In the event of an electrical fault it's important that all power to the electrical system can be cut off so that cables and circuits do not remain live. This will enable you to control any overheating which could start a fire, allow repairs to be safely made on any blown fuse or damaged cabling and reset the tripped circuit breaker. A battery master switch or switches, capable of disconnecting the system - including the starter circuits - must be fitted in a readilyaccessible position, as close to the battery as possible.
You must be able to get at the switch without having to move anything out of the way, or use tools or keys to reach it. If you can't see the battery master switch, you must mark its position very clearly so that emergency services or other crew members can easily locate it in the event of an emergency.
The switch must be capable of carrying the maximum current of the system, including when the engine is started. If there are separate circuits connected to separate batteries, each of them must have a battery master switch. A combined switch can be used, for example in a two battery system, where one battery is used for starting the boat's engine and the other is used for domestic services.
Electrical equipment, such as bilge pumps, security alarms, fire pumps and navigation equipment - some of which may have electronic memories - can bypass the master battery switch, as long as they're protected separately by fuses or circuit breakers. This will enable them to carry on functioning safely but still disconnect in the event of a short circuit or overload fault. [3.5]