The alternator field disconnect on the battery switch is designed to prevent this unfortunate occurrence from happening by disconnecting the alternator field wire before disconnecting the battery switch. So the alternator is effectively turned off before the battery switch is turned off and no harm is done.
Why have I never seen this actually used in over 20 years of marine electrical work? Well, first it only works with externally regulated alternators because those are the only kind where you have access to the field wire. Secondly you have to divert the field wire on its way from the regulator to the alternator and make it take a detour via the battery switch. My guess is that people buy these switches with the best of intentions and then never get around to doing that extra bit of installation.
Is there an alternative way of protecting the alternator from accidents caused by turning off the battery switch? You bet! Run the output wire directly to the battery; no switch, no problem. (Don't forget to put in appropriate circuit protection) Do you have a battery switch with an alternator field disconnect? Is it hooked up? This is an interactive blog so please feel free to add your comment.
There is another use for the field disconnect terminals.
You can use them to wire in an indicator dash lamp to tell you if the switch is on or not. This would be especially useful if the switch was being used as an emergency combine switch and would remind you that it was combined.
The battery switches we sell with AFD terminals are the following standard duty (red) and heavy duty (grey) battery switches: