Page 17 - Application Guide Semiconductor Fuse Link

Current-limiting operation

Total clearing time = melting time + arcing timz
peak
current
For this waveshape the total let-through I t is
2

2
Ipeak Ttotal
--
3

melting arcing
time time
total clearing time
Fig.8 Approximate current wasveshape for a current-limiting fuse

Quartz fi ller

If the fuse element is surrounded by a gas such as air, the arc channel is not contained and is free
to expand radially. The arc resistance and hence its voltage is, very roughly, inversely proportional
to the cross-sectional area of the arc channel, and therefore the arc voltage produced is too low to
achieve current limitation. If however the arc cross-sectional area is kept small by confi ning it, the arc
voltage can then be kept high, and the circuit current can be limited. During interruption, the arc
power is very high. If the arc were confi ned within a narrow tube in a ceramic block, the very high
arc temperature and pressure may cause the block to shatter due to thermal shock.

Ferraz Shawmut semiconductor fuses use a granular medium (quartz sand) for arc-quenching. It
confi nes the arc and at the same time allows the pressure to be relieved. Vapors can escape from
the arcing region through the gaps between the grains of sand. The sand can also absorb thermal
shock by slippage between grains. As arcing continues, the grains of sand in contact with the arc
melt and form a glassy body known as a fulgurite. This process of making fulgurite absorbs energy
from the arc.

Premium quality Ferraz Shawmut semiconductor fuses use an agglomerated “solid” sand fi ller, in
which the individual grains of sand are bonded together to produce a type of artifi cial sandstone.
This “solid” sand fi ller further enhances the control of the arcs under high short-circuit conditions. It
also improves the cooling of the elements at normal currents, because the “solid” sand fi ller has a
higher thermal conductivity.

Peak let-through currents

10
For current-limiting fuses the peak let-through i = 2.5 Irms
current (or cut-off current) is a very important
i = 1.414 Irms
parameter. Fuse data is presented in the form of
peak let-through (cut-off) characteristics. These 1
peak current (KA)
characteristics are published for specifi ed test
conditions, typically AC voltage, frequency and
power-factor. 0,1

Fig.9 shows a peak let-through characteristic for
a typical semiconductor fuse with a current rating
of about 30A. For low prospective (available) 0,01
currents, the fuse takes several a.c. cycles to melt, 0,1 0,1 1 10 100
and the highest value of current is equal to the
r.m.s prospective current (KA)
peak current in the fi rst half-cycle. This is 1.414IRMS
for a symmetric wave and about 2.3-2.5IRMS for Fig.9 Peak let-through (cut-off) characteristic
an asymmetric wave, depending upon the circuit
power factor (see Fig.5). These limits are shown by the faint lines. However, above a certain level of
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