Quick breaker question... oh, and a grounding question.

TucsonAZTucsonAZ Solar Expert Posts: 136 ✭✭
So, assuming I have this correct, DC breakers work slightly differently than AC due to the arc issues, hence the positive markings on the bottom of the breaker. So in my combiner box (in an RV, no code to deal with so it's mounted inside next to the cc), I have the three PV strings going into the bottom of three 15a breakers, out the top via the bus bar and off to the charge controller. However, now that I have a better understanding of how the system works, should I instead go from the busbar into the top of a 50a breaker with the bottom (or positive) of that going to the charge controller or is that not really needed?

Also, since I'm already in here and I'm sure this is an easy one. In a mobile application where there is no earth grounding and even my lights are all chassis ground. Do I just run everything to the chassis or a ground bar tied to the chassis?

Thanks a lot!

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Quick breaker question... oh, and a grounding question.
    TucsonAZ wrote: »
    However, now that I have a better understanding of how the system works, should I instead go from the busbar into the top of a 50a breaker with the bottom (or positive) of that going to the charge controller or is that not really needed?

    It is a good idea to have a PV disconnect, but is not necessary for safety. Most controllers require that the battery be connected before the PV. When turning off the controller you must disconnect the PV before disconnecting the battery. A PV disconnect makes that easy, but since your combiner is right next to your controller you can disconnect your PV by just flipping the three string breakers.

    Regarding the ground... yes, the chassis can be your ground. There may be some issues if you are connected to shore power... most shore power has the neutral bonded to ground (as it should), but if you also have a neutral-ground bond you can have problems because there should only be one neutral-ground bond in a system.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • TucsonAZTucsonAZ Solar Expert Posts: 136 ✭✭
    Re: Quick breaker question... oh, and a grounding question.
    vtmaps wrote: »
    It is a good idea to have a PV disconnect, but is not necessary for safety. Most controllers require that the battery be connected before the PV. When turning off the controller you must disconnect the PV before disconnecting the battery. A PV disconnect makes that easy, but since your combiner is right next to your controller you can disconnect your PV by just flipping the three string breakers.

    Regarding the ground... yes, the chassis can be your ground. There may be some issues if you are connected to shore power... most shore power has the neutral bonded to ground (as it should), but if you also have a neutral-ground bond you can have problems because there should only be one neutral-ground bond in a system.

    --vtMaps

    Thank you, that makes perfect sense. I will never really be connected to shore power unless I end up finding that need at some point down the line which I'm doubting. Maybe to a charger for my batteries but never to power the coach.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Quick breaker question... oh, and a grounding question.

    A couple of notes on these questions:

    1). For polarized DC breakers protecting PV strings the string they are protecting is considered a potential load (shorts; draws current from other strings) and so the breakers should be connected as such. For polarized DC breakers used as a disconnect the array is the source and the controller the load. MidNite also says it doesn't really matter because the breakers will actually work either way, just not ideally if reversed.

    2). On a vehicle chassis can be used as common point for one side of DC. Don't think of it as ground even though that term is used because it isn't; it has not got the electrical mass of Earth. Do not use it as ground for AC power; that will be 'floating' just as a portable generator would be.
  • BB.BB. Super Moderators, Administrators Posts: 30,178 admin
    Re: Quick breaker question... oh, and a grounding question.

    Polarized DC Breakers/switches can be quite different... One method is to install a small permanent magnet next to the arc path between the switch contacts. When the switch opens, the magnetic field "blows" the DC arc sideways (possibly into an "arc chute") extinguish the arc. If the polarity is wrong, the magnet can "pull" the arc into the guts of the breaker--Not good.

    Regarding Marc's (Cariboocoot) waring about grounding an RV and shore power... Pay very close attention. It is very easy to mix-up polarity between "Neutral" and "Hot" from shore power--And if there is a Neutral Bond in the RV + wrong polarity--You can make your RV+tow vehicle 120 VAC "Hot" and never notice. Wearing rubber shoes, floor mat on ground outside RV door, etc... No shocks. Rain, kids wet from swimming lean against RV's metal skin--Electrocution. Does not happen often--But always a concern as the installations become more complex (shore power, generator, AC inverter, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TucsonAZTucsonAZ Solar Expert Posts: 136 ✭✭
    Re: Quick breaker question... oh, and a grounding question.

    Thank you all, sometimes the technical data while nice, is an overload for which I can't install a breaker to protect against while still on the shallow end of this learning curve. Your knowledge and understand is however invaluable to many of us, myself included.

    So just to clarify, the fourth breaker is really unneeded yes? It would be there to protect the PVs and no other use but I'm guessing by the time it tripped the PVs would be toast anyway.

    The ground isn't a huge issue, I will have no AC so to speak other than maybe a simple surge protector plugged into my inverter, I'm trying to use little to no AC with no gen or anything else.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Quick breaker question... oh, and a grounding question.

    A breaker between the array and the charge controller is somewhat redundant as the current the array can produce will not exceed Isc and all the wiring should be rated for that. So in theory if the controller turns into a dead short the PV can just pump full power to the wiring and nothing fries. If you have breakers on each PV string they not only will protect against a PV short starting a fire but can also be used to shut down all input to the controller, although they may not be in the most convenient location. Again some jurisdictions require an (easily accessible) array disconnect.
    The ground isn't a huge issue, I will have no AC so to speak other than maybe a simple surge protector plugged into my inverter, I'm trying to use little to no AC with no gen or anything else.

    Okay, that doesn't actually make sense. If you have an inverter you have AC. A surge protector isn't a load, but neither is it going to perform properly without a ground.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Quick breaker question... oh, and a grounding question.
    TucsonAZ wrote: »
    So just to clarify, the fourth breaker is really unneeded yes? It would be there to protect the PVs and no other use but I'm guessing by the time it tripped the PVs would be toast anyway.

    If by fourth breaker you mean a breaker between the combiner output and the controller input, no it is not needed except as a convenience. It should never trip because it is rated higher than the greatest possible PV output.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • TucsonAZTucsonAZ Solar Expert Posts: 136 ✭✭
    Re: Quick breaker question... oh, and a grounding question.
    Okay, that doesn't actually make sense. If you have an inverter you have AC. A surge protector isn't a load, but neither is it going to perform properly without a ground.

    I meant I will have no AC wiring or outlets beyond what's already on the inverter and I may plug a surge protector into that if I need more outlets due to trying to plug in 3 wall worts or something. Otherwise, the RV will have no hard wired AC.
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