Battery Bank Negative Grounding

[mjp24coho]

Count me in as another one lost in the maze of grounding questions around grounding.  As a bit of background, I have a pre-wired E-panel with midnite controllers and a magnum 4000W 120V inverter, which charges a 24V forklift battery.  This system is used to power my off-grid cabin, fed by 2000W of solar panels on my roof.  The e-panel and battery bank is in a detached garage.  The benefit of the pre-wired epanel is that the install was very simple - connect the panels and battery bank to the e-panel, and then connect the e-panel to my main AC distribution panel (also in my garage).  My question relates to grounding of the negative side of the battery bank.  The ground bus bar in my e-panel is connected by bare copper wire to the ground bus bar in my main A/C distribution panel.  That ground bus bar in the AC distribution panel is then grounded (via bare copper wire) to the earth ground rod (only point of connection to the ground rod).  I'm struggling to see how the negative side of my battery bank is grounded.  Below is the link to the wiring diagram for the pre-wired e-panel, which may help clarify the question.  In the wiring diagram there appears to be a ground wire from inside the battery bank (not not connected to the battery from what I can tell), that is connected to the ground bus bar in the e-panel.  I'm unclear as to what/how it's connected to the battery bank.

http://www.midnitesolar.com/pdfs/MNEMS4024-CL200-REVB-120VAC.pdf

Another thing I'm trying to confirm is whether or not my battery and certain devices running off my A/C distribution panel share a common 0V ground, but I'm unclear if that is the case.  I'm clearly no electrician, but had an electrician oversee my original installation years ago.

Any direction/help you all can provide would be greatly appreciated!

[Robin]

Our wiring diagram does not show the battery minus grounded. There are a few different places where the ground can be made when required. When a DC-GFP is included in the system or charge controller, the grounded battery minus is done in the DC-GFP. To add an additional ground would bypass the DC-GFP. I am not a big fan of grounding battery minus, but since all systems are supposed to have a Dc-GFP, that would seem to indicate all systems will be grounded. The NEC requires 48V systems to be grounded, but not 12 or 24V. The DC-GFP pretty much forces them all to be grounded. I am also not a big fan of DC-GFP's although I would NEVER tell a person that the DC-GFP doesn't stop fires in these systems, but may contribute to a shock hazard situation. If I said something like that, it would go against the NEC, so it would not be a good thing for me to advocate going against the outdated NEC rules. The fact that we now have to have arc fault detectors makes the DC-GFP unnecessary in my opinion. Arc fault detectors do stop fires! Hmmmm. Maybe I should add a DC-GFP in my home system? Than again if I never get around to adding a DC-GFP to my own system, I guess that means I should ground the battery minus too? While I am at it, maybe I should add a PV combiner instead of the wire nuts that were installed 20 years ago before I knew what a combiner was?

[Mtn Don]

 

[Vic]

mjp ..,

Believe that that ground shown on the linked e-panel diagram is,  most probably,  a connection to the/any metallic battery box or cabinet.   MN does sell such boxes for batteries,  and in that case am certain that it would be Code,  and a good idea to ground that metal box,  just as are all other metal boxes,  gutters,  cabinets,  frames,  and so fourth.

However,  I am not an expert,  so YMMV,  just my opinion.   Good Luck,    Vic

[mjp24coho]

I do not have a metal battery box, so I guess that grounding is not required.  Since I have a DC-GFP installed with the system, then I guess the battery minus is grounded through the DC-GFP.  Also, with the the AC and DC busses being the same in the e-panel, I also have a 4 awg bare copper wire going from there to the main AC distribution panel ground bus, and from there to the main earth ground rod.  Would that be duplicate grounding?  Also, my other question would then be whether that would make it such that the battery and any devices plugged into receptacles tied to the A/C distribution panel are both "sharing the same common 0V ground?

[Robin]

You want to run the ground from the e-panel to your distribution box and then to the ground rod. I believe that is what you said you did. Everything is at the same ground potential.

[zoneblue]

Your dc ground is provided by the GFP unit, most likely to the epanel ground bus. If you have a GFP unit then grounding the DC bus is wrong and defeats the GFP. The primary risk with DC GFPs is that when they trip they disconnect the ground bond. Leaving the system open to other short failures. So keep an eye on it as part of your routine service procedure. The GFP units built in to the classics have the advantage that they beep and flash warning lights are you.