Sure most cars have neg earth chassis return .Boats have single point ground so there are no hull currents to cause electrolytic corrosion .My question is about busses .They have been known to have isolated starter motors and isolated alternators and an insulated neg return wiring system.What is the reasoning for this when it clearly increases weight and cost?
Some buses and many heavy trucks have a 12V negative ground system just like many cars but their larger usually diesel engines require more starting power than is common with a 12V system. So many have a 24V or even a 48V starting system which requires that the starter be isolated from the 12V system.
Yes, using 2 or even 4 batteries adds weight and cost but the market for buses and trucks is not the same as for automobiles and a truck or bus that won't start reliably is not all that popular among customers.
The most important reason that vehicles are designed with Insulated Earth Return is so that the chassis is not carrying an electric current at any time.
This is because many vehicles get used in environments where there is a risk of explosion, such as mines or fuel tankers. Even food tankers delivering powders have this risk. If you watch a fuel delivery tanker being set up then you will see a grounding cable being attached.