If you take your alternator output directly to the starter motor terminal and then accidentally turn off your battery switch with the engine running there will be nowhere for the alternator output to go. Without anywhere to go the alternator voltage will rise to a sufficiently high level that something will break, usually the diodes in the alternator. 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:
2 Comments
Dan Pease
Only problem with running Alternator hot wire direct to battery is that that wire will always be hot. What if you want to do work on alternator? I suppose just be careful?
Mark Cole
Disconnecting the fuse to work on the alternator doesn't take into account that one might forget to reconnect the fuse before turning the engine on - at which point the alternator goes into full field and is damaged. Using a switch with AFD to disconnect the alternator prevents any accidents like this.
Peter Kennedy
You should be putting circuit protection on the alternator connection to the battery. This would be right at the battery end of the wire and protects the wire. If you need to work on the alternator you can remove the fuse.
James Cross
It is fairly common knowledge that one should NOT disconnect an alternator from the battery while the engine is running, as bad things can happen. My boat manual talks specifically about this. No problem there. However, there is a 60 A circuit breaker in the alternator + lead heading to the battery, and the only wire connected to the alternator output passes through this CB. If disconnecting the alternator from the battery with the engine running is bad news, why would there be a CB in the output lead since it is a distinct possibility that the breaker could, in fact, open while the engine is going. For some reason on my boat (Jefferson 52 with Cat 3208's) the CB on both port and starboard engines operated at some point. Port engine CB was reset and is fine - starboard engine CB was reset without effect. CB must be toast. What's the theory behind this arrangement? s
Peter Kennedy
There has to be a circuit breaker in the line to prevent a catastrophe in the event that there is a short circuit. The circuit breaker should be a considerably higher value than the maximum possible output of the alternator to prevent accidental tripping. I often make it 150% of the rated output. The wire should be of an appropriate rating to match the circuit protection. So if you have a 100 amp alternator use a 150 amp fuse at the battery end and make sure the wire is good to carry 150 amps. If the alternator is not self limiting the ABYC requires circuit protection at the alternator end also.
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