Main Fuse
The main fuse is the top-level element that protects the whole system from a single point at the supply input. Its rating is chosen above the total current the circuits below it can draw simultaneously and below the ampacity of the main supply cable. Together with the sub-circuit fuses it forms a staged protection structure.
The logic of the two-stage protection structure is a division of responsibility. The lower fuses protect their own cables and their own consumers; the main fuse covers the distribution busbar, the main supply cable and the cases where the lower protection fails.
- Choosing the rating: the basis is the realistic simultaneous load, not the sum of the lower fuses. Taking the sum leads to a main fuse larger than necessary.
- Position: as close as possible to the battery output.
- Accessibility: the main fuse holder must be at a serviceable point and its rating must be labelled on it.
In systems where you install AXI remote controls, drivers and tractor kits together on the same vehicle, give every device its own sub-fuse and gather them all behind a single main fuse. This arrangement both makes sure that only the relevant circuit is cut in the event of a fault and lets you test the devices by energising them one at a time during commissioning.
Context in which this term is used: Installation, redundancy and field continuity