The alternator gets turned off.

[mestrezat]

The day we started a 12 day cruise through the San Juan Islands I noticed that my battery monitor only showed our batteries at 12.4 with an amp drain still happening on the batteries even though the engine was running. It ended up that the oil pressure switch had failed and when that happened, it turned off the current from the alternators to the batteries. If I installed a "jump" over the pressure switch, my pressure gauge came back up and my alternator began to pump out the amps.

When I stopped the engine, I had to remove the "jump" wire because it was draining over 3 amps from the batteries. With the wire removed, they stopped getting drained. I finally set up a toggle switch to turn it on(engine on) and off(engine off)as we moved through the islands

I've been told that it was set up that way to protect the electronics on the boat in case the pressure failure resulted in a run away engine producing far more amps and power than my system could absorb. That surge could burn out equipment.

Anyone ever hear of a system being set up this way? Sounds strange to me.

[Blue Stocking]

Oil pressure switches were originally set up allow the engine to start up without the load of spinning the alternator. Modern regulators allow a "no-load" start for the engine. I guess there is the remote possibility of "spike " damage, but with the battery acting as a buffer, I have never seen it happen in thirty years of wiring boats.