Grounding

Discussion about the Power System PV Combiner (PSPV)

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BAM
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Grounding

Post by BAM » Thu May 11, 2006 4:39 pm

I am grounding my pv array/rack to a double rod earth ground. I am bonding the combiner box ground to my ps2 system ground (also double rod earth ground, separate from the pv array/rack).

I noted that the Outback wiring diagrams show a separate ground for the array/rack and separate ground for the combiner box/BOS. But, I see that virtually all Home Power diagrams show the combiner box bonded to both the array/rack and the BOS whether the array/rack were separately grounded or not.

So, do I need to bond the combiner box to the pv array/rack as well as to the BOS?

It would seem to me that not bonding the array/rack to the combiner box would be beneficial. For example, if there were a lightening strike, the most direct ground would be through the array/rack ground, so why give it the opportunity to go through the BOS as well?

Thanks for your thoughts, Bruce

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Post by Jim L. » Thu May 11, 2006 6:48 pm

This is a fairly complex question. There are different levels of protection in here, starting from the building structure, to the electrical system, to the electronics.

The short answer would be that all rods need to be connected together. The direction for doing that would be NEC Art 250.58 and 250.106.

However, given a lightning strike, there will be a ground potential rise over the local area. It will not be confined to the immediate surface about either pair of rods. Therefore, surge protection by shunting will also be required to partially protect electronics and electrical system.

The effort and expense to completely protect the electronics with a true equipotential grounding system and single-point reference is more costly than replacing the electronics for most residential setups.

Best regards
Jim L.

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Post by sparky » Thu May 11, 2006 7:58 pm

And to quote Jimmy Buffett "you don't reason with Hurricane Season and Lightning." There may have been one more thing not to reason with but he would not say over the radio....... Jim's advice is good ! The cost of protection is a big factor when comparing the cost to replace !

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Post by Kent Osterberg » Thu May 11, 2006 8:32 pm

The NEC is very clear that the earth is not a grounding conductor so proper grounding requires a connection between the main grounding system and any grounding system at the array. While PV module frames should always be grounded, a grounding system isn't always required at the PV array. For systems with just one or two strings of modules, a grounding wire run with the PV output probably meets code requirements.

For lightning protection it is always best to have a ground wire from the array to a nearby grounding system!!!!

I've brought this issue to Outback Power Systems attention before and they said they would correct their drawings to show that all the grounding systems should be connected together. That was over a year ago, the drawings still show separate grounding systems and undoubtedly some installations are being done incorrectly.

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Post by BAM » Thu May 11, 2006 8:38 pm

Thanks much for your quick and well reasoned responses. Please let me know if I understand this correctly:

Though the Outback wiring diagram on the inside of my PS2DC cover shows a separate ground for the pv array, and a separate ground for the BOS, code calls for them to be bonded, and, in my case, that bond would be done through the PSPV.

If that is correct, would it be compliant to bond one ground cable to the supplied ground lug and tap a terminal lug to the box for the other ground cable?

Thanks again.

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Post by BAM » Thu May 11, 2006 8:42 pm

Just saw your post after mine, Kent. Thanks. That does clarify. The question remains about how, exactly, to connect the two "grounding systems" through the PSPV. Thanks.

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Post by Kent Osterberg » Thu May 11, 2006 9:34 pm

The grounding lug in the PSPV is only rated for one conductor. You can put two ground lugs in the PSPV by using the one intended for the second output bus. That is still insufficient for many installations. I recommend installing a ground bus bar. The ground bus bar used in the PS2 or PS4 is a listed accessory for the PSPV, however any manufacturer's ground bus bar will work as well.

For a poletop installation you should end up with a ground wire that runs to the MX-60 that is 4 gauge sizes smaller than the conductors (#6 ground works with #2 conductors), a #6 wire to your ground rod system, a #6 wire to your PV module frames, and one to the steel pole.

I'd sure like to see a four position ground bus as standard equipment on the PSPV rather than the ground lug it has. Making the second output bus an option should save enough to cover a decent grounding bus.

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Post by BAM » Fri May 12, 2006 5:25 am

Perfect. I believe I have a spare bus from the PS2. Thanks much. I'm all set now.

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Post by rock » Fri May 12, 2006 7:15 am

As an engineer it part of my job to determine if code is "good enough" there are times when putting things in to "code" (although perfectly acceptable) my not be enough.

My issue here would be quality of the ground itself. Double copper rods will do quite alot and that is good, but if you are wanting to protect aginst direct lightning strikes that will be tricky if not impossible.

From my experience growing up on a rural farm I noticed at an early age that no matter how well a facility is "grounded" a near lightning strike will almost always take out a deep well pump or its attached electronics.

My theroy here is that too good of a ground will attract the potential discharge and not a good enough ground will also attract a potential discharge.

The key is to get all you equipment and wiring to the same "potential" charge as everything else that surrounds your facility. Then create a place for lightining to "want" to strike that will not hurt you "stuff."

This is what I did. I wired everything to code (hopefull, its hard sometimes) and then some. I used a 200 square foot copper fabric, buried in some moist soil and ran a large copper gound wire to that. The purpose of this is to move electrons to and from my "important stuff" to equlibrium with its environment. Please note that more than 1 ground could create a potential difference between grounds and that could cause problems.

