Grid tie grounding question

davidwillis · March 2014

I am installing a PVPowered inverter, and am not sure how the grounding works with this. I have attached some images from the manual. Do I connect the Array ground to the same ground as the AC ground, and also to the GEC? That Is what I understand, but I am not certain.

Here are the images.

Cariboocoot · March 2014

Re: Grid tie grounding question

I can't see the images very well, but it doesn't matter; all grounds go to the same place ... eventually.

I believe what they want you to do here is follow the NEC rules, wherein the PV ground terminates on the grounding bus bar in the inverter case as does the AC output ground. A wire from the case itself would also be connected here. Then another wire would go to the service panel's ground bus bar which would be connected to the Earth grounding rod.

Around here we prefer the PV frame/mount be grounded outside of the residence to reduce the possibility of it giving a pathway for high Voltage from lightning strikes to enter the home. This is somewhat more complicated if the main ground rod is not located at a convenient place to tie into outside.

davidwillis · March 2014

Re: Grid tie grounding question

Thanks. I think I get it now. However from what I read, the actually array ground will be grounded through the GFI? Meaning I connect the AC Ground to the AC Ground terminal, and the Array ground to the grounding bus bar in the inverter case, but they are connected through the GFI...?

You should be able to see picture #4 ok, it is zoomed way in. To me it does not look like the AC ground goes to the same bar as the PV ground, but I could be wrong. Also it is set to ground the negative PV, but I can also change it to ground the positive. I am not sure what difference that would make, but I can change it if I want to. I think that also grounds through the GFI, as it says not to Tie the GEC to the PV Array positive or negative.

vtmaps · March 2014

Re: Grid tie grounding question

davidwillis wrote: »

Thanks. I think I get it now. However from what I read, the actually array ground will be grounded through the GFI?

I can't read most of those images either... are they available on the internet? You could post a link... then I could read the whole thing. I was able to read something about not defeating the GFI by connecting PV negative to ground. That's a clue:

There are major issues with the way DC Ground Fault Interruption is usually done. A common approach (and I think this applies to your equipment) is to bond the DC negative (-) to ground through a small circuit breaker. When fault current flows through this 'circuit breaker bond', the breaker trips and DC negative is no longer bonded to ground.

The small circuit breaker is mechanically ganged to larger circuit breakers that disconnect the PV array from the system.

If you make another connection (bond) somewhere between DC negative and ground, you have 'shorted out' that small breaker and it will never trip.

It's not a good or safe system. Depending on the source of the fault current, it may exacerbate an electrical fire. Negative wires may be at dangerously high voltage above ground. Unfortunately, it is still code. Our moderator 'BB.' has written some important papers on the subject, and many of the smartest folks I know have defeated their DC GFIs, but I do NOT suggest you defeat yours because that would violate code. btw, this provision of the code applies to rooftop arrays, and may apply to some ground mount systems.

Better techniques have been developed to squelch dangerous faults. IMO, the state of the art is an 'arc fault combiner'.

--vtMaps

davidwillis · March 2014

Re: Grid tie grounding question

Yes, that does sound like how this is setup... I didn't think of finding an online version. Here it is.

http://www.soligent.net/uploads/products/25242_2.pdf

It's not a good or safe system. Depending on the source of the fault current, it may exacerbate an electrical fire. Negative wires may be at dangerously high voltage above ground. Unfortunately, it is still code. Our moderator 'BB.' has written some important papers on the subject, and many of the smartest folks I know have defeated their DC GFIs, but I do NOT suggest you defeat yours because that would violate code. btw, this provision of the code applies to rooftop arrays, and may apply to some ground mount systems.

If it is not safe, then I am more concerned with having a safe system than one that meets code. My array is mounted on a detached garage, so even if it did catch fire, and burn down, at least it should not burn down our house (about 100 feet away). But I still want to make it as safe as possible (within reason).

Cariboocoot · March 2014

Re: Grid tie grounding question

Negative tied to ground is standard for DC systems. A few require positive tied to ground owing to the panel design. When DC ground fault circuitry is used neither is connected directly to ground. If the DC GFCI is built in to the inverter you must following the wiring diagrams from the manufacturer to have it work and meet code.

The issue is that that NEC DC GFCI itself is not safe for all circumstances. On a standard grid-tie inverter it's not much of a problem. When used with batteries it becomes a different issue.

Arc fault protection is not the same as ground fault protection, but it is more valuable for PV arrays as arc faults are more likely to cause a problem. So far very few implementations of such are available. But don't worry as the majority of PV arrays, even those with no ground at all, work fine. Grounding, ground-fault protection, and arc-fault protection all do absolutely nothing while the system is running. They are there to minimize the dangers should something go wrong.

Your inverter is a bit different than most in that it has an integrated DC and AC disconnect. It also has built-in GFCI which you can see indicated on the Power PCB in Fig. 4-3, page 19. The green line indicates the ground wiring, and that includes a line from the PV via the DC disconnect and a line from the AC via the AC disconnect all meeting at the AC connection terminals and going to the ground rod from there. The difference is that the negative is not bonded to the ground directly but rather through the GFCI circuit. The option is given to have the positive connected to ground via the DC GFCI if positive-ground PV's are used. Most PV's are not positive ground.

If you follow that diagram it should work, pass code, and be safe.

davidwillis · March 2014

Re: Grid tie grounding question

Thanks, but I am still a little confused. So does the ground wire from the pv frames just go into the DC ground (in the DC connection area of the inverter) and to no other ground? And what connects to the Disconnect Grounding Lug (it looks like it goes to the same place as the DC ground in the DC connection area). Then the AC ground would go to the AC ground terminal.

I just worry that there is no direct ground for the pv array... but maybe that is ok.

Thanks

Cariboocoot · March 2014

Re: Grid tie grounding question

davidwillis wrote: »

Thanks, but I am still a little confused. So does the ground wire from the pv frames just go into the DC ground (in the DC connection area of the inverter) and to no other ground? And what connects to the Disconnect Grounding Lug (it looks like it goes to the same place as the DC ground in the DC connection area). Then the AC ground would go to the AC ground terminal.

I just worry that there is no direct ground for the pv array... but maybe that is ok.

Thanks

Yes, the PV, including the frame/mount ground, connects to the DC Disconnect Board. There is no need for any other ground connection to it. Follow the green line in the diagram.

Unfortunately the physical images in the manual are clear as mud. I've tried enlarging them and they just get fuzzy and even more indistinct. Maybe you could take your own picture?

davidwillis · March 2014

Re: Grid tie grounding question

Thanks... I think I have it figured out now.

Grid tie grounding question

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