Grounding an Off Grid Dock Solar System

System · May 2009

I've recently installed a solar power system with a charge controller with digital readout, the two Trojan T-105 deep cycle batteries, then to an inverter, then to a distribution panel with GFCI breakers (However the GFCI won't trip when the Test button is pushed). I tied all of the ground wires and neutral wires on the 12v side and the 120v side back to the battery negative, without an earth ground.

The system operates my boat lift OK, but I've noticed the digital readout on the charge controller digital display goes black, or blanks out then appears to reset if I turn the inverter on or off.

Do I need to separate the 12v and 120v grounds? Do I need an earth ground? I've read conflicting statements on the need for an earth ground for and off grid system like this. Any suggestions would be greatly appreciated.

Thanks

icarus · May 2009

Re: Grounding an Off Grid Dock Solar System

A couple of things,,, and I really don't know much so I'm sure that others will chime in with better answers.

First, Many small inverters don't play well with grounded neutral wiring systems and in fact some go up in smoke right away. I would look at the inverter install instructions as to how they should be grounded.

Second,, because of the previous,, GFCI's don't always play well with inverters because they have trouble referencing ground. Once again,, I would research the inverter instructions.

Finally,,, I have had trouble with ac grounding not playing well with dc grounding in a electronic voltage control on a genset. It turns out that the problem was far mor obscure than my knowledge to understand,, but in short,, the dc was ground loop feeding dc into the ac control unit of the genset, burning it up over time.

As I suggested,, others will chime with better technical details.

Good luck,

Tony

BB. · May 2009

Re: Grounding an Off Grid Dock Solar System

As Tony/Icarus says--grounding of an inverter is "special"...

A MSW (Modified Square/Sine Wave (the "cheap" type) Inverter generally will self destruct if its AC output is grounded back to the battery "-" (or probably "+" too) terminal. Do not ground either AC output wire on a MSW inverter unless the manual says it is OK.

A TSW (True Sine Wave) Inverter (the expensive type) will generally allow the grounding of one of the AC output wires to copy a grounded neutral like in standard US Wiring (per NEC). Again, read the manual, or contact the manufacturer, to confirm.

Regarding the GFI--two issues. One, I am not sure how well a GFI will work with a MSW inverter--it may work fine, it may damage the internal electronics, or it may false trip.

The second issue with the GFI--the self test circuit requires that the breaker be connected to safety ground, and a grounded neutral AC circuit to work correctly (neutral grounding must be "near inverter" before its output goes through the GFI). The GFI places a test ground fault (small amount of current) on the Hot Lead of the AC output. If the switch body/green wire is not earthed and the Inverter Neutral is not earthed--there is no connection and therefore, the test failes (because there is no complete electrical circuit for the self test current to flow).

Now, the real reason you are doing this--safety and limiting damage (if you are in a lighting prone area).

1. Usually a TSW AC inverter output is isolated (not referenced to ground). So, if you touch one wire, there is no shock because there is no circuit path for current flow.

2. If you do not safety ground the battery output on a MSW inverter--its output will also be isolated too.

Isolated outputs are a good way to provide safety. Also, using an isolated transformer (120 to 120 VAC) would work too.

If you are also protecting against lighting... I am not sure that grounding the neutral or the battery really helps (safety or limiting damage). Grounding the metal frame work, motor housing, and running grounded metal conduit (and/or with safety green wire with the AC wiring) is probably the best you can do.

Regarding grounding the battery bank--leaving it floating may be the best bet (if "against code"). But if you do float the battery, realize that many 12 vdc loads you may use (for example, an old car radio) can use their metal chassis as 12 return--and you may have to safety ground the battery to your metal framework to prevent other problems.

The above are my personal suggestions/observations. I do not know your exact installation--so understand what you need (and ask more questions) before you proceed.

-Bill

dwh · May 2009

Re: Grounding an Off Grid Dock Solar System

Yea, GFCI is pretty simple - little current sensors (like a little amprobe) on the hot and neutral. If they have the same current flow, then all is well. If they are different, then the current is not following the prescribed path...and the GFCI trips.

The test button dumps some current to ground - thus creating a difference between hot and neutral - and the GFCI trips.

If there is no ground, then the current doesn't flow (nowhere to go) when you hit the test button, and it won't trip.

Now, I have had to test GFCI on non-grounded systems before (don't ask). Since there is no ground, the test button wouldn't work. The simple way is to just use a solenoid tester (Wiggy type) and touch one probe to the hot on the load side of the GFCI, and the other probe to any handy ground. This will create the imbalance and if the GFCI is good, it will trip.

Dunno if that would work with a non-grid tied inverter or not. I doubt it but I've never tried it.

Grounding an Off Grid Dock Solar System

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