I am assuming that you cannot use an actual piece of visco fuse, so i will go in the non-pyro direction. I would say its gonna be pretty custom but what you really need is a foot long piece of 1/2 fiber optic bunch. A simple flickering red and/or amber led inside the "bomb" will make the fuse look likes its burning. The actor can turn the unit off when they "blow" it out.
When we were doing Joseph, we had to contruct a similiar effect for "Those Canaan Days".
Basically, the fuse was constructed with the help of a few LEDs placed into the base of the bomb and fiber-optic strands appearing out of the terminal of the fuse. It worked rather well and was fully readable on stage.