If you ever got off a car or airplane that has had a static load build, you'd appreciate the desire to ground vehicles. (rare these days because so much carbon is in the tire rubber.) If God forbid you ever have a basket dropped from a helicopter the LAST thing you want to do is touch it before it touches the water, because the discharge can knock you flat. We really don't want the vehicle to have a charge different than what it comes in contact with, yet are running aluminum and stainless rigging through the air mounted on plastic boats. Having static occasionally discharge itself can also really do a number on radios and other electronics. Another reason is that boats may receive energy from lightning or other sources without even being directly hit and we want the easiest way for that to move to ground, especially if there are possible fuel vapors. A professor specializing in static electricity told me during the TWA 800 investigation that if you have a flammable vapor, nature will find you an ignition source.I'm comfortable with those concepts and agree. But why then, aren't our DC systems a "closed loop" between the batteries and the gadgets/load that use power? Couldn't we only have the the AC system grounded to the water via the engine block? The shore power battery charger would be the only AC/DC crossover point.
It's all again part of trying to not have a difference in potential between different grounded parts or wires in a vehicle. Remember that even 1/0 battery and ground cables have a unique resistance in each piece. If you have two lengths of 16 AWG both hooked to the same ground, there will be a temporary transient charge difference. For example if one is to a nav station incandescent bulb and one to an incandescent bulb at the top of the mast, flipping the mast light switch off could flicker the other bulb. Or create a pop sound from a radio speaker.The Radio Frequency Isolation (RF drain wire) issue remains mysterious to me.
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I find my black DC ground wires connected to a common post connection, and then I run my new green "RF drain earth ground" wires to the same common connection, isn't this just redundancy? Maybe that's the whole point.
Some electronics manufacturers do put more emphasis on preventing ground loops than others. My own guess is that the ones with more emphasis are going to be the ones which have experience in more of the larger or more complicated systems, such as commercial use, while the lesser have more of a consumer/simple background or focus.