this is a switch used to switch back and forth between 12v battery banks,deep cycle. they are used on boats and house solar systems when there are 2 different types of batteries or older/newer batteries. the top 2 lugs are for the hot leads to each battery bank. the bottom lug is labeled common.
I have run into this before and I'm not sure why this confuses me but........ what wire goes on the common..?? the black neg.
wire from each bank..?? or just a wire to ground, like a wire to
the grounding rod on a house or the chassis of a motor.??
could you provide diagram..??thanks..
my thinking was just run a suitable size wire to the chassis/motor mount from this common bolt..
now I find out it wont take a jpeg more than 244 kb.
skip the photo ,you can prob. figure what I mean.

 

If you have a meter, you can use it and see that the switch common connects to the top lugs, one at a time. You use it to switch a bank of batteries to the load, whatever that is. Those black wires are from the battery negative terminals. They should be tied together and go to the chassis.

 

If I read you correctly, it would be as in the attached: