Proper grounding

adasadas Solar Expert Posts: 136 ✭✭✭✭✭
Aloha, I am having some issues with grounding and think I need to go through the system from square one.

1: can the PV and the 110v inverter power in the office have the same ground?
2: should neutral be grounded to ground, ie to the conduit then to ground spike?

My new inverters use GFI's and these trip off with very little provocation, should I direct wire or use regular plugs for my 110 v inverters? Or heed the warning of the GFI's



  • BB.BB. Super Moderators, Administrators Posts: 33,140 admin
    Re: Proper grounding


    The leakage current for a typical GFI trip point should be around 5 milliamps (IIRC)... Anything over that is considered potentially life threatening...

    But... That is usually defined for hand-held equipment. If, you have permanently connected/grounded equipment (say a pedestal grinder, or lath/mill/welder) then higher ground currents are allowed (upwards of 15-30 mAmps???). The theory being that the equipment is safe because it is physically grounded and if there is ever a short then the safety ground will keep everything safe.

    Also, normally GFI's are installed in outlets near water (outside, pool, sink, etc.) where a hand tool could possibly drop in the water and energize somebody standing/swimming--again, not usually a problem with fixed equipment.

    Another issue with grounding neutrals--the typical MSW (modified Sine/Square Wave) inverter does not like it when the battery bank is earth grounded and the neutral is earth grounded... You would probably need an isolation transformer to be able to ground the neutral...

    One way that you could monitor a "floating" AC output is do what I have seen old US Navy ships do... Float the AC output and then put a small filament lamp (say 1+ watt type indicator lamp) from line-A to ground, line-B to ground (and line to line if you want). What you will see is Line-Line a bright light (circuit energized) and the LineX to ground lights will be glowing (basically 1/2 AC voltage across each). If you have a line to earth short, one light will glow brightly and the other will turn out--indicating a short in your distribution system (I assume that the Navy panel had a relay with counter wound relay coils--as long as the current was balanced, the relay was open. If one current failed, then the relay was energized for alarm/trip).

    Personally, at most, I would only install GFI protection on a per branch outlet near water... But, unless you have an earthed neutral, a GFI would not reliably indicate a ground fault (no hot to earth unbalanced current flow in shorted circuit).

    If your inverters are more than 15-20 amp output--then the "typical" branch GFI may be too sensitive for your usage (lots of leakage current through long runs, filter caps, etc.).

    Just a guess from 4,000 miles away. Only you and your electrician can work out the correct answer on site.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • adasadas Solar Expert Posts: 136 ✭✭✭✭✭
    Re: Proper grounding

    Aloha and thanks for your reply Bill. Here is further explanation and troubleshooting thoughts:

    Both warehouse and office systems are mounted on metal. As both Whs and Office are made out of multiple ocean/sea 40' containers. Office and whs are 125 feet apart and are not touching each other.

    I am trying to isolate the 24v system from ground and put a proper ground on the 110v side. But I have sooo much wire and conduit and circuit breakers and JB.s, it is really hard.
    Right now I do not ground the 110v side at all, but am getting a neutral to ground reading..... not a "beep" on my VOM but an Ohm reading... so trying to track that down. (if I have a beep on the Ohm setting then the GFI will trip, but they will not trip with only an ohm reading on continuity)

    I think I will keep using the GFI's that are on the inverter for the time being as they really help me find the potential weak links in my system. But as I mentioned in a previous post I am on base and 1/4 mile from the control tower, (even my rader/laser detector goes off all the time here in my car, and with all metal buildings, who knows what the heck is coming in and tripping the GFI's periodically? F-117's flyby, practice and land next door all the time, as well as Coasties training, touch and go maneuvers.

    So, given the circumstances above, Is isolating the battery side from ground and grounding the 110v side the best road to take? thanks
  • adasadas Solar Expert Posts: 136 ✭✭✭✭✭
    Re: Proper grounding

    Aloha, on the inverter issue. The Warehouse uses a MSW 3600 W, super heavy duty kick butt unit. One day a week or two ago I was running 45-50 amps 110v the whole day and my PV/FX80's were both logging 75-80 amps each at 28v the whole day and not a peep or overheating out of the inverter.

    So my grounding problem started in the office system (or became evident) when a couple of days ago I replaced my Solid state 5000w switching MSW inverter with the 3600 watt inverter type that I have in the shop. The 5000w unit did not have GFI outlets and I guess did not care about the problems in the office wiring, but was not running the fridge well (loaded freezer items were a bit soft, etc) and the microwave was making a bit too much noise. 2 hours after the installation of the new MSW 3600, the stuff in the freezer is rock hard and microwave times are cut in about 1/2.

  • BB.BB. Super Moderators, Administrators Posts: 33,140 admin
    Re: Proper grounding

    As far as I know, the GFI outlets/breakers just take the Hot+"Neutral" and wrap a couple turns of wire around them to make a current sense transformer...

    Basically, as long as every electron "goes out" one wire and comes in on "the other wire", the total current through the sense transformer is Zero MilliAmps.

    If, either of those current paths finds a path to flow current back through some other electrical circuit (typically the safety ground/earth path)--then the current sense transformer measures a non-zero sum of current and trips open.

    I would suspect that your current paths may be a combination of "surge suppressor" power strips and electronic equipment with filter caps on the inputs that connect to earth/safety ground... This is done many times to reduce radio noise interference (especially for Class B equipment used in home settings).

    While one or two of these devices (power strips, computers) is usually not a problem--if you have a bunch all plugged into one GFI branch circuit--it is possible that you will get nuisance tripping of the GFI... It is not that any once device exceeds the limits--but all of them added together do.

    You can do a test with a clamp meter... Just clamp the black and white wires in your meter clamp (leaving any earth/safety ground) of the clamp... Depending on the accuracy/sensitivity of the clamp, you should see near 0.000 amps (near zero down into the near milliamp range). Play with your circuits and see if you can identify which one is causing you the leakage current... Unfortunately, from what I recall--the standard 100+amp current clamp is not usually capable of measuring milliamp current... So that may give you random results unless you have/can find a smaller/accurate current clamp to give you those readings.

    If you cannot find any bad wiring--then I would bypass the inverter GFI--and if you still need GFI protection (outdoors, near water), just install GFI outlets where you need them... That would give you the point of use protection you need without false tripping a whole distribution circuit (nothing like 100% dark because the main GFI breaker tripped everything just because an electric drill got wet).

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