Need to know (and understand) grounding the system

showme · January 2022

As I whittle down the list of things that confuse me, I need to ask about grounding this set up. Someone mentioned in one of my previous posts that grounding had not been mentioned yet. My grounding knowledge is pretty limited from some of the posts I've read on this forum, so bear with me. My experience with grounding is with the main ground rod in the half dozen re-hab houses my wife and I have worked on and my 40 years as a welder. That's basically grounding my work to with the machine's ground lead, and rarely grounding the machine to earth ground if it's an indoor grid fed welder.

Though I thought I knew 'enough' before I started this project, I'm apparently pretty ignorant of the subject. Up until now, I thought I'd need to have a ground rod at each of the arrays, one at the charge controllers that would also ground the batteries, and then somehow ground the inverter, disconnects and breakers to that. From what I've read here, I'm way off. So I re-read the manual for my Trace SW5548 inverters, DC disconnect and AC disconnect. It hasn't helped much. So, with my system of 7680w of panels, two combiner boxes for the arrays, two MS Classic 150's, two Trace Xantrex SW5548 inverters, a Trace DC disconnect and a trace T-240 AC disconnect, what do I need per grounding?? My head is spinning (and aching) from what I've read about different types of grounding, neutral and "bonded/non-bonded ground to neutral". Slowly but surely I'm catching up on this stuff, but it ain't easy for a geezer like me. (And I thought I was pretty intelligent- until I got here!) Thanks for any help on this subject. Please be aware that I'm not a master electrician. Just an old Ironworker. Lee

MJSullivan56 · January 2022

Assuming a grid tied system (I.e.not off grid) then the only place you bond the neutral with the ground is at the main service panel. Everywhere else you keep them separate. Your solar panel frames should be grounded to earth no different than a lightning protector. By logic (but not by code ironically, go figure), this should share the same grounding rod as the main service panel.

BB. · January 2022

I am guessing that an Ozark Hill top is a relatively prime target for lightning strikes?

https://duckduckgo.com/?q=Ozark+lightning+strike&ia=web

So grounding and lightning discussions are more than just an interesting read (???)...

There are lots of issues with grounding in these cases... The basics are outside metal structures (array racking, etc.) should have a minimum 6 AWG cable from solar panel frames/metal structure straight to earth ground (if array is mounted on home, lightning grounds run on the outside of the building to outside the foundation--You don't want to bring lightning into your home, and lightning,because of magnetic fields, tends to "force" the strike current away from the "center" to the outside edges of the home/wiring/etc.). Straight runs, no "sharp turns". You want to send lightning to earth. (Just one element of the multiple design guidelines for lightning control).

Regarding what MJ said... The concept of "single point" grounding your power system is (mostly) to ensure that all of your load currents flow through the load wiring (120/240 VAC L1/L2/Neutral, even 12/24/48 VDC Hot and Return current flow) and no sustained current flow through green wire safety grounds.

When you tie 120/240 VAC Neutral to the Ground bus in your main AC panel... This is so that L1/L2/N current only flows in the Load Wiring. No current flows in Green Wire/grounds except for "fault current". In the case of a fault, that "single point" ground allows a "short circuit" from L1/L2 to grounded metal (electrical box, kitchen sink, plumbing, furnace, solar racking) to green wire safety ground--Back to G+N bond and back to Neutral to carry the fault current back to the source and trip the L1/L2 circuit breaker.

This is also typically done for 12/24/48 VDC Hot (+), Return (-), and Return tied to safety ground bus (typically the common ground rod). Again, this allows Hot fault current to safety ground to Return (-) and trip the Hot fuse/breaker. The single common ground point (can be one "master ground" rod, or a DC and AC local ground rod tied with 6 AWG cable--Takes care of the basics. Local direct grounding for lightning, and 6 AWG cable stitching local ground rod to local ground rod (i.e., home to racking to well pump house, etc.) does the basics for safety.

Now--The idea of "separate grounds" vs all grounds "tied together"... Tying all the grounds together (main ground rod to lightning rod for array, etc.) with 6 AWG wire (minimum) allows any short circuit (say you have a security current 120 VAC L1 shorts to ebox which is tied to array racking--The 6 AWG wire from Array ground rod back to Home ground rod, carries the fault current back to the main panel and trips the circuit breaker for the security light branch circuit.

