The provisions of this section of Part 3 in the 2002 BSS Standards are mandatory for non-private boats where applicable.
To provide efficient circuits and minimise the risk of fire or personal injury in the event of a short circuit or power overload, all electrical equipment - except engine circuits - should be 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]