Page 16 - Application Guide Semiconductor Fuse Link

Current-limiting operation

An important advantage of the current-limiting fuse is its ability to break high fault currents rapidly,
2
which limits the peak current fl owing in the circuit, and consequently limits the let-through I t.
Fig.7 illustrates the operation of a fuse interrupting a short-circuit fault current in an a.c. circuit.
During the prearcing (melting) period the current closely follows the prospective (available) current
wave and the voltage drop across the fuse is quite low. When the fuse element melts and arcing
begins, the voltage across the fuse increases rapidly and the current is forced to zero well before the
natural zero crossing of the prospective (available) current wave. For clarity the degree of current
limitation shown in Fig.7 is low. In actuality, a typical 200A semiconductor fuse, when subjected to a
prospective (available) current of 230kA peak, can limit the peak current to about 8kA, reducing the
2
stresses on the circuit by a factor of (8/230) , or to 0.12% of the level without fuse protection.
The current-limiting behavior of fuses can be explained by reference to the equivalent circuit, which
shows a fuse in a circuit under a short-circuit fault condition. Application of Kirchhoff’s Voltage Law gives
di
V (t) = Ri + L- +
s
dt F
Rearranging this to give the rate-of-change of current
di V (t) - Ri - F
s
- = --
dt L
During the prearcing (melting) period the
120
fuse voltage is almost zero and may be
neglected, so when VS(t) is positive, the prospective
current
circuit current grows as shown, with di/dt 80
VS(t)/L. When arcing begins, vF increases i,KA fuse
current
rapidly, and if > VS(t) – Ri, the rate-of-
F 40 time (s)
change of current becomes negative, and
the current is «forced» down towards zero.
0
0,005 0,01 0,015 0,02 0,025
The presence of the inductance in the 0
-40
circuit prevents the current from changing
instantaneously. At the instant the fuse
changes its state from the prearcing (melting
or low-resistance) state to the arcing (high-
resistance) state, the current stays almost 1200
constant, and the voltage developed across fuse voltage
the fuse (arc voltage) increases rapidly.
V
time (s)
The higher the arc voltage, the more rapidly 0
the current will be driven to zero during the 0 0,005 0,01 0,015 0,02 0,025
arcing period. If the design objective for
semiconductor fuses is to minimize the let-
source voltage
2
through I t, the fuse must be designed to
-1200
generate a high arc voltage. However there
is a practical limit to the magnitude of arc
voltage, since in a power electronic circuit,
diodes, thyristors and other semiconductor
components can experience this arc voltage,
in the non-conducting state. In general the R L
peak arc voltage must not exceed the peak VF
inverse voltage withstand capability of the Vs(t)
i
associated semiconductor devices.

Note that the fuse current waveform has a
roughly triangular shape, and the total time
2
Fig.7 Limitation of peak current and I t
to clear the fault is the sum of the prearcing
time and the arcing time. This is illustrated in Fig.8.

16