Aux panel frame grounding and GFDI

BrentBrent Solar Expert Posts: 64 ✭✭✭✭
I have seen many roof and ground mount installs where the panel frames/racks are connected to a new ground rod for lightning protection. The lead to the ground rods were attached to the bare copper panel frame grounding conductor or bolted to the racking structure and are usually between 6-10 feet long. In most cases the conductor to the ground rod is heavier than the frame ground conductor size. According to the Calif. Elect. Code this is permitted.

The panel frame grounding conductor also connects to the inverter GFDI connection point in most cases.

Does this "dual" ground configuration reduce the effectiveness of the inverter GFDI? In some installations the run from the panels to the inverter exceeds 100 feet. It would seem at first glance that any panel "fault" currents might be shunted to the local ground rod and not be detected by the inverter GFDI ckt.

Any thoughts would be appreciated.

Brent

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Re: Aux panel frame grounding and GFDI

    Probably not...

    The typical DC GFI detection is performed by a ~1 amp fuse (or ganged breaker with many off grid systems/retrofits).

    The 1 amp fuse is connected between DC Return (typically negative, but sometimes positive--that is another discussion) and if there is a potential between DC Return and Earth Ground, then the fuse pops--which signals the GT Inverter (and some charge controllers) to "turn off".

    In the case of older controllers that use a retrofit kit--This is a 1 amp breaker ganged with a 60+ amp second breaker. If the 1 amp breaker trips, it also trips the high current breaker (or two breakers) in the PV array +/- lines (sometimes in the output section of the charge controller).

    Since you need > 1 amp (call it 2 amps or more) to "quickly" pop a fuse or breaker, and the minimum ground rod to earth resistance is ~25 ohms (as I recall), that would mean a voltage of:

    V=I*R= 2 amps * 25 Ohms = 50 volts maximum voltage required to "clear" a DC GFI fuse/breaker.

    So, while a remote ground does not necessarily cause a DC GFI to become less effective... If it was to be made "more effective" there should be a (typically) 6 awg minimum cable from the remote ground rod to the local (main) ground rod at the home where DC return (battery bank negative) and AC Neutral to Earth bond are all tied together.

    That should provide low enough resistance to allow enough current to flow to trip any standard protective devices (breakers/fuses).

    Personally, I have major issues with DC GFI and I believe placing a fuse/breaker between earth/safety ground and DC Return (negative battery terminal) is very unsafe (no matter that NEC and UL suggest to do exactly that). We have written a lot about that here and on the Midnite forum before.


    Some newer NEC standards and solar equipment should make the DC GFI an obsolete requirement in the long term...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BrentBrent Solar Expert Posts: 64 ✭✭✭✭
    Re: Aux panel frame grounding and GFDI

    Sounds good to me. The 100' ground run from the panels to the inverter will be only about 0.1 Ohms with #10 so the resistance is 250 times lower than the path through the ground rod back to the main ground not to mention earth resistance. Some installers will bond the grounds from the array ground rod to the main system ground if the run is 25' or less otherwise they rely on the earth ground for conductance.

    Ground conductance is an interesting area of study. I have 20 miles of electric fence on my property with a 8 kV/3 joule pulsed charger. The charger return is connected to 3 ground rods at 10' intervals and the area is kept damp. If you short the fence to one of the "T" posts 10 miles from the charger you get a 1/4" snap and if you touch the fence your arm won't work for several hours.

    Thanks,
    Brent
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Re: Aux panel frame grounding and GFDI

    Grounding is one of the most difficult subjects we discuss here... Lighting control throws a whole new set of monkey wrenches into the mix.

    I am no lightning expert--But I have been changing my mine too (i.e., more ground rods everywhere is a good thing--Even if it becomes multi-point grounding for AC neutral wires).
    BB. wrote: »
    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    And our host's consolidated FAQ page:

    www.windsun.com
    Lightning Protection for PV Systems

    From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

    Towards the end of this thread is a very nice discussion of proper generator grounding.

    -Bill :confused:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BrentBrent Solar Expert Posts: 64 ✭✭✭✭
    Re: Aux panel frame grounding and GFDI

    We live in Northern Calif. and have frequent cloud to ground lightning events in the summer [that's why a good portion of Calif. is on fire right now].
    The good news is that you can see them coming. When we start getting strikes in the area I open the 200A main breaker on the service panel and 60A AC disconnects on the inverters and run off batteries until the storm passes. Usually in a few hours.

    Nothing can protect against a direct strike. I have seen a telecom tower blown off the concrete support piers due to the lightning conducted through the piers [large!! several feet across] and the moisture in the concrete piers instantly turning to steam and exploding the pier. This pier had lightning suppression and was well grounded.

    Can't mess with mother nature!

    Brent
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Re: Aux panel frame grounding and GFDI

    Opening a breaker is little help for a direct strike (1/2 inch vs going through 2-5 miles of air).

    Interestingly, I asked our host years ago what seems to be killed most often by lighting--And he said it was AC Inverter's AC output that failed most often.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Aux panel frame grounding and GFDI

    bill,
    that was for gt systems connected to the grid. in totally off grid i would say the dc is more prone due to longer outside wires to attract the lightning and pick up the emp with as compared to shorter indoor ac wiring. totally off grid is far less susceptible than gt imo.
  • BrentBrent Solar Expert Posts: 64 ✭✭✭✭
    Re: Aux panel frame grounding and GFDI

    I agree that if you get a direct strike then you will have a lump of charcoal where the inverter used to be. Probably a big lump of charred remains where the building used to be.

    I was thinking more of a tertiary strike on a nearby PG&E pole where the surge impulse would be reduced by the 12 kV to 240 V pole transformer and would not jump the 200A open main breaker. This happened to a friend who lost his PC's, TV, oven etc.

    Brent
  • BB.BB. Super Moderators, Administrators Posts: 29,514 admin
    Re: Aux panel frame grounding and GFDI
    niel wrote: »
    bill,
    that was for gt systems connected to the grid. in totally off grid i would say the dc is more prone due to longer outside wires to attract the lightning and pick up the emp with as compared to shorter indoor ac wiring. totally off grid is far less susceptible than gt imo.

    Actually, as I remember, I asked the question specificity about off grid installations (charge controller, inverter, etc.)...

    I looked--But I think some of the earlier posts (over 5 years old?) were lost/scrubbed in a major upgrade.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: Aux panel frame grounding and GFDI

    How is that for a strike.

    http://bcove.me/735z90qe
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Aux panel frame grounding and GFDI

    dave,
    i heard of that one before, but didn't read into it much.

    bill,
    you may have asked once, but i'm remembering (maybe wrongly) a particular thread that was going on (i think with mangas in the conversation) in which windsun participated. he stated the ac side of the inverters he found to be blown out, but that was on hybrids connected to the grid if memory serves. i'm thinking what and how much wire is more exposed to the surges or emp in making them prone.
  • westbranchwestbranch Solar Expert Posts: 5,142 ✭✭✭✭
    Re: Aux panel frame grounding and GFDI

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