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  • Fused disconects on panel outputs

    I have been installing my solar panels with fused disconnects on their outputs. Now I understand they are current sources. Those fuses are stupid yes? Direct short or load current is the same. They can never blow or provide safety? And that means...dare i say it... That chassis ground the foundation of electrical safety is useless on PV panel outputs enclosures too?
    Last edited by 123easy; 01-10-2021, 11:42 PM.

  • #2
    Sometimes fuses are necessary, for example if three of more panels (or series strings of panels) are connected in parallel, each panel (or series string) needs to be fused since if one panel develops an internal short, the others can supply more current through the shorted panel than that panel's wiring is designed to carry. See NEC 690.9 (A), exceptions 1 and 2.

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    • #3
      Originally posted by 123easy View Post
      And that means...dare i say it... That chassis ground the foundation of
      electrical safety is useless on PV panel outputs enclosures too?
      Usually the chassis ground is NOT CONNECTED to the panel power wiring. If a panel or its wiring
      shorted to ground, it might connect dangerous voltages to the metal frame. But a safety metal
      ground shorts out any such fault, possibly blowing fuses or setting off ground fault detectors.
      Bruce Roe

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      • #4
        Originally posted by bcroe View Post

        Usually the chassis ground is NOT CONNECTED to the panel power wiring. If a panel or its wiring
        shorted to ground, it might connect dangerous voltages to the metal frame. But a safety metal
        ground shorts out any such fault, possibly blowing fuses or setting off ground fault detectors.
        Bruce Roe
        Thank you for the relies and particularily the NEMA reference . Im somewhat familar with the diference between a "ground system" and a chassis ground". I would say I am a bit of a fanatic about electrical safety and proper chassis grounding in particular.

        With a voltage source if power makes it to a chassis grounded enclosure current eill exceed breakers or fuse rating. With a current source current can not exceed that rating. (except in sbetween paralel and series striings as noted above)h No way for the fuses to blow.

        The standard voltage source safety mechanism of a breaker tripping and creating a open from the power supply should voltage find its way to a metallic enclosure can not occur with a current source.

        To my mind that leaves a bit of a vacuum in terms of best practices. My paradigm is upset .

        So lets go a step further. If the Systems are not "grounded" ie negative terminal power source is not tied to ground the only current path is from terminal to terminal making the chane of a human becoming a short to ground improbable. Which is why to qoute sunking "grounded systems are dangerous".

        So why do we tie ground to neutral in the USA with AC if thats true? Is it to establish references for the grid since the earth is the only common potential thus chassis grounding?

        I would seem to me the risk of shock would be neglible from these DC systems unless you manged to put some portion of your body as a load in between the plus and minus. I have no doubt the inspectors will want to see OCPD and chassis grounding but unless im missing somthing there is no possible way those standard practices will provide safety like they do in a voltage supply system.

        On the other end of the charge controller we have voltage supply but is there current path there on chassis grounded enclosures? Obviously a short across the batteries will blow the battery disconect fuses but will a voltage short to a chassis grounded enclosure? I say no no current path. No current patj but also no current path between a human to ground.

        So even on the voltage supply end chassics grounding of disconnects has no real safety benefit?

        Is that why you can work on 48 v safely? How about panels in string for 200+ volts? If you dont become the short is working with these power sources safe?

        We all know that if you grab the black wire coming off the grid or a inverter tour not going to be happy. And we all know that you can grab the hot terminal of a 48v battery bank and not get shocked if another part of your body is not contacting the negative terminal.

        I would appreciate any insight and best practice information in regards to safety on both the current source and voltage source ends of these off grid dc systems.

        I am in no way mentioning these things out of laziness or attempts to bypass electrical safety. It would seem that the things that keep us safe in the grid world do not do so in the PV world and i believe understanding things is paramount to safety.
        Last edited by 123easy; 01-11-2021, 02:01 PM.

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        • #5
          Main purpose of a safety ground is to assure any faults in system wiring WILL NOT elevate the metalwork
          to a significant voltage. It is NOT necessarily to assure a fuse will blow. Here the usual 6 gauge ground
          can absorb up to 60A indefinitely without fail, allowing only flashlight battery voltages on the properly grounded
          metalwork. Any fault large enough to threaten 6 ga will certainly blow a protection device in our residential
          service.

          Circuits of significant power and size are NOT ALLOWED to float at an undetermined voltage. In theory a
          100V system could be completely floating at 10,000V from ground. This may allow birds to safely land on
          power lines, but our systems have means to limit such offset voltages. My panel wiring can operate with
          a differential of 420VDC, but the neg terminal it tied to GROUND by a 1A fuse. That fuse blowing will
          immediately result in equipment shutdown and report of a ground fault. But externally the system is always
          made safe by that 6 ga ground wire.

          Tying the neutral AT ONE POINT to the area ground prevents any floating voltages to occur. They are not
          tied at multiple points, because then load return current would be splitting between the 2, ground is NOT
          for carrying regular load currents.

          While grounds and GFIs try to protect us from undiscovered equipment faults, they do little for those
          who open the doors and get inside.

          I suspect 48V was chosen by my company (before I was born) because it was about the highest voltage
          that would not present a significant threat to human life. It can still be dangerous. The phone co has
          mostly been grandfathered to setting their own rules in house, but better safety regulations have been
          developed for the rest of us. Bruce Roe

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          • #6
            Originally posted by 123easy View Post
            So why do we tie ground to neutral in the USA with AC if thats true? Is it to establish references for the grid since the earth is the only common potential thus chassis grounding?
            Yes, but also to let the breakers or fuses clear faults. A short to ground alone wouldn't trip a breaker, it needs to find a way back to neutral to complete the circuit.

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            • #7
              Originally posted by 123easy View Post

              The standard voltage source safety mechanism of a breaker tripping and creating a open from the power supply should voltage find its way to a metallic enclosure can not occur with a current source.

              .
              Once the current delivered by a Current Source, exceeds the trip threshold of a breaker or fuse, it will trip or blow. 12A should trip a 8A device.

              Voltage rating is only so the protection device does not become an arc lamp. ( AC & DC devices differ in ratings )

              Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
              || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
              || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

              solar: http://tinyurl.com/LMR-Solar
              gen: http://tinyurl.com/LMR-Lister

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              • #8
                Originally posted by Mike90250 View Post

                Once the current delivered by a Current Source, exceeds the trip threshold of a breaker or fuse, it will trip or blow. 12A should trip a 8A device.

                Voltage rating is only so the protection device does not become an arc lamp. ( AC & DC devices differ in ratings )
                Most circuit breakers have 2 ways that make them trip. The first is a short circuit (fault to ground) and the second is thermal due to too many amps going through it and heating it up.

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