High Voltage DC Disconnects

Hi there,

Just in the process of building another off-grid solar system and am currently planning the DC wiring/ breaker/ fuses.

I will be using 18 - 350 watt panels wired in 2 strings, combining in a MNPV3 using high voltage, finger safe fuses.

My concern is how to isolate incoming DC power when and if it is sunny and system is under load. Obviously this is why "rapid-disconnect" is on the market........ Especially since the SCP is discontinued and now would have to log in to the insight to put things in standby....

I was hoping to simply use an appropriate DC rated disconnect, as a point of isolation before the PV wire enters the PDP and into the 100A 600V Charge controller, and and as such am reviewing the options listed in the link below.


Would it be better to combine the two incoming strings first, and then run the one wire through one of the poles on a DC disconnect or run both string wires through the disco first and then combine? Or does it matter. The finger safe fuses have no open contact once the fuse holder is opened, however I would not think it would be wise to open the fuse holder while under load.......

Thanks for any insight or direction to any who may respond.........



Comments

  • PhotowhitPhotowhit Solar Expert Posts: 5,974 ✭✭✭✭✭
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • PhotowhitPhotowhit Solar Expert Posts: 5,974 ✭✭✭✭✭
    Fuses, regardless of the container, shouldn't be used as switches under load.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,296 ✭✭✭✭✭
    It is against code to run 600 vdc wiring in the PDP. It is only rated for 150vdc.
    "we go where power lines don't" Sierra Nevada mountain area
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • Dave AngeliniDave Angelini Solar Expert Posts: 6,296 ✭✭✭✭✭
    2 strings do not require a fuse. Join them at the array
    "we go where power lines don't" Sierra Nevada mountain area
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • ligwydligwyd Registered Users Posts: 191 ✭✭
    Hi Photowhit and Dave and thanks for your response.

    Photowhit. Would you suggest installing two din rail mount 600VDC breakers in midnite solar outdoor rated box or the like, providing both protection and point of disconnect?

    Also, I would not pull a fuse under load. Only was wondering if it would be a good idea to install an IMO disco before the fuses so that it could be isolated under load or not?

    Dave. I will study the single line for the MPPT 100A 600V, but I'll ask here too. Do the high voltage PV wires then come in first directly to the Charge Controller?

    Also, is it not good practice to have each string individually protected by breaker or fuse before combining?

    Thanks guys for the feedback.



  • bgcanibbgcanib Registered Users, Users Awaiting Email Confirmation Posts: 2
    It's always a good practice to protect each string individually before combining, at least with appropriated fuse, on each pole. In your PV panels' datasheet they recommend the value of the series connected fuse. Never open the fuse holder under load!

    Even better option is using breakers on each string.

    If you want to disconnect PV strings from the Charge controller under load you should absolutely use appropriate DC disconnect switch. Even if you put a DC switch, string fuses should be installed.
  • ligwydligwyd Registered Users Posts: 191 ✭✭
    Agreed. Midnite Solar has been waiting for high voltage breakers for some time now, but are in the works,....supposedly.
    I'll be using a dc Isolator:
    and then MNVP3 combiner with finger safe fuses.

    When the appropriate breakers are avail, this would achieve the same end with one product. Until then, this isolator and then a separate combiner seem to be the best path forward for now.
    Thanks for the feedback :)

  • BB.BB. Super Moderators, Administrators Posts: 32,921 admin
    Ligwyd,

    That is a good find. A high voltage DC switch (up to 1,000 VDC):

    https://www.solar-electric.com/lib/wind-sun/imo-si-range-brochure.pdf

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ligwydligwyd Registered Users Posts: 191 ✭✭
    I like the outdoor rated isolator since I'll put that at the Array first and then through the combiner. The switch would be fine for an inside 600v rated enclosure but since the isolator comes in an enclosure already I'll go with that, unless there is a better solution.......... Sure is nice to find the right stuff is out there. Just a little looking and learning......... Thanks Bill
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,296 ✭✭✭✭✭
    Just be sure and check the listing if that is important on those plastic switches. Several of my installer friends have had to go back to the industry standard unit by Schneider square D. They are in the store here and on sale. You get 3 poles and can add fuses if you want. As I said 2 strings do not need to be fused and by adding them you do get a new failure mode that is not required. Easy to troubleshoot these and they are the standard since high voltage solar began.

    https://www.solar-electric.com/sqd600vodcar.html


    "we go where power lines don't" Sierra Nevada mountain area
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • ligwydligwyd Registered Users Posts: 191 ✭✭
    edited 6:14AM #12
    Thanks Dave.

    At what point are strings required to be fused? Also if strings are ever required to have fuses, whats the harm of putting a fuse in place even if only using two stings? How often is a fuse found to have be a failure point and if so isn't that a good thing, to have a fuse fail as opposed to a wire melted or fire caused?

    I know there will be reasons why two strings don't need a fuse, but I guess I like taking all precautions, even if they are not required.....

    Also re: the DC Isolator/ disconnect  choices: Do you recall why some of the installers you know have had to go back to the Square D? The vid I watched on IMO islolators vs. conventional disconnects (granted it was put on by IMO) seem to highlight that the IMO is a superior choice. Does a better job and for less $$$?


    I guess an advantage of the Square D is that does both disconnect and fuse, if using one set of wires (combined). However, if I want each string fused separately, then I am back at a separate disconnect and then the finger safe fuses in the MNVP3 again........ (that is until these high voltage DC breakers are actually avail in the near future.......)


    Appreciate any feedback you are able share :)
    Thanks for the dialogue.



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