Then I used some "bait" for the lightning. Using an old water well, that was not being utilized for anything, I dropped copper wire down 400' to the bottom and then bonded the wire to the well casing. Then I attached a wire from well casing to a 20 foot copper pipe that I put up approximately 40 foot from my Utility entrance and about 40 foot from my copper fabric.

I live at a high elevation and high lightning area and have never had any lightning stirke my "stuff" but have had several VERY close hits and near misses. My "bait" has been hit at least 2 times maybe more. The copper wire in my old well has been replaced once due to a strike. Please note the noise generated from a strike to a well head is like an explosion and will wake you up if your asleep.

This is just what I did. What you can do in your area is to build to code. Install lightning arrestors and try to find some bait for lightning to want to strike. Not sure what someone with out an old well could do, but I think my theroy of baiting lightning is a solid one.

Hope this helps.

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Post by DavidB » Fri May 12, 2006 8:52 am

I believe it was John Wiles that had an article about grounding a while ago in Home Power and when the array was a considerable distance from the charge controller ( I believe he said at least 60ft away) then it was acceptable to use a separate ground rod and ground it right at the array and NOT tie it back to the grounding system at the charge controller. There were several caveats that must be met.. one was that you could not have any other wires going to the array except the dc negative and positive. The DC negative is already indirectly tied to the BOS grounding.

That is how I grounded my array which is 220 ft away along with a Delta lightning arrestor on the PSPV... The ground rod is 16ft deep. Plus it does not make sense why I would want to provide an easy path for lightning to follow right back into my house to the BOS components.

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Post by Jim L. » Fri May 12, 2006 11:25 am

It's not an easy problem to work, considering that you have to think in DC, AC, and electromagnetic fields.

I had a Sony stereo back in the days when the cover was real wood. The electronics were in a metal chassis. One day when storms came, I disconnected the stereo and TV from the electrical outlets and antennas.

Didn't matter. The stereo quit working, so I opened it up for a look. The low voltage power supply was a pass-through transistor with a capacitor to smooth the output. The field from a nearby lightning strike had charged the capacitor, putting reverse voltage on the transistor, and killing it. Replaced the transistor and the stereo worked fine.

That's the gate that's left open with consumer electronics. Electronics designed for an EM environment will be metal boxes, sealed seams, through-wall filters, damage limiting shunts, etc. Wiring will be run in shielded raceways.

Even with good grounding, the fields problem can still bite.

Best regards
Jim L.

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Post by BAM » Fri May 12, 2006 12:07 pm

Hmph... Give all of that, since my array is on my metal roof, and the cable run to the BOS is only about 30', would I be better off NOT grounding the array separately, and just running the array ground through the BOS ground?

I am on a bit of a rise, but surrounded (except on the south) by trees roughly 20' to 30' taller than my house. Also, I'm in Maine, and we don't actually have a lot of electrical storms. But, having said that, I can see two lightening damaged trees from my house.

I ask this thinking that if I don't ground the array separately, it will be less likely to bait lightening. Thougths?

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Post by Jim L. » Fri May 12, 2006 12:57 pm

Not a direct answer to that, but some more things to consider.

The metal roof is a non-issue with lightning, but a safety consideration for electrical faults.

From a risk perspective, it would seem that Code issues should take precedence.

NFPA 780, which is the standard for the installation of lightning protection systems, says in 3.9.5 that no bend of a conductor shall have a radius of less than 8 inches. This is because a lightning current will induce a significant amount of torque on the conductor when it changes direction. Whichever you decide, the conductor should be securely fastened at least every 3 feet.

Since the path will be to ground, NFPA 780 also forbids "U" or "V" pockets in conductors. In other words, keep the conductor moving down, or horizontal, but not up. Since the lightning bolt has moved down miles through air, and the conductor is only a wave guide, if the conductor is less attractive than a more direct path, the direct path will be taken.

Bonding the rods with bare copper in the soil will provide a bit more path for discharging the energy.

So the physical configuration should be taken into account also.

Best
Jim L.

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Post by Kent Osterberg » Fri May 12, 2006 4:33 pm

BAM,

With your system on the roof, I think it is acceptable per code to ground the array frames with a ground wire running with your PV conductors. That's typically how it gets done. That is the same level of protection that most roof mounted equipment gets. However, a direct path from the array to a ground rod will improve the lightning protection. None-the-less a direct hit will probably ruin everything you own.


DavidB,

John Wiles has written many grounding articles. The one you mention is one of the early ones, I think in the later ones he mentions that the DC negative should only be connected to ground at one location. Making a ground connection to the DC- (or DC+) at two points is allowed per code but it is a bad idea. The voltage drop in the conductor that is grounded at two points provides enough potential to cause some current to flow through the earth. The grounding system, at one end or the other, will slowly be corroded away by electrolysis. This doesn't happen with ac because the polarity is always reversing.

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Post by Martin » Mon Sep 25, 2006 5:12 pm

Back to grounding questions again.

