According to his story it happened when his deep cycle battery shorted internally when both were switched in. From what he is saying he had a 1,2 or both battery switch that fused itself in position and he could not cut the power despite his quick response.

I think it is a case for double pole battery isolator switches.

................. and here i was thinking it was a thread on how hot you like your chili con carne!

I have these on my batteries as well as the double isolator switch and they are open, disconecting the batteries when i am not on the boat. As the isolater switch is in a compartment next to the battery compartment, I might look at placing it further away after looking at the above scenario.

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PhoenixStar said..

www.pantaenius.com/au-en/insights/journal/news/

Had a battery do this on "Nacooma", but lucky it didn't create this much devastation.

Is this a case for fusible links?

I'd imagine that a sensibly sized fusible link (blows at twice what your starter draws) would go a long way towards instantly shutting off this sort of damage. There's a reason that they're installed on cars.

Way more reliable than slow manual switches that either weld themselves shut, or draw a plasma arc as you're trying to shut them off. High current electricity doesn't behave in the same way as normal sized loads. I don't know of any normal isolator that would be able to break those sorts of fault currents. Most isolators, even dual pole ones, are there purely to isolate batteries when you're not using them. They're not designed to break 1000 amp fault currents. The only switches I've seen that would do the job were some knife switches I saw when doing my apprenticeship at the steelworks, that had spring loaded terminals that have a very fast 'snap' to break the arc. Even then I wouldn't bet my life on them not welding themselves shut at that sort of fault current.

Just put a fusible link inline with your battery and you're done.