[Elecraft] [K3] Why fuse the negative lead from a battery?

[Dave, G4AON]

With car electrics (and a boat may be similar), the reason is that if 
the engine grounding strap fails (or is removed and not replaced), the 
starter motor can draw current via the radio negative lead which leads 
to damage and possibly fire.

73 Dave, G4AON
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 >I wonder why the manual recommends that the negative lead should be
 >fused? Can anyone explain this for me?