Originally Posted by Erin
I think you will find proper isolator switches can easily handle this. Diesel engines in many motorboats pull 300 to 400A for the starter motor and the switches are usually rated up to about 750A IIRC.
A decent switch in a sensible position should not get accidentally turned off. Everything bar the bilge pump should be fed via an isolator IMV.
For petrol engines "a good starter will normally draw 130 to 150 amps when cranking a four cylinder engine, 175 when cranking a V6, and as much as 200 to 225 amps when cranking a large V8" so do you fit a switch rated appropriately to the engine fitted or one that might handle a straight short? We know what the boat builders will do......the cheapest they can get away with.
You have to be wary of how you interpret the manufacturers rating figures.
Many switches I've seen quote "intermittent" figures in the order of 500-1000 amps but that's only for 5 seconds and relates to flowing current. They're not rated to "switch" or "interrupt" that current. (that's left to the starter solenoid)
Also, having done a quick bit of research and it looks like my 500-600amp educated quess at the short circuit current of a 12V lead acid battery was a fair bit short of the mark. The actual figure appears to be over 1000amps --http://www.battcon.com/papersfinal2003/korinekpaperfinal2003.pdf
I'm not advocating pulling out switches or standing on a soap box on this, it's just food for thought. Most "straight shorts" happen when we're working on something and the battery should have been disconnected before we started.
It could happen in normal use but I imagine (with proper maintenance) fairly rarely and I'm still not convinced the switch is going to work. Everything, with the exception of the starter motor, is fused anyway so really it is only the starter and it's feed that constitutes a risk factor.