Any fire alarm installation prior to 2008 will most likely have a fused spur installed as means of safe isolation from the building electrical supply.
In 2008 BS5839 was amended and the method of isolation was changed. The new isolation point now requires in addition to a fuse, a double-pole authorised-access switch and indicator light.
Why the changes?
With previous methods of fused-only spur, there is a risk that if the wiring to the spur is inadvertently swapped, i.e. live and neutral are reversed, this will leave the potential of the 230V AC supply to the fire panel remaining live even though the fuse has been removed.
To visually explain this, here is a video from Barry going a little more in-depth.