This switch is used ONLY for paralleling the house and engine
batterys for starting so a field interupt is not needed. It is switched only while no load is applied, the engine
is started and then it is switched back to separate the house battery.
The diagram the was offered by the last poster is not as good as it could be and mine is wired differently. I would NEVER put both the house load and the engine starter on the same buss. My engine starter is on its own battey buss that is NOT shared with the house battery.
The #1 terminal on the switch is connected to the house battery, the #2 terminal is connected to the engine battery and the output is connected to the house load.
Under normal opperation placing the switch in #1 puts the house battery on the house load and isolates the Engine battery which is connected directly to the engine starting solenoid.
When the switch is in #2 position it isolates the house battery and connects BOTH house and engine loads on the engine battery. IT WILL NEVER be used in this position.
When the switch is in BOTH it parallels both batteries so that both the house load and the engine load are supplied by BOTH batteries. This position is used for starting the diesel
only in an emergency
when the engine battery hasnt got the ability to do it on its own (hopefully never) while the main house panel is switched OFF so it is NOT a load during starting.
Once started the switch is placed back in the #! position once again issolating the engine and house batteries from one another and the hous panel is then switched back on.
there is a battery combiner (electronic circuit to make sure the engine battery is first at full charge before connecting the house battery and isolating the engine battery if the house battery develops a fault)
All the above is designed to preserve the ability to start the engine in all possible fault scenarios......of course, if both batteries are dead then the only solution is to break out the crank and hope you dont break your arm in the process...#8-)