From a BR Training Booklet:

The Fast Fuse
The purpose of this fuse is to protect the rectifier diodes against reverse polarity connection of the vehicle battery which would destroy them.

It consists of a precision made high purity silver strip, surrounded by fine quartz sand enclosed within a cylindrical ceramic body. This design of fuse ensures that the current restricting section of the link ruptures at high speed and at precisely the pre-determined current level. This point will occur at the beginning of the fault current rise and this the current will be prevented from reaching a damaging level.

It is essential that only specified fuses should be used as replacements. If such a fuse is not readily available the alternator should be kept out of service until this fuse can be obtained.