Fuse Proximity to the Battery Terminal (Protection Proximity)
The positive conductor leaving the battery carries a short-circuit risk for as long as it remains unprotected; that is why the fuse is placed as close as possible to the connection point. Low-voltage DC system guides typically give a 175 mm limit for the unprotected length; if the conductor is inside a protective sheath, this distance can be kept longer.
The piece of conductor before the fuse is under no protection at all. If this section touches the chassis, all the short-circuit current the battery can deliver flows; the cable glows red within seconds and a fire starts. Shortening the unprotected length physically reduces the area in which this risk can arise.
- Application: place the main fuse right on the cable connected to the battery terminal, using a terminal-mounted fuse block or a short fuse holder.
- Sheathed conductor: for a conductor inside conduit or an additional sheath, the limit can be extended somewhat; still aim for the shortest possible distance.
- Crossing points: if the unprotected section passes over a sheet-metal edge or a sharp point, always use a grommet.
In caravan, construction machine and tractor installations the battery is often inside a metal compartment; letting an unprotected positive cable run loose in that compartment is a serious risk. Placing the fuse at the battery output is the common safety foundation for every sub-circuit added later.
Context in which this term is used: Installation, redundancy and field continuity