RP 111C - Circuit Protection: Design

Battery cables, starter motor circuits, battery-to-generator and/or alternator circuits are normally wired without circuit protection. These circuits should utilize cable insulating materials and routing techniques that provide adequate protection against the possibility of a short circuit.

All other power feed circuits should have adequate circuit protection. It is recommended that each circuit have both primary and secondary protection devices. In every case, the circuit protection device should be located at a point in the circuit which is between the power source and the device being protected. It is also preferred that the protection device be as close to the power source as practical.

Each circuit protection device should be sized as needed for the circuit it protects. Circuit analysis should assure that the device provides adequate protection. The circuit protection should provide both high and low resistance short circuit protection, while at the same time allowing normal overload conditions (for example, light bulb inrush current or motor startup).

Circuit analysis should be performed to verify that the circuit protection device is not defeated by the design of the wiring system.

The vehicle lighting circuitry should not be protected by a single device. In normal practice, the headlamp circuit is independently protected.