[Alberg30] Floating Ground on Diesel Engines

[Brian and Elaine Timmins]

Can anyone explain to me why my Diesel (Perkins 4-108) has a floating ground?  I am just about to start to rewire the controls and instrumentation, and realised that all of the senders, the starter circuit, & the charging circuit all have additional wires creating a Floating Ground. 
Seems odd since there is a connection of the ground circuit to the engine anyway. The very oddest is the ground lug on the starter has a jumper to ground to the engine on a mounting bolt for the starter! What's up with this?
Thanks,
Brian
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