I have an old Leitz REDYX variable AC transformer for 6V and 8V tungsten microscopy lamps. Unfortunately it does not contain a fuse, so in other words if the transformer has an isolation failure it will start smoldering until the main circuit breaker for the room trips. So I figured to improve the safety I should add a fuse. In this case I’ve only fused the primary side, it might be a good idea however to fuse the secondary sides as well. But since I only run tungsten lamps from this there isn’t much of a danger of a prolonged short circuit on the secondary side (not without me noticing anyway).
So I added a small european standard 5x20mm 500mA (slow, ceramic w/ extinguisher) fuse on the input tap that goes to the switch.
Also I want to give credits to Leitz for the manufacturing quality of this, the internal cabling is beautiful.