I was carrying my Mag Charger 61 to the car, it was not turned on, it had a cool bulb and I dropped it on the sidewalk. The shock broke the bulb at the pins. I did not know that and I turned it on and nothing happened. I changed the bulb and found a twisted pin in the socket. The new bulb did not illuminate. I found the fuse had been blown. After the fall the bent pin somehow was grounded. The fuse at least saved some wear and tear on the battery pack and maybe some burned out wiring! I have room in the tail for a spare fuse and bulb. I was back in business very quickly.One Mag Charger, a MC61, that is a 900 Torch-Lumen light, Phillips 5761 on 7.2 volts, 5.4 Ah, 12X Sanyo 2700s in the pack has the pack soldered and wired by leads directly to the positive contact and to a ground screw. (see pict.) The ground lead has the mini fuse in it on the pack side of the connector. When I built this light and was working on the 4x boring I had a similar pack and I twisted it some. The wrappers were damaged. After a half-hour after setting it aside and there must have been some contraction due to cooling, the pack shorted while it was on the desk and nearly caused a fire. That inspired me. I fused the light and got a new battery pack and that was an expensive lesson.
I have built several basic hot wires now. And I fused some of them. I added an automotive mini fuse, 10 amps to the tail cap ground.
For an ROP I isolated the ground on the spring by placing a plastic cap on the spring and then put a contact, a U.S. penny, on the plastic and wired it to ground where the spring contacts the tail cap. In that circuit I put a mini fuse.
Isolating a ground wire to many types of packs, holders and tail caps does not appear to be too difficult and these could be easily fused in a number of designs. I will continue to do this after seeing the success.