The 12 volt Fuse is designed to protect your vehicles wiring harness in addition to working as a normal fuse. The thermal fuse is unique as it measures resistance. Resistance generates heat (heat and wiring don't mix). The amount of resistance will vary depending on the length and thickness of the cable used in the installation. If the cable has too much resistance the fuse will blow, protecting your vehicle. These fuses are also directional, the arrow marked on the side of the fuse should point in the direction of the cigarette lighter socket. If you find that the fuse has blown we would suggest that you consider having your vehicles wiring tested and fitting an Engel pos-fit connection.