Page 9 - Application Guide Semiconductor Fuse Link

real-world working conditions

High-frequency effect coeffi cient CPE

If the fuse current contains signifi cant high
frequency components, (above about
1kHz), the current distribution within the f (Hz) CPE
fuse changes, due to skin and proximity
effects. This is due to electromagnetic fi eld 0 < f 100 1.0
interactions, and depends upon the position 100 < f 500 0.95
of the fuse relative to the return conductor 500 < f 1500 0.9
or other current-carrying conductors. An 1500 < f 5000 0.8
unequal sharing of current between the 5000 < f 10000 0.7
fuse elements is produced, which results in 10000 < f 20000 0.6
additional heating.
Table 1. Frequency derating coeffi cient CPE

Table 1 gives the de-rating coeffi cient CPE
which should be used if the fundamental
component of fuse current is in the frequency
range shown. For the more common case, the current waveshape contains a mixture of harmonic
components and the reduction in current rating will not be so great.

In an application the total r.m.s. current including harmonics must be used as the basis for fuse
selection. If the total harmonic content is more than 15% of the fundamental, contact Ferraz Shawmut
Technical Services.

Current-variation coeffi cient A’ 2

In a real-world application the r.m.s. current through the fuse is not constant, but varies depending
on the nature of the load, and its duty cycle when the equipment is in operation. In addition the
power conversion equipment may be switched on and off from time to time, for example during
the night time. The heating and cooling of the fuse elements due to this variable loading produces
expansion and contraction (thermal straining) of the fuse elements, which can cause mechanical
fatigue and premature nuisance opening of the fuse. This condition is of particular importance for
semiconductor fuses, which have very small notch zones on the elements in order to obtain the
required high speed of operation. Variable loading causes temperature fl uctuations ( ) in these
notch zones. It is essential to ensure that semiconductor fuses are adequately sized, to cope with the
temperature fl uctuations and resulting thermal strains produced in service.

Variations in the load current and duty cycle are taken into account by using a de-rating coeffi cient
A’2. Loads are classifi ed as either continuous, or cyclic, as defi ned below.

(a) continuous loads

Continuous means that the load current is steady when the equipment is in operation, and there are
no overloads. Appropriate values for A’2 are given in Table 2.

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