Q & A—Why Double Pole Breakers?

JHH5II-16535

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.

{ 23 comments… add one }

  • RDE August 27, 2012, 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.

    Reply
  • Matt Marsh August 27, 2012, 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.

    Reply
  • Ron August 28, 2012, 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.

    Reply
    • John August 29, 2012, 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 negative 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.

      Reply
  • Matso June 23, 2013, 6:45 am

    Hi all,
    Looking to rewire my Freedom 44 in the near future. Have you or forum members any experience/advise on the CZone system from BEP Marine?http://www.bepmarine.com/home-mainmenu-8/productcategory-190/overview

    Reply
  • John June 23, 2013, 8:44 am

    Hi Matso,

    As it happens, we have a post coming up in the next few weeks on distributed power systems.

    Reply
  • Phil Streat November 15, 2013, 2:22 pm

    I was interested to read of your experience with the grounded VHF antenna. I had the same issue and was surprised to find that my new Icom VHF (purchased in the UK) has its case and RF ground internally connected to DC negative, which seems to contradict their published advice about RF grounding. I was told that some, though not all, of their radios are wired this way. They sell an isolating antenna cable link, which isolates the antenna from the case. Part number OPC1758 in Europe.

    Reply
    • John November 16, 2013, 9:19 am

      Hi Phil,

      I never knew that Icom made an isolator, great tip, thanks.

      Reply
  • SteveA November 17, 2013, 3:55 pm

    Hi John,
    I recall AAC discussion on a device made by VDO I believe, and available in France to monitor this. Has that been imported to North America to your knowledge?
    Maybe a typo on your Aug 29 comment? States one bulb should be connected to positive buss, the other to negative, then further explanation has them both to positive. Or am I confused?

    Reply
    • John November 17, 2013, 5:00 pm

      Hi Steve,
      Oops, typo, fixed now.

      The gadget you speak of is not, as far as I know available in NA. However, all is not lost. I bought one from the French distributor, without problems.

      Reply
  • Norm January 18, 2014, 1:18 am

    John, how many electric shorts per year do you experience? Is the average metal boat owner cruising around out there leaking unknowingly?

    Reply
    • John January 18, 2014, 1:56 pm

      Hi Norm,

      An interesting question. I would guess that over the years I have had to track down an average of about two hull shorts a year. I would estimate that about half were self-inflicted, in that I installed a new piece of gear, or worked on an old one, and did something to short it to the hull. These were the easiest to find, since I knew where to look. The rest were caused by some insulator that wore through, usually as the result of vibration.

      And yes, I would guess that many metal boat owners who think they have isolated ground systems don’t. It takes constant vigilance to keep the system fully isolated.

      The good news is that with some simple gear and a bit of logical thought, and time…sometimes lots of time, it is pretty easy to track down a short, even without double pole breakers. In short (ouch), it’s just another maintenance task like checking the integrity of the bond system for fiberglass boat owners.

      Reply
  • Marty Vanwinkle March 3, 2014, 4:48 pm

    Are there any books that are dedicated to metal boat wiring?

    Reply
    • John March 4, 2014, 11:49 am

      Hi Marty,

      I’m sorry I don’t know of one. I own and like the Boatowner’s Illustrated Electrical Handbook. However I don’t think it has any specific information on floating ground systems for metal boats, although I have not seen the latest edition, so that may have changed. Anyway, said book, would be a good place to start.

