Page 7 - Application Guide Semiconductor Fuse Link

The time-current characteristic

If a fuse is subjected to a current
10000
greater than the minimum current
required to produce melting, the
fuse element will melt. The higher 1000
the current, the shorter the melting
time will be. This inverse relationship
C
is shown graphically by the time- 100
current characteristic (TCC). C’
Fig.2 shows a typical time-current
characteristic for a semi-conductor 10
fuse with a nominal current rating prearcing time (s)
of 450A. 1

In = 450A
The time it takes for a fuse element
0,1
to melt is often referred to as the
prearcing time, since melting is
followed by a period of arcing. 0,01
Melting of a fuse element is due to 100 1000 10000
the heating effect of the current, r.m.s prearcing current (A)
which depends on the r.m.s. value of
the current actually fl owing through
the fuse before melting occurs. For Fig.2 TTC for a typical 450A seminconductor fuse
operation in times less than one a.c.
cycle, the melting time of the element
is greatly affected by the waveshape
2
of the current. In this case it is necessary to use I t values for checking the system protection. (See
section 8). The standard time current curve shown in Fig.2 is for a symmetrical sinewave.

The boundary C-C’ shown on Fig.2 indicates that the fuse will safely interrupt currents at times below
this limit. The fuse must not be applied to interrupt current levels which produce melting times
longer than this limiting boundary. Sustained overloads which persist for longer times may result in
failure of the fuse, and must be cleared by other means. There is a limit to how long a temporary
overload can be tolerated by the fuse. This limit is shown by sloping part of the C-C’ boundary. (New
style gR class fuses do not have a C-C’ limit, and can be used for overload protection.)

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