Lights out...

Under the heading of When Things Go Wrong...  [Or Sh.tuff happens...]

In summer 2018, peacefully at anchor one sunny afternoon, our 12V DC power shut down.

Everything off. 

No sound. No sparks. No smoke. No smell. No alarms. Zippo... Nada...

And no battery power... anywhere...

Ruht Roh...

And no high load before this occurred [just the usual stuff averaging ~12 amps 12V DC…]

It was as though the 350 amp primary feed fuse at the batteries silently blew [which was my first fleeting thought…]

I decided to do some tracing and grabbed a VOM to test for voltage on the [few] 4/0 positive [and negative] cable connections starting at the battery bank.

12.7V DC at the batteries— before and after the 350A fuse [which protects the boat from a direct short in the 4/0 feed from the battery bank.] Good.

Hmmm… the alternator on the engine still has 12.7 volts…   [This also means the high amperage fuse connecting the alternator output to the battery bank isn't blown.] Good.

Next in line is the emergency DC power disconnect switch in the pilothouse. This switch kills all power from the house battery bank to everything on the boat except the engine starter and alternator... so I continued there [after first confirming it had not been accidentally switched off—  by no one in particular...]

OK; 12.7 volts on the input [battery] side of the switch… Good.  
But zero voltage on the output side... Hmmmm?...  Not even a fraction of a volt? 

Not good... [Yup, the switch was on— and cool to the touch…]

Hmmmm?…

I toggled the switch off and on a few [dozen] times [Hoping for what exactly?... To remove corrosion on the contacts? Or toggle contacts fully into position? Or make is magically start working by touching it...?] 

No such luck... No change... Still no battery power.

Apparently the switch just suddenly [and silently] died— and with just the ambient 12V DC load… [e.g., No surge of current from running a welder off the inverter... I exaggerate…]

Having no spare switch onboard [but several in our storage facility...] I ended up just bypassing the switch to put us back online. [Thankfully, the switch was the only culprit— the root cause of this issue.]

This seemed absurd at the time because one never expects a high amperage switch that has maybe been cycled a handful of times in its life to catastrophically fail out of the blue— especially with no appreciable electrical load…

On the plus side, that is the right way to fail… [vs. discovering you cannot disconnect battery power... in an emergency…]

Remember, this is the switch to be quickly turned off in the event the symptoms listed above present themselves... [Smoke, sparks, 'electrical smell', lights dimming and flashing, etc...]

What happened?

After I removed the switch, I noticed one of the 3/8″ cable connection studs on the back was loose; the other wasn’t. I suspect a connection inside the switch [solder/weld?] just suddenly yielded, unceremoniously breaking the circuit. [I didn't care enough to tear it down as it was to be replaced with something different... more below...]

Perhaps the  4/0 cables hanging on it induced a strain that over time broke the bolt’s connection to the switch? [Inadequate strain relief? The cables were firmly secured mere inches from their attachment point on the switch... Perhaps it was over tightened when installed in 1999 by the previous owners? Manufacturing/design defect? Poltergeist? Who knows...?]

But— remember our story started out that it was a warm and sunny afternoon… so it wasn’t all that bad… 

Doesn't Murphy mandate events like this are supposed to happen when it it is pitch black and stormy, the boat is bucking at anchor like a bronco, and you have only been asleep for an hour after a long day, and perhaps you are even coming down with the flu, or are seasick…?  [Oh, and the first 3 flashlights you grab have weak batteries...] Catch my drift?...

Looks like Murphy gave us a pass this time... and...

The dead switch has since been replaced…

The new 300A [continuous; 500A ittermittant] emergency cut-off switch [shown with key removed...]
[Different manufacturer from the one being replaced...]

Our lessons learned include: 

• Always troubleshoot from the bottom up [To determine the root cause, start with the roots... and don't ignore other possible contributing factors...]
• Never assume you immediately know what has happened [Don't believe everything you think...]
• Be prepared for any component to fail without warning or apparent cause at any time [AKA Cruising...]

Are there any proactive measures or any preventative maintenance to help mitigate something like this from re-occurring?  

Search me... Leave a comment... I'm all ears...

This post began life as my reply to the question:

"What’s the most ridiculous repair you’ve done on your boat?"  

[Asked by our friends on SV Galapagos in their We're in Hot Water post;  7-Mar-2019]