[taneq]

It doesn't have to be anything fancy - you could just fit a hasp to the fuse box (if one's not already attached) to allow you to lock it closed with a padlock. Then you keep the only key for that padlock on your person until you've finished the job.

If you want to get fancier or if others are going to need to access the fuse box while you're working, you can get "lock dogs" which let you lock individual circuit breakers. Then you just lock off the one for the circuit you're working on.