value,
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and the coordination of system protection is done using I t values, which are much less sensitive
to the waveshape of the short-circuit current.

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The lower curve in Fig.10 shows how the melting I t of a typical semiconductor fuse varies with
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the r.m.s. prospective (available) current. For very high currents, the melting I t is constant. This
adiabatic region is so called because the rate of heating is so high that heat losses from the
element notch zones can be neglected. For lower prospective (available) currents the melting
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time is longer, and heat losses from the notch zones cause the I t required to produce melting
to increase.
105

I2t total clearing I2t curve
(max)
arcing I2t
104

melting I2t curve
(A2s)
melting I2t adiabatic melting

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1 10 100
RMS prospective (available) current, KA
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Fig.10 I t characteristics
Total clearing I t
2
2
The upper curve in Fig.10 shows the total clearing I t, which is given by

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2
2
Total clearing I t = melting I t + arcing I t
In applying fuses for short-circuit protection, the total clearing I t of the fuse must be less than
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the I t damage level of the device or system being protected.
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Published data gives the worst-case total I t at the fuse’s rated voltage, frequency and circuit
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power factor. However in real-world applications, the fuses are used in systems at lower voltages.
When the applied voltage is lower than the fuse’s rated voltage, the current-limiting action is
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more effective, and the total I t is reduced.
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