Originally Posted by dfelsent
Sorry but I have to disagree here. You may have a bonding system separate to a 12v DC system, but you will not have a DC ground in any shape or form. There is in effect, no such thing as a DC ground or it is very rare in boat systems and would require a three wire DC system similar to an AC one to be implemented.
Why is there is no such thing as a DC ground? It is incorrect terminology and at the risk of great flaming here is why:
When you look at a simple DC circuit you have a wire leaving the positive terminal of a battery going to (through) a load then returning to the negative terminal of the battery. That is, you have a complete path from positive to negative and no ground circuit involved.
circuits AC or DC have to leave the source and return to the source to make it work
. Yes your negative return wire will have zero volts (because the load used it all up - this is true of all complete circuits BTW).
It is exactly the same in a boat. When you refer to grounding a load on a boat you are simply connecting it back to the Negative battery terminal. Prove it to yourself simply by "grounding your load" as you refer to it, then disconnect the negative battery terminal and check if your load still works. Of course it doesn't because there is no return path to the battery.
You don't have a DC grounding system on a boat - you have a complex path back to the battery negative terminal.
Completely different to an AC circuit which has a hot and neutral wire (analagous to positive and negative DC) and you have a ground wire for safety
. Which conducts a shorted neutral wire back to earth (ground, same thing).
One thing you should NEVER do is connect anything related to your DC circuits to your bonding system. Otherwise you are creating a path for electrolysis
You are correct when you say you need to connect the DC case "ground" wire back to the boat, but it will go to the DC negative circuit. There is no alternative.