Electrical Systems

Manufacturer Requirements

The purpose of this requirement is to prevent accidental passage of the battery supply current through fuel systems and smaller electrical conductors that may be common to engines. If one of the grounded cranking motor circuits accidentally opens (breaks) due to corrosion, vibration, etc., the accidental passage of starter motor current could melt fuel lines or burn up smaller conductors such as instrument wiring. Both of these hazards could lead to fire and explosion accidents. The common conductor circuit referred to in the regulation is a circuit made up of jumper conductors and may include a common bus bar.

Definitions:

Ground: ground applies to the potential of the earth’s surface. The boat’s ground is established by a conducting connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.

Grounded Conductor: a current-carrying conductor connected to the side of a source which is intentionally maintained at boat ground potential.

In a two engine installation, a jumper conductor will satisfy compliance if connected between the negative sides of the grounded cranking motor circuit.

In installations of three or more grounded cranking motor circuits, the common conductor circuit can be of several different configurations. Installing a common bus bar to which a conductor from each circuit is attached is one approach.

Common and convenient points of attachment to the negative side of the grounded cranking motor circuits are the engine negative terminal, any point on the engine block, and in direct contact with the negative side of the cranking motor.

In all cases, proper sizing of the conductor leading from each grounded cranking motor circuit is imperative. A rule of thumb to use when selecting the conductor is to use one as large as the conductor used to supply power from the battery to the cranking motor (the battery cable). Table 5 of the regulation can also be used as a guide when selecting the conductor.

If there are two or more starting battery installations and the negative terminals are connected by a common conductor, that common conductor does not satisfy this grounding requirement. Additional conductor(s) are necessary. Figure 11 diagrams typical circuits.

FIGURE 11 - Cranking Motor Circuits

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