I Have a European Boat and the Battery Switch That Has a Third "Black" on-off - What Is the Purpose of This Position on the Switch?

Question:

Hi Jeff, I have a question about main battery switches. My old boat had 1 'red' battery switch with positions : Off-1-2-Both. My new boat (European built) has 1 `red` battery switch labeled 'house' and on and off positions. It has a 2nd red battery switch labeled 'start' with on and off positions. It has a third 'black` switch with on and off positions. What is the purpose of the `black' switch? I want to connect a Victron 712 battery monitor and am thinking to connect it between the black switch

Answer:

Many French boat builders, install a DC negative disconnect for the whole boat. The black on/off switch, when installed properly, is supposed to disconnect all loads from the battery by disconnecting the DC negative return. Caution: many boaters and installers don't understand the purpose of this on/off DC negative switch and bypass it. Unfortunately, these modifications, post-factory, render the purpose of the DC negative switch to be only partial. Also, many French boat builders do NOT provide the boater the option of paralleling / combining the house and engine battery in the event of an emergency.