What is an ‘AFD’ type battery switch?
Automatic Field Disconnect.
If you have a battery isolator switch between an externally regulated alternator and the battery bank, then you need to cut the field current to the alternator before disconnecting the batteries to reduce the very real risk of damaging your alternator.
The AFD is actually a separate, isolated, single pole switch within the main battery switch itself. The alternator field current wire from the external regulator is connected to one pole, the other pole then goes on to the alternator.
The AFD switch itself, opens slightly before the main switch contacts open. This ensures that the field current is interrupted, so the alternator will stop generating, and ultimately, there won’t be any large voltage surge. If you don’t cut the field current before disconnecting the battery bank, the charge current will have nowhere to go, and the resulting surge will more than likely blow the diodes.
Blue Sea Systems have a selection in their ‘e’ and ‘HD’ series with AFD’s, below is a diagram showing how their e-series 11001 is configured. The AFD terminals are labelled F1 and F2.