Lightning itself will only "follow" 6 AWG wiring for a few 10's of feet before it finds a "better path" to earth (lower impedance).

Then there is lightning protection for your 120/240 VAC circuits and yoru 12/24/48 VDC circuits. Generally placing appropriate surge suppressors at the "edge of the home" (such as solar array wiring into home) and inside the AC main panel (hopefully also on an exterior wall with ground rod "outside" the foundation. Example of some very nice surge suppressors:

https://www.solar-electric.com/search/?q=surge+suppressor

The idea being that any high voltage surge (lightning) goes through the suppressor to the local/main ground rod and helps protect your AC mains, AC inverter, genset, DC wiring.

There is a lot more to the discussion (lighting arrestors, types of grounding, and one member here in lightning capital Florida would disconnect his array DC wiring from the home and move the plug&cord at least 10 feet from home if lightning is forecast)--And here is more information:

https://forum.solar-electric.com/discussion/comment/84242#Comment_84242

Your 60 HZ VAC and DC currents follow wiring as expected (current follows the lowest resistance path).

Lightning is an "RF" event (radio frequency). And it follows the lowest Impedance path... That is a combination of Resistance (Ohms) and inductance+capacitance. For example, a sharp right angle bend--The (upwards of 7,000 Hz lightning current) will see the bend as "high impedance" and leave the bend and find a better path to "earth". Flat braided cable has a lower impedance than a single strand solid copper wire (lots of math and electrical engineering stuff involved).

Also, don't think of a ground rod as anything "magical". It is not a direct short circuit to "earth". A "good ground rod" insulation can have upwards of 25 Ohms of resistance from rod to "earth". Short 120 VAC L1 to that, your current flow is:

120 VAC / 25 Ohms = 4.8 Amps

Not even enough current flow to trip a 15 Amp breaker in your branch circuit wiring.

"Ground" is more or an "idea" for 120/240 VAC wiring. The "actual safety grounding" is tying all the metal in the home (ebox, plumbing, etc.) together and single point bond to Neutral (and DC return for solar) so give you low resistance and the ability to carry high current to "fault" a breaker or fuse.

Earth grounding is not a player in conducting "low voltage" current--It is there, mostly, for absorbing lightning energy if a nearby strike occurs (direct strike--Almost impossible to limit damage to electrical circuits).

There are other things ground is involved in: Cathodic protection, better radio wave propagation (from antenna), and things like spark ignition for gas flames (stove, water heater), starting tube type fluorescent fixtures.

What are your needs and concerns?

-Bill

showme · January 2022

Wow, that's a lot of info. MJSullivan- it's an off grid set up. The arrays are top of pole, away from the 'future' house. The house will be an earth home with soil surrounding all but the south face.

BB-give me the morning to try to absorb all this. It's good information. So, are you saying then that I SHOULD ground the arrays to their own rods, or no? It sounds like that's what you mean, but I'm not sure. I've got 6 gauge solid copper on the way for grounding the two arrays (30' apart) and whatever else I may need it for. Thanks for the heads up on not using 90* bends. I would have tried to keep it neat, as in plumbing and indoor electric wire, so I'll add some broad bends to those spots. Aside from that, the technical information is great, but I still don't know where to put the ground rods and how many (or few) are needed, total. I'll make a drawing of what I've got and try to post it on here. Thanks again. This subject just confuses me to no end. Lee

mike95490 · January 2022

Often, a sturdy pole mount in concrete, can become it's own Ufer ground. That's what both of mine are. There's some details on arranging the rebar and tieing it together, but that's the way I like it. Distances are too far at my site to safely daisy chain all the possible ground sites

BB. · January 2022

Ground the arrays' frames/racking to local ground rod (or UFER as Mike says--Basically rebar in concrete).

And run 6 AWG ground wire from "remote grounds" to "master ground rod/point" in home.

Local grounding for lightning control... Ground wire back to "power source" to prevent electrifying metal structures (such as security light metal pole, solar racking). A short from "hot" lead to racking and somebody walks by on wet ground/tall wet grass and "touches" energized metal and gets shock/electrocuted.

