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Choosing fuse holders for PV combiner box

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  • Choosing fuse holders for PV combiner box

    Why would you choose to use a 1-pole fuse holder for each string as opposed to a 2, 3, or 4-pole?
    So if I have 4 strings (each are 2 in series) coming into the box, what would be the difference in sending each string through a single-pole fuse holder as opposed to a 4-pole fuse holder?

  • #2
    I would NOT use fuseholders in a combiner box. Why ? They cannot be opened under load, as when the sun is up. Their contacts will arc and burn when opened under load. When the fuse blows, the fuse opens the circuit and disconnects the load. Almost all modern fuse holders are "Touch Safe" meaning they won't shock you, but that's not the same as opening up under load.

    Part of your troubleshoot routine will be to compare each panel's output to the others. Easily done with circuit breakers, but not so easy with fuses. And circuit breakers are about the same cost as fuse holders, fuses, and some spare fuses - you aren't going to find 150VDC fuses at home depot
    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

    Comment


    • #3
      Ok, thanks. I'll go with circuit breakers. So would I use a single pole breaker for each string (positive lead) or a double-pole for each string (pos + neg leads)?

      Does this sound right?:
      1. From the breakers --> to the positive and negative bus bars. (Or neg leads straight to bus bars if using 1P breakers
      2. From the positive & negative bars --> to a surge protection device --> which connects to ground
      3. From the positive & negative bars --> to a main breaker /shut off --> out to the charge controller

      Am I missing anything?

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      • #4
        Hmm, I thought at least for 400VDC string systems, specific fuses and touch safe fuse holders
        were the norm in the combiner box. Whatever is used, I would not open their circuit under load.
        Rather if I must, the inverter AC output is turned off to stop current flow, before opening a DC circuit.

        There is also a DC disconnect on the combiner output to the inverter, which I would not open
        under load (except in an emergency). From experience, I do recommend it break both the +
        and - solar leads.

        Bruce Roe

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        • #5
          Yeah, I kinda prefer fuses just in case I decide to reconfigure the panels in some way in the future, I can just replace fuse with a different rating. Circuit breakers are nice not having to replace fuses, but how often would I expect this thing to trip anyways? Now I'm unsure again. Can anyone add to what might be needed for the PV combiner box? I also still don't understand the use of multipole fuse holders compared to the singles.

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          • #6
            If you had 3 or more panels (or strings of panels) wired in parallel, a short in one would result in
            the short circuit current of the others adding up and flowing over to the fault. This current is
            more than panel wiring is designed for, but it can be limited by having a fuse on each panel.

            I have never blown a combiner fuse, but they can also be useful in trouble shooting. Bruce Roe

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            • #7
              Is this for a grid tie system or an off grid battery charger ? I saw "2 panels in series" which I assumed a vow voltage system.
              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

              Comment

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