Main Battery Fuse
The high current fuse placed as close as possible to the positive terminal of the service battery, protecting the line that feeds the whole installation. Its purpose is not to protect the devices but the thick cable between the battery and the distribution panel in a short circuit, and its breaking capacity must be above the short circuit current the battery can deliver.
A lead-acid service battery can deliver thousands of amperes into a short circuit. This current melts the insulation of the cable within seconds. When the main fuse is placed close to the battery terminal, the unprotected length of cable comes down to a few centimetres and the risk is minimised. Low voltage DC system guides typically give a limit of 175 mm for unprotected conductor length; if the conductor is inside a protective sheath this distance may be kept longer. If the fuse is placed next to the panel, all the cable in between is left unprotected.
There are two quantities in the selection: the rated current and the breaking capacity. The rated current is chosen below the value the supply cable can carry continuously. The breaking capacity is the highest current the fuse can interrupt without arcing; in battery systems fuse bodies with high breaking capacity are used.
In an RV, if there are separate thick lines such as the inverter, the levelling system and the distribution panel, each is taken from a fuse block close to the battery with its own main fuse. This arrangement both protects each line separately and prevents the whole system shutting down on a single fault.
The context in which this term is used: RV automatic levelling guide