Fuses in this type of gear are for the power supply. Under normal operating conditions, the current draw isn't high enough to blow the fuse. When something goes wrong (in the power supply) it'll draw more current and blow the fuse. Let's say the power transformer or rectifiers have shorted, and there was no fuse to open and remove the power, a fire would occur.
This is why fuses are called "protection".
Your fuse blew because something else has gone wrong and it did it's job by opening and removing power from the power supply.