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Do I need double-pole breakers? If so, why?

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  • Do I need double-pole breakers? If so, why?

    So for some year now, I've had all my parallel panels on my van, protected against a short circuit dump by running the positive of each through their own 10A breaker, just before the controller

    Today, I had someone inform me however, that this is a No No - all floating panels (ie negative not grounded) in strings of 3 or more must have Double Pole breakers (ie on + and - ).

    When I pressed him on why, got the usual "Er....um...safety...best practice...regulations say so", but nothing in terms of a technical explanation as to WHY you need both lines cut?

    I've been hunting all day and can't find anything conclusive, jut a lot of equally vague references about how this is the done thing with 'ungrounded' panels.

    Can anyone explain the actual hazard mechanism behind having only a single breaker string, on the positive line?

    Thanks!

  • #2
    I work in industrial electronics and machine building. The Japanese were the first to put circuit breakers on the positive and the negative and it is slowly becoming the standard. The thought is even though the breaker being on the positive trips there is still a chance that the negative could be caring current and be a potential for fire issues. Thus 99% of the time not needed but that 1% is what kills you....

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    • #3
      Originally posted by NewBostonConst View Post
      I work in industrial electronics and machine building. The Japanese were the first to put circuit breakers on the positive and the negative and it is slowly becoming the standard. The thought is even though the breaker being on the positive trips there is still a chance that the negative could be caring current and be a potential for fire issues. Thus 99% of the time not needed but that 1% is what kills you....
      Thanks, but this is exactly what's confusing me.

      The explanations always seem to be some vague variation on 'the negative might do something' but I dont understand HOW?

      The circuit is broken. No current can flow, even if the panel -ive voltage is floating way above you, the ground, the vehicle chassis...right?


      I must be missing something?

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      • #4
        It is a nice to have thing....I don't thing the major auto makers have bought into it because of cost.

        But, I would bet that electric cars have breakers on both the neg and pos..... with solar and batteries you have allot of power.....might be wise.

        They are thinking in a wreck wires could be cut/shorted together sending power to the say gas tank through the negative.

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        • #5
          Originally posted by VanHausen View Post

          Thanks, but this is exactly what's confusing me.

          The explanations always seem to be some vague variation on 'the negative might do something' but I dont understand HOW?

          The circuit is broken. No current can flow, even if the panel -ive voltage is floating way above you, the ground, the vehicle chassis...right?


          I must be missing something?
          If the negative side of the pv system in not grounded then you run the risk of "shorting" the negative to ground which is as bad as a shorted positive to ground.

          So using an over current device (breaker) on both the positive and negative side of an "un-grounded system" is not only smart but safer.

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          • #6
            Why are you using any fuses? Not required if you limit to to two parallel strings.

            However if you have a Floating System and use over current protection, then YES you must FUSE Both Polarities.

            If one polarity goes to ground, nothing happens, the system becomes Grounded, If the other polarity then has a fault to ground, both fuses operate. Otherwise no fuse would operate.

            Now having said al that, in a vehicle, all equiupment is Grounded to the Chassis including battery which means your panels wil be grounded (negative polarity to chassis so the fuses can operate). Hoiwever none are required between panels and controller if you keep to 1 or 2 parallel strings which is no problem in a vehiclee unless you do not know what you are doing. Sola rPanels are current sources, not voltage like a battery with unlimited fault current. As olar panel has a spec called Isc aka current short circuit. On a 100 wat panel shorted out is 6 amps, not enopugh to even make a 14 awg wire warm. In fact 2 panels in parallel are 12 amsp and not enough to heat up a 14 AWG wire. That is why no fuses are required or needed. They just cost money and something else to fail.

            MSEE, PE

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            • #7
              Originally posted by Sunking View Post
              Why are you using any fuses? Not required if you limit to to two parallel strings. "

              However if you have a Floating System and use over current protection, then YES you must FUSE Both Polarities.

              If one polarity goes to ground, nothing happens, the system becomes Grounded, If the other polarity then has a fault to ground, both fuses operate. Otherwise no fuse would operate.

