What should be done first when replacing a junction block or fuse?

When replacing a junction block or fuse, the immediate priority is to ensure safety and prevent any electrical hazards. Unlocking the battery ground cable is critical because it disconnects the electrical system from the battery, ensuring that there is no risk of electrical shock or short circuits while you're working on the components. This step effectively prevents any live current from flowing through the wires connected to the junction block or fuse, which is essential before any disassembly or replacement occurs.

Once the ground cable is disconnected, the technician can work on the electric components with reduced risk of accidental energization and injury. Following this step, other actions like removing the old fuse or checking for power can be safely conducted, but the first step must always prioritize safety by removing potential power sources.