Page 5 - Application Guide Semiconductor Fuse Link

Continuous current rating In

The rated current (In) of a fuse is a value assigned by the manufacturer, and is the r.m.s. current
which the fuse is designed to carry continuously under specifi ed test conditions. In this respect it is
no different from the rated current of any other electrical device. In is established by testing the fuse
under standard conditions specifi ed in fuse standards (such as UL or IEC). Test fi xtures with standard
sizes for the connecting cables or busbars are used, with the fuse cooled by free air.

The current rating alone does not provide the user with any information about a fuse’s protective
characteristics. The temperatures of the various parts of a fuse must be kept within acceptable limits
when it is carrying rated current. For example, the temperature at the middle of a silver fuse element
must be much lower than the melting point of silver (960.5°C), and this means that the rated
current will be considerably lower than the minimum current required to cause the elements to
melt. For semiconductor fuses the maximum allowable temperature rises are determined by the
manufacturer.

In real-world applications, the working conditions of a fuse inside equipment are rarely the same
as those used in the standard type test. Since the rated current is based upon the temperature-rises
produced under steady-state thermal conditions in the lab, operating a fuse under different thermal
conditions may change the current carrying capability of the fuse. The user must take this into
account when applying the fuse. (See section 5).

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