              Now having said al that, in a vehicle, all equiupment is Grounded to the Chassis including battery which means your panels wil be grounded (negative polarity to chassis so the fuses can operate). Hoiwever none are required between panels and controller if you keep to 1 or 2 parallel strings which is no problem in a vehiclee unless you do not know what you are doing. Sola rPanels are current sources, not voltage like a battery with unlimited fault current. As olar panel has a spec called Isc aka current short circuit. On a 100 wat panel shorted out is 6 amps, not enopugh to even make a 14 awg wire warm. In fact 2 panels in parallel are 12 amsp and not enough to heat up a 14 AWG wire. That is why no fuses are required or needed. They just cost money and something else to fail.

              Yes, we have more than two parallel strings. The system was built using salvaged and 2nd-hand components at extremely low cost (in some cases free). Consequently we made it work with what we had, which meant some non-ideal design aspects.

              I'm quite aware the low current of 100W panels makes fuses almost entirely unnecessary in this specific scenario, my question however is more about the general theory than this specific build, for the purposes of future understanding.

              Regarding this line: "in a vehicle, all equiupment is Grounded to the Chassis including battery which means your panels wil be grounded (negative polarity to chassis so the fuses can operate".

              If the panels are simply running both leads directly to the controller, with no other connection to the vehicle body, would they not be considered floating?

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              • #8
                Originally posted by SunEagle View Post

                If the negative side of the pv system in not grounded then you run the risk of "shorting" the negative to ground which is as bad as a shorted positive to ground.

                So using an over current device (breaker) on both the positive and negative side of an "un-grounded system" is not only smart but safer.
                Thankyou for the reply. However this is the part I'm struggling to understand.

                If I have a floating system with only a breaker right up at the positive terminal, and it shorts to "ground"....nothing should happen, right? There's no circuit there, it was a floating supply after all. The ground is not 'hot', but it's also still downstream of the positive fuse

                If I then get a second short, from negative to ground, then there's a dead short ....and the positive breaker will trip anyway. So whats the need for e breaker on the negative?



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                • #9
                  Originally posted by VanHausen View Post
                  If I have a floating system with only a breaker right up at the positive terminal, and it shorts to "ground"....nothing should happen, right? There's no circuit there, it was a floating supply after all. The ground is not 'hot', but it's also still downstream of the positive fuse
                  In your case you do not need two breakers, because you do not have a floating system.

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                  • #10
                    Originally posted by jflorey2 View Post
                    In your case you do not need two breakers, because you do not have a floating system.
                    I don't? None of the panels are connected to ground at any point. Is this not floating?

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                    • #11
                      Originally posted by VanHausen View Post
                      Yes, we have more than two parallel strings.
                      Why would you do that? That means a lot more expense, wire, labor, and more things that can go wrong. Wire them in series of two strings or less and all that goes away.

                      Originally posted by VanHausen View Post
                      If the panels are simply running both leads directly to the controller, with no other connection to the vehicle body, would they not be considered floating?
                      Because every piece of equipment including the batteries have the negative polarity terminal bonded to the chassis and there is no way for you to prevent that.

                      MSEE, PE

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                      • #12
                        Originally posted by VanHausen View Post

                        I don't? None of the panels are connected to ground at any point. Is this not floating?
                        A BIG FAT NO!

                        A Grounded System has nothing to do with the panels, or the panels wires. A Grounded System means one of the Source Polarities is BONDED to DIRT or something in place of dirt like a Vehicle Frame/Chassis or Ham Sandwich. In an Automotive application that would be the Battery Negative Term Post is BONDED to the vehicle frame making it a GROUNDED SYSTEM.

                        Here is a diagram detailing both a Grounded System, and Floating System. Completely different.



                        Here is an great example of a Van or RV

                        Last edited by Sunking; 03-27-2019, 01:13 PM.
                        MSEE, PE

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                        • #13
                          I think I understand the OP's confusion, if so it's also something I'm not sure about: In the charge controller, is the panel negative input bonded to the battery negative terminal and also the ground terminal?

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                          • #14
                            Originally posted by VanHausen View Post
                            I don't? None of the panels are connected to ground at any point. Is this not floating?
                            Power down the system and measure resistance between battery - and chassis ground. If it is zero, it's not a floating system.

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