Hi, I agree, this sounds like a large voltage spike. As noted, a direct lightning strike (and even an EMP from close strike) can wreak havoc on electronic without either being turned on or blowing the fuse. Since, there was no mention of weather, I agree with those suggesting an alternator spike.
It is possible to be an intermittent fault within the alternator, but given the marine environment, it’s more likely a connection problem, either positive or negative. Don’t over look a bad negative connection. An alternator can have a good ground connection but have a bad negative terminal connection. This is why companies like Balmar recommend a dedicated negative cable, same size as the positive cable, connecting the alternator to the negative terminal. That said, I knew of a person who tried to make things better by installing a dedicated negative cable. Things worked well for a long time until he upgraded his alternator to a high end brand. He was really pissed when he found out that his new system would only charge his house battery and not his start battery. He originally had a working system, and the thought was that the new alternator somehow messed with his battery isolator by damaging it or just keeping it from working. The problem turned out to be a bad connection between the negative terminals of the house and start batteries (maybe they got disconnected and forgotten). The new alternator had an isolated ground whereas the old one did not. The old alternator system worked because the two batteries' negative terminals were connected through the dedicated negative cable to alternator to engine to start battery negative. With the new isolated alternator, the alternator negative was only connected to the house negative terminal. - Paul E. 1981 C&C Landfall 38 S/V Johanna Rose Fort Walton Beach, FL http://svjohannarose.blogspot.com/ > On Sep 27, 2019, at 8:11 AM, cnc-list-requ...@cnc-list.com wrote: > > Message: 3 > Date: Fri, 27 Sep 2019 10:08:18 +0300 > From: Matti Airas <mai...@iki.fi <mailto:mai...@iki.fi>> > To: cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com> > Subject: Re: Stus-List Interesting electrical problem > Message-ID: > <cahfg+fn-gl0unp8mrnbrep+nsjrprhkfkly1jrv_7zzxbpd...@mail.gmail.com > <mailto:cahfg+fn-gl0unp8mrnbrep+nsjrprhkfkly1jrv_7zzxbpd...@mail.gmail.com>> > Content-Type: text/plain; charset="utf-8" > > Hi, > > I'd check the battery and alternator connections carefully. And the main > switches, too. If the batteries get disconnected while the motor is on, the > alternator will create a load dump that can be even over 100V for a > fraction of a second. In principle, automotive and marine electronics > should be protected from that but you never know. I'm only speculating, but > if that was the reason, maybe the voltage spike was short enough that the > fuses didn't have time to heat up? > > Google for "load dump" if you want to learn more. > > Cheers, > > Matti Airas > C&C 36 "Hurma" > Helsinki, Finland
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