Page 20 - Application Guide Semiconductor Fuse Link

Interruption of d.c. fault currents

When a d.c. fault occurs the rate of rise (di/
dt) of current is dependent on the circuit 600
time constant (L/R). For a typical d.c. motor
armature circuit, the L/R time constant is
around 40ms, which yields a lower rate Vdc
of rise of current (di/dt) under short circuit
conditions as compared to an a.c. short 300
circuit. With the lower di/dt, the fuse takes
longer to melt compared with a.c. conditions,
which also leads to a relatively long arcing
time. These conditions, together with the
absence of natural current zeroes, make the 0
0 50 100
interruption of d.c. faults more diffi cult for
time constant l/R (s)
the fuse than a.c. faults, if L/R is high. As the
L/R time constant in a circuit increases, the
DC voltage capability of the fuse decreases. Fig.14 Effect of L/R on d.c voltage rating

Therefore, for d.c. applications it is essential to know the value of L/R. Fig.14 shows how the d.c.
voltage rating of a typical semi-conductor fuse is affected by the circuit time constant.

It is often diffi cult to obtain a precise value for L/R in practice. In the absence of better information,
Table 4 gives some typical guideline values.

Type of equipment Typical L/R , ms

Battery supply/capacitor bank < 10
Bridge circuit < 25
DC motor armature 20 - 60
DC traction systems 40 - 100
DC motor fi eld* 1000

* It is never recommended to fuse a DC motor fi eld circuit.

Table.4 Typical values of L/R

The arc energy produced in a d.c. circuit also varies with test current in a manner similar to that shown
in Fig.13. So for a given L/R there exists a maximum breaking current, minimum breaking current,
and current which gives maximum arc energy. The Vdc s L/R characteristic (Fig.14) is published
for the maximum fuse arc energy condition. Minimum breaking currents for d.c. applications are
published separately.

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