To provide efficient circuits and minimise the risk of fire or personal injury in the
event of a short circuit or power overload, it's recommended that all electrical
equipment - except engine circuits - is installed using a two-wire system, with one
wire out from the positive and one back to the distribution box. For each circuit the
positive must be connected to a fuse or circuit breaker, while the negative is taken
to a common terminal connected to the battery negative and often earthed.

Some steel-hulled boats do have a one-wire system, using the hull to make the
return circuit. But, as well as the risk of accidental short circuits, stray current
can seriously damage the metal of the boat, and other boats in the water,
contributing to hull corrosion.

Engine circuits are fitted with a low-resistance return conductor between the
engine and usually the negative pole of the battery. This provides adequate
protection to carry the starter current. The connections must be secure to
prevent overheating and starter motor failure. [3.8]