Thank you for your quick replies
Yes Tony I see your your point of the danger of running the alternator into an open circuit should the fuse fail.
Yes Loddon the isolation switch dillema I think still remains, Am I right in thinking BSS requires the alternator charging cable to connect to the cabin side of the isolation switch ?
So if the cabin battery switch is OFF and the engine is started , there is a possibility the alternator can put its full charging volts down the 12 volt cabin supply !!