Page 10 - Application Guide Semiconductor Fuse Link

Adjusting the fuse rated current to allow

Working conditions A’2

A few stops per year (e.g. aluminum rectifi er plant). 0.95
1 stop per day (e.g. overnight stop 0.90
up to 12 stops per day 0.80

Table 2. De-rating coefficient for continuous loads ( A’2)

(b) cyclic loads

Cyclic means that the load current is variable, with a repetitive cyclic nature. In this case, the value of
A’2 may be lower than the values given in Table 2, and depends upon the nature of the cycle. For a
cycle with long duration, for example 1/2 hour ON followed by 1/2 hour OFF, the fuse cools down
almost completely during the OFF period, resulting in a large element temperature fl uctuation ^0
, and A’2 must be 0.6 or lower. Contact Ferraz Shawmut Technical Services for information on the
value of A’ 2 which needs to be used in these circumstances.

If the cycle contains overloads in excess of In it is also necessary to check the magnitude of the
overload current in relation to the melting time-current characteristic of the fuse. See section 6 for
further details.

Fuses connected in parallel (by the user)

In North America, the Electric Codes do not permit the paralleling of overcurrent protection devices
in the fi eld, although paralleling is permissible in a factory-built assembly. When two fuses are
connected in parallel it is necessary to ensure that they share the current equally, by making the
connections to the terminals symmetrical and of equal impedance. A de-rating factor of 0.9 may be
necessary, depending upon the fuse type and the installation. (In some cases the voltage rating must
also be reduced by 10-15% for certain fuse designs).

Contact Ferraz Shawmut Technical Services when considering connecting fuses in parallel.

Combining the factors

For a single fuse with a nominal current rating In the maximum r.m.s. continuous current allowable
in a real-world application is given by the expression

I n’ = In x A1 x Bv x C1 x CPE x A’2

This is illustrated in the following simple example.

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