Electric Fuse

A fuse is the key before the key. Most card rooms at monuments sit behind a fuse box, and until a fuse goes in, the reader ignores your blue keycard no matter how far you ran to get there. So the fuse goes in your inventory the moment you plan a monument run, not into a box at base.

Forgetting it hurts most on a red keycard run. You crossed the radiation, cleared the scientists, and now you walk home with a card that only had two uses in it anyway.

Crates hand out most fuses, all wipe long. The tables also list a recipe of 50 scrap at a Level 3 bench, but the game files say fuses cannot be crafted or researched at all, so crates and recycling are the only source. Four fuses go into the Test Generator, which normal servers do not let you build. Restoring the Power Plant is a job for the heavy fuse.