There are three reasons:
1) The most common thing is that the selection is unreasonable. The rated current of the system is close to the rated current of the fuse, or even higher. This situation usually occurs shortly and it is easy to find.
2) When the system starts, there is a high pulse current flowing through the fuse. This pulse current will cause the fuse to aging. This situation occurs for a long time, and the fuse appears when the system starts.
3) Instant wave current is too large, and the number of microseconds leads to the melting of the fuse. This situation occurs occasionally, and it appears more in the surge test.