The reason the cut-off is preferred on the ground side is that in a crash, a
battery might come loose and be suspended by its cables. Its possible that
the positive terminal could come into contact with ground and short, running
the risk of starting a fire. With the entire body grounded, any damage that
gets to and exposes a hot lead in the wiring harness also creates this
possibility.
By disconnecting the battery from ground (i.e. the entire chassis), this risk
is minimized. If you disconnect only the positive, then the battery could
potentially still contact ground and short.
The issue with the alternator has to do with the fact that while the engine
is running, the alternator is putting out electricity, even with the battery
disconnected. There's the chance that even if you disconnect the battery,
the engine (ignition and fuel pumps) might continue to run even if the
battery is switched off by power provided from the alternator. Thus, a
positive disconnect from the alternator is also recommended (and/or required
by some groups I suppose). So, a cut-off switch with an alternator terminal,
or an alternator cut-off relay fed by the main cut-off switch is a good idea
for added safety. Then, cutting off the external cut-off switch is sure to
kill power to the engine without having to reach the ignition switch itself.
Regards,
Myles H. Kitchen
1965 Lotus Cortina Mk1 #128
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