When is fuse fatigue a concern?
Fuse fatigue is deterioration due to repetitive thermal stress caused by short-term overcurrent pulses that are part of the normal operating cycle of the fused system. For example, with full-voltage starting of an induction motor, there is a short period of starting and accelerating current that greatly exceeds the normal operating current. Fuse manufacturers provide application guidance for a variety of such situations, but there may be a concern that that information does not cover every eventuality.

The harmonic content of the input currents to power electronic equipment may be another factor that could cause fuse fatigue. In addition, power electronic equipment often has electronic protection that "instantaneously" shuts off the output in the event of a short circuit, but may let through a very short high fault current pulse. The electronic circuit may be designed to withstand those situations, but it could still cause fuse deterioration. As a result, many electronic designers have concluded that electric protection should be designed to protect the equipment from output short-circuits, overloads and most other abuse, but fuses should be selected to prevent fire, not to prevent the failure of electronic components.

Whenever the normal operating current contains pulses or repetitive surges, the guidance of the fuse manufacturer needs to be carefully applied. If the guidance doesn't adequately cover the situation, there could be a risk of a fuse fatigue problem.