Page 8 - Application Guide Semiconductor Fuse Link

Adjusting the fuse rated current to allow

Ambient temperature correction coeffi cient A 1
`

Fuse current ratings are established by standard type tests with a reference ambient air temperature
o
o
(00) of 25 C or 30 C. For most power electronic applications the ambient temperature is higher than
o
this, usually in the range 40-65 C, and the fuse must be “de-rated.” The temperature rise of the fuse
depends upon the internal power generation, which is a function of the square of the current. For an
ambient temperature 0 the current rating must be multiplied by a de-rating coeffi cient A1 given by

a -
A = -
1
a -
0

o
where a is the maximum allowable fuse temperature (typically 130-150 C ).

Forced cooling correction coeffi cient B v

If forced air is used to cool the fuse in service, the
continuous current rating of the fuse may be increased,
by multiplying the rated current by a coeffi cient, Bv.
The value of the correction coeffi cient used by Ferraz
Shawmut is shown in Fig.3. Bv increases linearly with
air speed up to 5 m/s. Further increase in air speed
does not improve the fuse cooling. The limiting value
(B1) is typically 1.25, but values higher than this are
possible for fuse designs which have been optimized
for cooling.

Fig.3 Correction coeffi cient Bv vs. air speed

Terminal conductor size coeffi cient C 1

In the real world, the fuse may be used with cable/bus sizes which are smaller than those used in
the standard type test conducted in the lab. Since heat is conducted away from the fuse through
the connection between the fuse terminals and the cable/bus, the effect of using smaller cable/bus
sizes reduces the cooling and causes the fuse operating temperature to increase. Furthermore the
fuse is usually located in an enclosure and its temperature may be infl uenced by other nearby heat-
generating components. To account for these effects, Ferraz Shawmut uses a multiplying coeffi cient,
C1, to reduce the current rating of the fuse. C1 varies with fuse design confi guration and is typically
in the range 0.8-1.0. However, more heat can be conducted away from the fuse if the fuse terminals
are liquid-cooled, and values of C1 greater than 1.0 can be used. Typical values of C1 for several types
of Ferraz Shawmut fuse are given in the Appendix.

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