Question: Do people always use double pole breakers on a floating DC 24 volt system, on the branch circuits? If so why? I understand they are now two ungrounded conductors, but the return path to the batteries is the same. This is for a steel boat.
Answer: No, unfortunately people don’t always use double pole breakers on floating ground DC systems, but they should.
The reason that this is a good idea is that if you get a short to the hull on the negative side of the circuit—the most likely isolation failure because the negative conductor is often connected to the case on DC devices—you will have compromised the floating ground. If there is only a single breaker in the positive side of the circuit, the current (electricity) can be leaking through the hull, even when the circuit is turned off.
But even more important than that, on a boat without double pole breakers, it is hell to trouble shoot a negative short to the hull. The only way to find the offending circuit is to disconnect each circuit from the negative buss until you find the one that has shorted to the hull—no fun, trust me.
On the other hand, if you have double pole breakers, it’s simply a matter of turning everything off and then turning each circuit on until the hull short warning light goes on—much easier.
Having said all that, I have lived with single pole breakers on a metal boat for 20 years, and still managed to maintain a floating DC system. But seeing the hull short light glow red results in Phyllis heading for the tall timber—she knows that the next few hours will not be pretty…or quiet.
The worst short to the hull I ever had to trouble shoot was when the plastic sleeve isolating the VHF antenna at the top of the mast cracked allowing the outside of the VHF cable plug to touch the mast, thereby compromising the entire isolated DC electrical system. Took me a whole day to find that one.
By the way, all of the above applies in any isolated ground DC system, regardless of voltage.