Page 19 - Application Guide Semiconductor Fuse Link

Current-limiting operation

The actual I t values which apply at voltages less than rated voltage can be determined by multiplying
2
the published values by a correction factor K, obtained from curves such as that shown in Fig.11.
1,5 1500
I2t correction factor peak arc voltage
Vpk
K1
1000

0,5 500

0 0
0 100 200 300 400 500 600 0 100 200 300 400 500 600
r.m.s voltage r.m.s voltage

Fig.11 I t correction factor v.s applied voltage Fig.12 Peak arc voltage v.s applied voltage
2

Peak arc voltage

The peak arc voltage developed across the fuse during short-circuit interruption decreases as the
source voltage decreases. Fig.12 shows this characteristic for a typical 500V fuse.

Rated breaking currents

A current-limiting fuse can be thought of as an active device during its operation. When a current
limiting fuse is subjected to a short-circuit fault,
it produces a high arc voltage which forces the 100
current to zero well before the natural current max energy point
zero. During the arcing period a large amount
of energy is absorbed by the fuse and its fi ller.
Fig.13 illustrates the variation of this arc energy
with current for a typical semiconductor worst-case energy (KJ) 10
protection fuse. The maximum breaking
current or interrupting rating, of this fuse is
100kA, and the fuse is type-tested at this level.
However, it can be seen that there is a level
of test current (about 7.5kA in Fig.13) which 1
0,1 1 10 100
produces a higher energy. This is known as the
“maximum energy current” or critical current, r.m.s prospective current (KA)
and semiconductor fuses are also certifi ed at
Fig.13 Energy absorbed by fuse v.s IRMS
this test level, to insure that they can safely
interrupt all possible short-circuit faults.

As the available current is reduced below the critical current, the energy falls, but at very low currents
it begins to rise again.

This corresponds to the overload region. Most semiconductor fuses are short-circuit protective devices,
and are not designed to interrupt below a certain level of current, known as the minimum breaking
current. For a.c. faults this limiting condition corresponds to the C-C’ lines on the time-current curve,
as previously explained in section 4.

19