      Reply
  • Simon Churchill March 23, 2014, 12:48 am

    I work for a builder of both steel and aluminium boats.
    The simple truth is that electrical systems on metal boats are no different to any other boat, wood GRP whatever.
    What is different is the need to protect the metal hull from electrical faults that could harm it. But with electrics you have one principal concern that should always take precedence over protecting a hull and that is life. This is either through electrocution or fire.
    Fully isolated systems with multi pole MCBs are fine from a fault finding or a fault based point of view and this is where in ship building there use has come from. On a ship safety includes the whole ships safety i.e. if the steering gear fails the whole ship and all those in it could be at peril. Thus it’s the norm for a ship to be wired where no one fault can cause the failure of a critical system. So lets say a steering pump motor cable, single conductor shorts to the isolated hull, what happens? Nothing, the motor runs and no one gets electrocuted etc. But if now another system or another cable in the same system has a fault that causes a short to the hull a system might fail. So one fault will not cause a system to fail. In this case we are normally talking about larger craft and motors powered by three phase generators. Ships rely on electrics, it just is not practical to have a hydraulic steering system.
    But what most don’t understand is on that same ship there will be 240V or 110V sockets used by crew and passengers in cabins or galleys etc. These voltages are much lower than the ones used by the ships main motors and will be transformed down, at this point a neutral potential is created by connecting intentionally one of the transformers output poles to the hull. This means in case of a power socket one pole is live and the other is at the same potential as the hull i.e. neutral. This now means if a metal electric toaster has a fault were the live touches its case it will blow the fuse. Just think what might happen if the toaster was sitting on a stainless galley counter and live, when someone touches it while being in contact with the earthed counter top? At the moment we are talking AC voltage and I’m not going to go deeper here but basically a safe AC system requires a neutral which means a solid connection to earth. Here in UK this is important as we always have a earthed neutral and all appliances come with a plug with a fuse only in the live conductor.
    Therefore as a result of all the above when we wire a 240V single phase or 400V three phase generator we will always create a solid neutral earth connection to the hull, for safety. After this there is only a need to fit a fuse or a breaker in unearthed poles thus on single phase circuits only the live phase requires a breaker, though as others have said a two pole breaker is better for fault finding but that’s all in this case.
    Now with DC we also follow the grounding of the negative wire rule. This allows for single pole fusing and thus we can use the likes of Empirbus which only protects the positive pole.
    What is important is that if your going to go the fully insulated route is you have a very good monitoring system that tells you that you have a fault, both on AC and DC systems. In my experience people just don’t understand earth lamps or just don’t maintain them. I have rarely seen insulation monitoring systems on yachts or workboats either. What also happens is that with a double pole fully insulated system is if you get a positive fault in a high current circuit to the hull it can have a devastating effect on equipment that for one reason or another has been fitted without a fuse in the negative pole. One item that is high in this list is your engine harness. Very few engines are truly double pole fused and I have seen burnt out engine harnesses a few times because the engine harness negative is not isolated from the hull when a positive fault occurs. If you have a solid ground to the hull this will cause the live fault to fuse and thus it can not exist.
    I think both AC and DC neutrals and negative poles should be earthed to the hull for safety. Check wiring often and fit a Electroguard product like this perhaps? http://www.boatcorrosion.com
    Safety comes first and I have yet to see any boat built by us damaged because of the principles we chose.
    Please note this is just a quick overview of my thoughts and there is more than one way to skin a cat but there are some odd and misconstrued interpretations on this subject out there. But I would firstly stick to the relevant rules i.e. E-11 in US or BMEA in UK for instance, the fact that the boat is metal does not change the rules. This in it self may explain why you might find books for metal boat electrics, because there is no difference at all. All the rules or method is about the electrics not the material of the craft.

    Reply
    • John March 23, 2014, 8:51 am

      Hi Simon,

      Just to clarify I was not suggesting and have never suggested that the AC system should be floating. In fact I banned a person from commenting for suggesting that an AC system should anything other that grounded (earthed) to proper code.

      Having said that, it is perfectly possible and safe to have an AC system that is installed to code and properly grounded (we used ABYC code) and still have a DC low voltage system that is floating. I know this because we have exactly that on our boat and have done so for 20 years. You do need an isolation transformer, but then every metal boat should have one of those anyway.

      The key, as you point out, is to properly and regularly monitor that the system is in fact isolated. This easy to do with the right leak meter.

      By the way, we don’t recommend impressed current systems, like the one you link to, due to their complexity and potential to damage the boat if they go wrong. These systems might be fine on a commercial vessel, but our opinion is that they are overkill on a small (relatively) yacht and that an isolation ground system is the simplest and best solution given its long term track record of preventing stray current damage.

