What is the appropriate method to disconnect all electrical power to prevent discharge while a fire apparatus is not in use?

Using the battery selector switch is the most effective method to disconnect all electrical power and prevent discharge when a fire apparatus is not in use. This switch is designed specifically for isolating the electrical system by either connecting or disconnecting the batteries from the apparatus's electrical system. By utilizing the battery selector switch, you ensure that all power drawn from the batteries is cut off, which extends battery life and reduces the risk of electrical fires.

Other methods may not provide a complete disconnection of the entire electrical system. For instance, removing the fuse only interrupts power to certain circuits, which does not address the entire electrical configuration. Turning off all lights merely reduces the load on the battery; it does not prevent discharge from other systems. Disengaging the fuel line is unrelated to electrical disconnection and primarily pertains to fuel system safety, but it does not address battery power management.