Here's the question: if you have a pole-top array that's about 250 feet from your inverter, is it OK by code to ground the pole top array at its location with a series rods that brings resistance down to the sub-25 Ohm range, and then at the PSPV bond that ground to the DC- cable run down to the inverter (where it is bonded to the house ground and to local ground rods)?

I understand this isn't the best thing in the world to do (running a separate ground line is best), but by NEC is it acceptable? If so, can anyone help with the specific NEC chapter and verse that says so?

I'm still trying to understand grounding - so thanks for your help!
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Post by Martin » Mon Sep 25, 2006 5:13 pm

Back to grounding questions again.

Here's the question: if you have a pole-top array that's about 250 feet from your inverter, is it OK by code to ground the pole top array at its location with a series rods that brings resistance down to the sub-25 Ohm range, and then at the PSPV bond that ground to the DC- cable run down to the inverter (where it is bonded to the house ground and to local ground rods)?

I understand this isn't the best thing in the world to do (running a separate ground line is best), but by NEC is it acceptable? If so, can anyone help with the specific NEC chapter and verse that says so?

I'm still trying to understand grounding - so thanks for your help!
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Post by sparky » Mon Sep 25, 2006 6:15 pm

Just went thru this with help here from Kent ! To meet current building code which may or may not be accepted in your area do this

#1 the only connection from the negative on a negative bond system is at the DC side of the inverter where it goes to ground.

#2 run a ground from all panel frames, the pole, and the PSPV to ground bar at the array.

#3 run a ground wire from the Array to the DC inverter ground at the house

That is it in a nutshell.

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Post by Kent Osterberg » Mon Sep 25, 2006 9:19 pm

Martin,

Take a look at 2005 NEC 250.164 (B) it specifically says "a grounding connection shall be made at one of the following ..."

What you are proposing may have been okay in the 1999 code, but not now. The intent is for the dc system to be connected to earth at one point.

When the dc negative is connected to earth at two points, some dc current will follow the parallel path through the earth. This causes electrolysis of the grounding system that will slowly deteriorate it.

The only way to ground the remote PV array and keep it "to code" is to run a dc grounding conductor between the PV array and the rest of the system.

For a long circuit, I'd put a surge arrestor at each end.

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Post by VolvoFarmer » Tue Sep 26, 2006 4:31 am

Kent Osterberg wrote:
The only way to ground the remote PV array and keep it "to code" is to run a dc grounding conductor between the PV array and the rest of the system.
Yeah, I got called on this one on my inspection. My array is approximately 80 feet from my power shed. I need to pull the wires out of the conduit and add a ground. So... any suggestions as to the wire size needed to bond the array ground to the power shed ground? Is #8 overkill? This wire needs to be insulated right? Is a bare ground wire in the conduit a no-no?

<edit> I just re-read the thread. My conductors are #6, so #12 is okay for the ground? Still unclear on bare vs. insulated.

Thanks for all the code help!
Thanks Outback!
Bob W.

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Post by sparky » Tue Sep 26, 2006 5:47 am

Bare or insulated ground are fine but the some inspectors want to see insulated and insulated stranded is easiest to snake thru conduit. Your #8 is fine but if you have the money and room in the conduit use #6.

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Post by DavidB » Tue Sep 26, 2006 7:43 am

Help me understand this then:
If you have the situation where top-of-pole mount array is 200+ft away
  • - The negative and ground(connected to ground rod at pole) are both tied together at the PSPV
    - There is no ground wire connecting ground at PSPV to ground at PSDC
    - The negative and ground are also tied together at the PSDC
Why the need for separate ground wire from PSPV to PSDC then?

I'm reciting this from memory the last time I looked in the PSPV so might be wrong in my description :roll:

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Post by Kent Osterberg » Tue Sep 26, 2006 8:19 am

VolvoFarmer,

You should use a #10 ground wire for #10, #8 or #6 conductors. I think either bare or insulated is okay, but I usually use insulated in conduit, especially if it underground.


DavidB,


If you have the situation where top-of-pole mount array is 200+ft away

- The negative and ground(connected to ground rod at pole) are both tied together at the PSPV
No, the negative and ground should only be connected at the PSDC!
- There is no ground wire connecting ground at PSPV to ground at PSDC
No, you do need a ground wire with the conductors.
- The negative and ground are also tied together at the PSDC
This should be the only place that negative and ground are connected.
Last edited by Kent Osterberg on Tue Sep 26, 2006 8:26 am, edited 1 time in total.

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Post by Kent Osterberg » Tue Sep 26, 2006 8:22 am

Deleted duplicate post.

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Post by Martin » Tue Sep 26, 2006 10:59 am

Thanks Kent and Sparky for your answers! I was hoping that there was an exception that would let me go with what I've already got installed, but if I have to run an extra ground wire, it's not the end of the world. Copper wire sure is getting expensive these days, though.... :sad:
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Post by Martin » Tue Sep 26, 2006 10:59 am

Thanks Kent and Sparky for your answers! I was hoping that there was an exception that would let me go with what I've already got installed, but if I have to run an extra ground wire, it's not the end of the world. Copper wire sure is getting expensive these days, though.... :sad:
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