-Bill

showme · January 2022

Thanks, Mike. I've got 3'x4' rebar mats at 4-5' below ground level (where i hit a rock plate), then a 32" rebar cage tied to that inside the sonotube form, with a couple of stabilizing rods tied from the cage across the 8" pipe, making contact there. Since I'm running the 6g ground wire and have to come down the pole with conduit, I'm running one for each array, since this is how the inverter manuals say to do it. I'm just not sure if I'm suppose to ground them seperately at the array or send them onto the dc disconnect/charge controllers and ground them with the conrollers/batteries/inverter. I'm not sure where most people doing their own installs are finding this important information, but it seems to have been left out of the many books and articles i've researched. BB gave a great in-depth explanation on grounding, but I'm still confused as to where I place my grounds.

showme · January 2022

Ok, BB, I've got some questions concerning electrical vocabulary that I'm not sure of.

What do the terms "local ground rod", "remote ground rod" and "master ground rod/point in home" mean? I'm guessing the master is the ground rod for the service panel/power panel at the house (which will be the shed till the house is built)?

"power source"? The array and the generator, I'm guessing, again.

I've been under the impression that, although grounding is necessary as close to a possible problem point, that there is a possibility of 'too many' grounding points(??). I've also read somewhere that all grounds are to be tied together, which to me sounds like I may have hundreds of feet of ground wire between the arrays and the future house structure (270' away), the shop I intend to build, which is where I want to position my back up generator/welder and possibly the Bergey wind generator, which will be a couple hundred feet beyond the house from the array. Where do I find the reference material to find these things? I've looked everywhere, but you and this forum are the only source I'm finding so far. How can so many people do this with such limited information available?

mike95490 · January 2022

The frames of the solar panels gets grounded to the rack/pole/concrete
the power leads of the panels ( - + ) go to your power system. Usually.

BB. · January 2022

Look at it this way... A ground rod at every location where lightning strikes can be an issue (the tower for the wind turbine, the ground mount solar array, your AC main panel+surge suppressors, well pump house, generator frame/chassis, etc.). This is (mostly) to direct lightning energy to earth and away from your electrical wiring.

Usually, for a home--The main AC panel is where Utility Power comes into the home, with N+G bonding to a ground rod outside/next to your foundation (don't bring lightning into the home).

At this ground rod, you will also have 6 AWG cable to your hot and cold water pipes (typically at the water heater)--And this days, natural gas and propane piping will also be grounded to the "master" ground rod of your home (every home this days has one, or NEC will require a new ground rod when remodeling/etc.).

When I talk about "local" ground rod... That means the ground rod next to the thing needing lightning grounding.

"Remote ground rods" are those ground rods not at the AC main panel (remote from the "master/main panel ground rod).

"Master Ground Rod" is not a proper technical term... In Central Office for the phone company--They have something called the "Ground Window" were all green wire grounding wires go to (this is the "building ground"). That ground run next to your AC main panel--Is this "ground window" for your home. The N+G from the main panel goes to this rod. The Hot/Cold water pipes/gas pipe ground/etc. all go to this ground rod. As well as your DC Battery Return (typically - negative battery bus) goes here too.

The rest of the grounds in your property that receive power from your home (AC mains panel, inverter, etc.) such as the pump house, shop/garage/"remote ground rods" have their (one or more) 6 AWG cables terminating at the "master rod/ground window" for your AC panel. This way, if there is a 120/240 VAC or 12/24/48/Solar panel wiring VDC short to tower/racking/pump house electrical box/etc., the return current follow the 6 AWG safety ground back to your Mast/Ground Window and then back to the AC or DC panel and trips the breaker/fuse to prevent energizing the "remote" metal and causing shock or electrocution.

For AC and DC power wiring, we talk about single point grounding so that AC or DC load current does not flow through both the return (neutral/white wire for AC power, negative cable for DC) and the green wire safety ground (current flow required "two points" on wiring--One common N+G bond--No parallel current paths).

Multi-point grounding--Is the answer for Lighting and Radio Frequency energy grounding. At each location where lightning can strike (wind turbine tower, Radio Antenna, etc.)--Each has its own ground rod to take energy to earth.

A quick read on "fixing" an AM Radio station that had lots issues with grounding and lightning... Probably overkill for a home--But the ideas on grounding are pretty clear.

https://www.copper.org/publications/pub_list/pdf/a6189-wtmn.pdf

-Bill

Need to know (and understand) grounding the system

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