      Reply
      • Philip Streat June 1, 2014, 11:59 am

        Hi John,

        Grounded AC and floating DC seems to be a common arrangement on aluminum boats. I think it is the conventional wisdom among the French builders from what I have read and is the way my own boat is wired. However, Nigel Calder is adamant that the DC system should also be grounded and this is required by ABYC when AC circuits are on board. I have been sitting on the fence on this one by I think I am going to ground my DC negative.

        Reply
        • John June 1, 2014, 2:17 pm

          Hi Philip,

          That’s fine. Many boats are done that way.

          Just realize that if you develop a high, or even medium resistance short to the hull you will have no way of knowing that and no way to trouble shoot it. (A full short will blow a breaker). The first indication will probably be fast zinc wastage. If the zinc go and you don’t notice it, you will start eating boat, so it will be really important to monitor the zincs, I would say at least once a week.

          Further, at least when I wired my boat to ABYC, there was no problem with grounding the AC, but not the DC.

          Finally, no offence to Nigel, but as far as I know, he has never owned an aluminium boat, or even one of any metal.

          Reply
        • John June 1, 2014, 3:00 pm

          Hi Philip,

          Rethink. I just had a shower and hot water hitting my head helps me think. And no, I don’t want to talk about what that might indicate!

          Anyway, I still think a floating DC system is the way to go because of the issue with high resistance leaks from positive to hull. Having said that, there is, I think, a way that you could test for leaks with a grounded negative system. And that is with a sensitive volt meter and a silver/silver anode. (Every aluminium boat owner should have one anyway.)

          The procedure would be to disconnect every battery on the boat at the positive. Take a reading between anode in the water and hull. Then connect each battery back and then turn on each circuit, all the while monitoring for a jump on the meter. Get a jump…got a problem.

          I would recommend once a week since a medium resistance short could eat your zincs that quickly.

          Hope that helps.

          Reply
          • Philip Streat June 1, 2014, 6:19 pm

            Hi John,

            I also had a hot shower and I think I am back to sitting on the fence again. I thought I could make the DC to ground connection easily removable or even switched and then use the hull short test light but the risk remains between checks. Continuous current monitoring on the hull ground connection might do it but how reliable would that system be? My Garcia has one tiny and ineffective hull zinc, which is apparently the way they built them, so I have no protection. I plan to add hull zincs.

          • John June 1, 2014, 7:05 pm

            Hi Phillip,

            Hot water will do that to you. Still think that floating is way easier and no downside. Seriously, make sure you actually look at the ABYC regs, not just let someone tell you how it should be. I went through exactly that when I did mine: Everyone telling me this and that, and then I actually got the book and all that stuff I was told all turned out to be…how should I put this…crap!

            Also, be careful adding zinc. Just because some zinc is good, more is not always better, make sure you use a meter to determine the right amount. I saw an aluminium boat the other day from a “reputable” builder with enough zinc on her to make her fizz! Blowing the bottom paint right off her.

            Bottom, line, a lot of people in boat yards, particularly in north America, who should at least know what the don’t know about electricity, shoot their mouthes off anyway.

  • A2B2Sea May 31, 2014, 6:46 pm

    Hi John,

    I’m interested in installing some kind of leak monitoring system for our aluminium sailing yacht. Thanks for the link to the French site. Do you recall if the device came with English instructions? I don’t speak French and am a bit of an electrical luddite, so I’ll need all the instructions I can get!

    Having upgraded from your old “warning bulb” system, do you consider it worth the additional expense?

    Regards,

    Bryce.

    Reply
    • John June 1, 2014, 6:38 am

      HI Bryce,

      Sorry, I don’t remember, and I’m not on the boat right now, so can’t check.

      However, I wouldn’t worry about it too much. There are only three connections to make: positive, negative, and to the hull. All are marked on the case.

      The only slightly confusing thing is that the unit only works for about 20 seconds after power is supplied to it. So if you want to run a check, you need to cycle the power. The reason is that by being designed that way, the unit is not acting as a current leak in and of itself.

      Reply

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