Why is the fuse placed at the point closest to the battery terminal?
The piece of cable before the fuse is unprotected; nothing interrupts a short circuit occurring there and the battery's full short-circuit current is loaded onto that cable. ABYC E-11 requires overcurrent protection within 7 inches, that is 175 mm, from the connection point for every positive conductor connected to a battery or charging source; if the conductor is inside a protective sheath, this limit is extended.
A cable shorting a battery glows red within seconds and ignites its insulation. Every centimetre before the fuse is exposed to this risk, because that region cannot be switched out of circuit.
The rule is therefore defined in terms of distance. According to ABYC E-11, every ungrounded conductor connected to a battery, an alternator or a charging source must have overcurrent protection within 175 mm (7 inches) of the connection point. If the conductor is inside a protective sheath or conduit, the limit extends to about 1.0 m (40 inches), and for a sheathed conductor connected directly to the battery terminal to about 1.8 m (72 inches).
Application notes:
- Mount the fuse holder as close as possible to the battery terminal, but at a point still accessible for service.
- Protect the short section between the fuse and the battery mechanically: run it through conduit, keep it away from sharp sheet-metal edges, use a rubber grommet.
- The fuse holder must be enclosed and resistant to battery acid vapour; a corroded holder creates resistance and drops voltage under load.
- Measure the connection under load; a drop exceeding 0.2 V across the fuse holder is a contact problem.
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