T&T: Engine Will Start Even if Selector Switch in "Off" Position

[Ken Bloomfield]

Normally, the selector switch is only selecting the power for the DC 
breaker panel.  That is to say, it is not handling the several hundred 
amps that the diesel starter motor needs.  So, in my boat you can 
disconnect everything except the starter motors with the selector switch 
in off.  Ideally, the starter motors should have their own high-current 
cutoff switches mounted outside the engine room, but many (perhaps most) 
don't.  My 50' Marine Trader has these switches mounted inside the 
engine room, and the switches have a warning to the effect that if they 
ever have to cut off a shorted starter, they should then be replaced.  
The PO wired around these switches, for reasons I am not sure of, but I 
went back to them and they work fine.  Probably he had some bad cable 
end terminations with high resistance and blamed the switches.  It is 
very hard to find these really high amperage switches, as most of the 
ones you find only handle up to about 200 amps and even my Perkins 
T6-354 can exceed this on a cold day.

Ken Bloomfield
MTOA 2062
AGLCA 3529
M/V Tellico Lady

Bob/Myrna Siegel wrote:
> In the latest MTOA magazine, Bob Smith advises running on either battery
> rather than both.  He then notes that often even if the selector switch is
> "Off" one can start the engine.  In my case this has been proved correct.
> What does this signify?  That is, does it mean that one cannot really
> disconnect both banks even in the "off" position or??  Bob Siegel Annapolis