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.
RDE August 27, 2012 at 10:14 am
Now if you really want a nightmare, start with a certain famous brand Italian motor yacht about 90 ft in length. Being Italian, its AC system is 220v, and being wired by laid-off electricians from Fiat, the circuit breakers are installed on the negative side. Now add some stupid Americans in Florida and ask them to install a 110v stereo. Simple— just grab one of the hot leads from a 220v circuit. Voila, a completely unfused system.
The boat burned so intensively that we finally had to bring in a giant jaws-of-death crane used to crush cars to rip it apart so the fire trucks could extinguish the remains.
Matt Marsh August 27, 2012 at 2:11 pm
I’m not aware of any safety concern with using single-pole instead of double-pole breakers.
But troubleshooting an intermittent ground fault is one of the most irritating, frustrating tasks in the world of electricity. It’s probably not a stretch to suggest that, with a metal boat, spending twice as much on double-pole breakers (simply so that you can properly shut everything off as John described) could pay off in reduced troubleshooting labour time within the first year or two of the boat’s life.
Ron August 28, 2012 at 5:32 pm
John,
I live on a 36 ft steel boat and am I intrigued by your hull short light. I have never heard of it and would like to know how to wire one. Could you help or direct me to a source that can. Love your blog and read it with great interest. Thanks Ron.
John August 29, 2012 at 12:50 pm
Hi Ron,
In its simplest form a hull short light is just a light bulb connected from each side of the main DC buss (negative and positive) to the metal hull through a switch. If the bulb glows when connected to the positive side, then something on the negative side is shorted to the hull, and if it glows when connected to the positive buss, then something on the positive side is shorted to the hull.
Traditionally, a standard incandescent bulb was used, but if you use an LED instead, you will have a much more sensitive device that will detect a slow current leak through a high resistance short. But be careful in that an LED requires so little current that it will glow a bit on a humid day, even if your electrical system is isolated.