Instruments Randomly Powering Up

[mdrsail]

Today I heard a beep that I only normally hear when I turn on the breaker for the instruments. For some reason, the port side helm  instruments (the ST60s) turn on and stay on for an indeterminate amount of time, then they turn off. The other instruments such as mast and plotter remain off. How can something get power when the breaker is off? Has this happened to anyone else? I turned the breaker on and everything else came on as normal, then turned off and they followed except the port helm!
2008 39i

[Don Reaves]

If I'm not mistaken, the ST60 instruments get their power from the SeaTalk bus (SeaTalk-ng, actually). Perhaps there's another device like an AIS buried somewhere that has power which is being fed onto the bus. You're right in thinking that the instruments can't power up if they have no power, of course. So you need to investigate where the bus goes and what might be connected to it.

Next time it happens, start turning off all the breakers one by one and see what happens. If the instruments are still on when all the breakers are off, turn the main power switch off too. Eventually, you should be able to narrow down the source.

[mdrsail]

Thanks- that makes sense. I think the AIS is wonky. It started causing a beeping alarm last month whenever I turned on the plotter.