Off-Grid, Non-Battery, Proper Grounding

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OK, I have a off-grid direct coupled pumping system. All DC. Depending on how many pumps, could be two or three panels. The panels are connected in series. I want to ground the panels and also the equipment.

1. By grounding the panels, is all that is required is to use a DC GFCI and run the ground on the back of the panel frames where the ground is indicated and run that straight to ground using a ground rod?

2. Equipment grounding. Is all that is required is to run a ground from the equipment negative to ground using a separate grounding rod than the PV rod? Or can I use the same rod for both?

Thanks,
Scott

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Off-Grid, Non-Battery, Proper Grounding

    I have a question for you: why does the title of your thread include the phrase "Non-Battery"? Basically 0ff grid no batteries = no power. The exception to this would be direct-solar equipment such as certain water pumps.

    NEC requires DC GFCI on panels, which basically removes the negative-ground bond normally used. Whether or not you must use this not-at-all-best system depends on your exact location and whether or not it is subject to inspection. Were it me, I would leave it out if at all possible.

    The panel frame/mount grounding is best run directly from there to a grounding rod without entering the building.

    Electrical safety ground connects all metal-cased equipments grounding together and brings it to a central location to be connected to Earth ground. You do not connect this safety ground to the negative connections of the equipment. At the battery, where all negative connections come together, you run a wire from (-) to the same Earth ground rod.

    The safety ground wiring creates a path to Earth for any power that may energize the outsides (cases) of things should the insides short to it. Normally it carries no current at all.

    I may not have explained this clearly. Feel free to ask for clarification.
  • Scottg4001
    Scottg4001 Solar Expert Posts: 47
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    Re: Off-Grid, Non-Battery, Proper Grounding

    Carib, its a daytime use only. No battery backup, etc. Panels directly connect to the load (pump). I do want to use the GFI because I have anywhere from 60-90VDC going into the linear current booster(depending on how many panels) and I want personal protection. With that said, since it is directly connected all the components run off the same negative. So if I tie in the GFI to the negative terminal, all equipment should be protected from a short, correct?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Off-Grid, Non-Battery, Proper Grounding

    I'm not certain the DC GFCI rules apply in this case. You'd have to check with a "code guy".
    In my opinion it wouldn't really add much protection, due to the nature of PV's (current source rather than Voltage source).

    A wiring diagram for this would have to come from the component supplier (both MidNite and Outback make DC GFCI systems). Basically it isolates negative from ground instead of connecting it. In the event of uneven current flow between the positive and negative wires the breaker is tripped disconnecting the power source (panels). This won't really do much but shut off the pump. I'd like to hear Bill's opinion on effectiveness of GFCI in this application.

    In either case, the case ground is still connected to ground. The only change is the isolation of negative from ground.

    As I said I wouldn't use it if possible. A standard system with negative tied to ground will behave much the same way. Just my opinion.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
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    Re: Off-Grid, Non-Battery, Proper Grounding

    DC GFI does not provide "personal protection" like AC GFI does...

    AC GFI trips around 5 milliamps (0.005 amps) and is designed to turn off the power before somebody gets enough energy to stop their heart.

    DC GFI has a trip point around 1 amp--A 100x or more current than is needed to stop your heart--So that type of personal protection does not apply. And, as of today, I don't know any technology for DC GFI that will trip on a 5 milliamp DC leakage (AC is easy--they just use a transformer to measure the differential current and pop a switch--With DC, you would need active magnetic sensors--which tend to have zero drift over a few minute time period--Very difficult to do cheaply/reliably/at such low current levels--probably can be done, but no market yet).

    Then we get into the whole reason for grounding theory and your needs.

    1) Ground a Return/Neutral (White Wire in AC power, typically negative in DC power). This allows us to use single pole breakers/fuses on the "hot lead" if there is a short from hot to ground or hot to return and limit maximum current flow. (neutral to ground shorts will never have current flow).

    2) Ground metal boxes/frames. Prevents those items from becoming "electrically hot" if there is a short internally to the box. Earth ground and Neutral grounds are usually tied together so that if there is a Hot to metal box short--There is enough current flow to trip a breaker (breakers/fuses typically do not trip with solar panel power--they are already current limited--so there is no surge current available to trip a breaker/fuse--Battery banks, on the other hand, have lots of available current and can easily trip protective devices in the event of a short).

    3) Grounding metal structures on roofs/out in years. Same as reason #2, but also for lightning protection. Generally, a heavy wire/cable from exposed solar panels on roof/tower/framework to metal ground rod(s) at base of structure. Lightning flows differently (it is more like radio frequency energy vs the 60Hz/DC of power wiring) and has to be done correctly for lightning grounds to work correctly/safely.

    4) there are other reasons to ground (cathodic protection systems to reduce the chances of electrolysis/corrosion for buried pipe lines and such--but ignore for now).

    If you are looking to ground for personal safety--You have the safety ground of metal mounts/solar panel frames/electrical boxes tied to ground rod (and/or well casing). This will prevent you and yours from getting electrocuted if there is a short to metal anywhere in the system.

    Grounding or floating the +/- leads of the solar power system... It is sort of your call... A floating circuit cannot shock anyone (if you are on a rubber mat, you can touch the hot leg of a 120 VAC circuit and not get shocked). It takes a return connection/current path for the shock to occur.

    But since a short/failed wire is always a possibility, even a "floating system" can become ground referenced--So assuming a system is floating today/tomorrow/next year, should always be confirmed with a small load and volt meter to ensure there is no current flow to ground.

    If you hard ground the DC return for the solar PV power system--You will "know" that the return wire is always near zero volts with respect to ground, and the other leg will always be "hot"... And can kill you if touched.

    In the end, if you choose a floating power system, you should use double pole switches to "kill the power" when servicing the system (break both legs--because you are never 100% sure that the floating circuit has been compromised somewhere/some how).

    If you choose to ground bond the "DC Return" you can use a single pole switch because the return wire should never be not zero volts with respect to ground.

    Note--If you use those little non-contact voltage sensors (used by electricians to sense if a wire is energized or not)--They only work with AC power systems... A DC power system will not trigger the voltage alarm. You need to use a standard volt meter to measure the actual system voltage before servicing.

    Long answer for a short question--Hope I was not too confusing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset