Yet Another PV Panel Grounding Question

This one is a little different as it involves two disparate panel locations.

I'm upgrading an off-grid system from 1 to 5 kW.  Most of the new panels (three strings) have to go on the roof due to rough terrain (no place for ground mounting).  There is one existing ground mount tracker array (An old WatSun with the tracking disabled) which I intend to put the other two strings on.

For the roof strings I'll do a exterior down rod to the house GE from the racking and a EGC (EBC) in conduit with the PV wiring to the roof combiner.  The combiner box ground will not be tied to the racking at the roof.  Trying to keep the lightning outside the house.

The issue involves the old tracker array which is about 100 feet from the house GE.  It has its own ground rod from a prior installation.  My thought is to ignore that ground rod and run what is essentially a down rod to the house GE from the array frame. (the array is actually a little higher elevation than the roof racks).  Then in similar fashion I'll run a EGC (EBC) to the combiner at the array which would be isolated from the down rod, as is the case with the roof.

Here's the question.  The tracker pipe mast is anchored in a big chunk of concrete with rebar which goes down maybe 6'.  It probably comprises a poor grounding electrode (Watsun instructions say it is not a good grounding electrode and to sink a proper ground bar).  It is impossible to isolate the array frame from the mast and anchor so there will be some grounding from the mast and anchor.  So, is it be better to ignore the tracker ground bar and just tie the array frame to the house GE or to tie the tracker array to its local ground bar and then tie that to the house GE?  My thought is that if there is a nearby lightning strike the ground voltage differential on the tie to the house GE would be less from the mast and anchor than from the ground bar.  Thoughts?

Thanks in advance.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,452 admin
    Welcome to the forum BillD,

    Before I try to get into specifics, just a general suggestion.

    Lightning grounds, they should be short runs of 6 AWG wiring on the outside of walls to a ground rod at the base. No sharp turns, but nice rounded turns instead (lighting tends to "jump" from wiring at sharp turns). You want multiple lightning grounds (one at the base of each structure to be protected). Lighting will not go more than a few 10's of feet down a 6 AWG cable before trying to find a different low impedance path. Lightning is a "radio frequency" current (very low frequency RF). So the "impedance" (inductance/capacitance & resistance) that affects the path lightning will follow--Not just resistance.

    For safety grounding, you want to tie all structures and power sources/electrical boxes/metal conduit together with (typically) a 6 AWG minimum cable... This is so that any short from "hot" to ground will not energize a structure, but will find a low resistance current path back to the "source" (main panel, fuses/breakers, etc.) so that energized structure will trip the breaker/fuse/etc. That generally means that each ground rod should be connected back to the main panel/ground via the 6 AWG cable (solid 6 AWG if exposed/direct burial, stranded OK if protected in conduit). For example you don't want a rack mount or light pole to become hot (solar panels, AC power to outlet/yard flood light/etc.) where somebody could grab the metal and get electrocuted through wet grass/puddle/water pipe/properly grounded metal electrical box. The minimum resistance of a ground rod to earth is 25 Ohms. A short to ground of 120 VAC is not usually enough to trip a breaker (I=V/R=120v/25ohms=4.8 amps of current). That is why the AC safety ground wire should be "stitched" to all outside structures/boxes/well heads/etc.

    "The combiner box ground will not be tied to the racking at the roof."

    My question is what do you mean by "combiner box ground"? Do you mean the metal box will not be tied to a local ground rod? Or do you mean the negative DC solar terminal--Negative ground--will not be tied to the ground rod?

    In general, you want to have grounds (chassis/box/structure) and power cables referenced to local ground rod. Because you cannot directly tie hot and DC power cables to grounds, usually some sort of surge suppressor is used instead:

    https://www.solar-electric.com/search/?q=surge+supressor (suggest looking at Midnite brand both AC and DC voltage units available--Not a spark gap, but MOV based).

    So, for your solar power, you would tie the combiner box sheet metal to structure+ground rod cable... And use a Surge Suppressor between the + and - solar power cables and local ground rod. Allows DC power to be "isolated" but high voltage lightning energy to be sent directly to local ground rod. And, there should be a 6 AWG cable from local ground rod back to the earth+safety ground connection back at the home (ground rod next to exterior of walls--As you say, don't want to bring lightning energy into the home).

    By itself, a concrete foundation and rebar can be an "acceptable grounding" system. UFER:

    https://www.ecmag.com/section/codes-standards/what-ufer-ground-concrete-encased-grounding-electrodes

    For lightning, there are lots of advantages to multi-point grounding (ground rod(s), UFER, metal cold water pipe, well casing, etc.)--You want to give the lightning the best path to local earth ground. Plus a 6 AWG back to the home main ground rod+AC safety ground connections (for DC and AC power/short protection).

    With a direct lightning strike--Probably anything subjected to a direct strike will need to be replaced. As you say, keeping it from entering the home is the best protection for safety.

    Using Surge Suppressors at the Combiner box (the array location), and another suppressor at the house wall where the array cables enter the home.

    Grounding the Battery DC negative bus (6 AWG minimum) to the local (outside wall) ground rod (sometimes "buried ground plates" are an option (cold water pipe/propane pipe.etc, tied together).

    And AC surge suppressor(s) at the AC main panel. And main panel safety ground/neutral bond (normally single point neutral/AC White wire tied to green wire ground in main panel--And run 6 AWG minimum from panel safety ground to house ground rod).

    As a very rough answer--Turns out that AC inverter output stage is more typical lighting induced failure than the DC side of off grid power systems (from WindSun, the original founder of our forum here and one of the owners of NAWS--Our host). So installing surge suppressors on the AC panel (and/or AC inverter AC output) is a good idea. All of the AC wiring in the home is susceptible to lightning too.

    Keeping lightning from hitting your system is good (lightning rods). One former poster here would run the last 10 feet from the array to the home with outdoor cordage... And unplug the array from his home if lightning was expected (at least 10 feet between array cable and home). Obviously, you do not unplug in the middle of a lightning storm.

    The issue of Neutral+Earth Ground bonding (and grounding in general) is not a simple answer... For example, many larger power sources (more than 3,500 Watt output AC inverters and Gensets) may default from the factory to have AC Neutral+Frame ground connection made internally to inverter/genset. Multipoint grounding of AC neutral+safety ground is not desired. The "parallel" connection(s) of green+white wire means that your safety ground is carrying part of the AC load current (not good). If you have ground fault breakers at the inverter or genset, this will trip the GFI breakers.

    However, in systems with multiple buildings (such as homes served by a pole transformer) or remote well shack/shop/etc... Sometimes (for lightning control) having neutral+ground rod bonds at the remote facilities is standard (and local surge suppressors a good idea). For example, you have 5 homes and 6 neutral+earth bonds (one at the pole transformer, and one at each home). The current will go mostly through the neutral connection, with little current through the earth itself (remember upwards of 25 Ohms from ground rod to earth vs a faction of an Ohm for electrical wiring).

    This post has links to multiple lightning FAQs and posts:

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

    I hope this helps. This is a complex subject and lots of "it depends" answers. Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BillDBillD Registered Users Posts: 4
    Bill,

    Thanks for the lengthy reply.  I have already installed a Magnum MS4448PAE, with a Magnum Mini Panel and PT-100.  These are all installed per Magnum's instructions and properly single point bonded with #6.  I have Midnight Solar surge suppressors on the AC in and out and DC In.  The DC is all floating except for a single point ground at the PT-100 GFDI.  I intend to install DC suppressors at the rooftop and array combiner boxes. Based on what you are saying above it seems like the suppressors are best connected to the down rod, not the EBC (EGC) (see below).  Unless I'm talked out of it I intend to tie everything into the single house ground bar.  Conveniently this is actually the shortest path to the earth from the location of the roof array.  However, based on what you are saying, there is merit in using the ground bar at the array and tying it to the house ground bar.  As I said, I'm concerned about ground differential from a local strike, unless the tie would in effect eliminate it.  Seems like it would be better to keep the array more like floating.  Is one approach superior to your mind?

    My big concern was the possibility that the array mast was effectively a Grounding Electrode and I didn't like the idea of tying two ground bars together which are 100' apart.  But I might be wrong about this.  So I just had the bright idea of disconnecting the mast bond to the ground bar and measuring the resistance between the two. About 3.5K, and the ground is damp from recent snow. So, I'm thinking the mast anchor really isn't a Ufer. So if I disconnect the mast from the ground bar and run a down rod (GEC) from the array frame to the house Grounding Electrode for lightning protection. The high resistance between the mast and the bar means I'm really not tying two ground bars together which are 100 feet apart, which bothers me for some reason. So, the situation at the array is essentially that same as on the roof. I'll run an Equipment Bonding Conductor (formerly EGC) in the raceway to the Combiner Box at the array and on the roof. However, I will not connect the Down Rod (GEC) and the EBC together at the Combiners. I'm thinking this would provide a path for lightning into the house through the EBC. Think I'm wrong here?  If they are tied together can I count on the lightning always taking the lowest impedance path?  I have seen conflicting information about this.  Some say it will take all paths, while others say it will take the lowest impedance path.  If the EBC and GEC are not connected and there is a short on the array the OCPD should still function properly due to the GEC.

    I'm using #4 for the down rod. Depending on the standard you consult, the down rod gauge for lightning protection ranges from #6 to #2. The Iron Ridge grounding clamps only go to #4, which seems a reasonable compromise. Good tip on avoiding tight radius bends on the down rods from the roof and the array.  I had not seen this mentioned in other posts.  They will both be run on the exterior of the building, away from any conductors, like EMT or pipe..

    For anybody who's interested, this approach stems from this post:
    https://www.solarpaneltalk.com/forum...d-gec-required
    and several others which indicate that a rooftop array is really an expensive Lighting Rod (Air Terminal) and should be treated as such, with a down rod connection to a ground bar.  Using the system Grounding Electrode provides lightning protection and also satisfies code.  As is pointed out in the video in the linked post, NEC 690.47(d) which specifies an independent grounding electrode for roof arrays is wrong and dangerous.

    Thanks again for the help.




  • BillDBillD Registered Users Posts: 4
    edited December 2020 #4
    Had some difficulties with posting as I didn't realize posts had to be approved.  Sorry for the confusion.

    After reading this:
    I'm now of the opinion that the tracker array should be bonded to it's ground rod and that ground rod connected to the house ground rod.

    So my big question now is whether a EBC (EGC) at the combiner box should be tied to the GEC from the array at the roof, if this makes any sense.

    Thanks Again.
  • BB.BB. Super Moderators, Administrators Posts: 31,452 admin
    BillD,

    Remember I try to give reasons why A/B/C... I am not an expert by any means. Just telling you my understand of the issues. Code/Others may have differing opinions and requirements (NEC was really not great for lightning control in decades past... Have not look recently to know how much, if any, has changed).
    Thanks for the lengthy reply.  I have already installed a Magnum MS4448PAE, with a Magnum Mini Panel and PT-100.  These are all installed per Magnum's instructions and properly single point bonded with #6.  I have Midnight Solar surge suppressors on the AC in and out and DC In.  The DC is all floating except for a single point ground at the PT-100 GFDI.  I intend to install DC suppressors at the rooftop and array combiner boxes. Based on what you are saying above it seems like the suppressors are best connected to the down rod, not the EBC (EGC) (see below).  Unless I'm talked out of it I intend to tie everything into the single house ground bar.  Conveniently this is actually the shortest path to the earth from the location of the roof array.  However, based on what you are saying, there is merit in using the ground bar at the array and tying it to the house ground bar.  As I said, I'm concerned about ground differential from a local strike, unless the tie would in effect eliminate it.  Seems like it would be better to keep the array more like floating.  Is one approach superior to your mind?
    If I understand you correctly... Generally the DC "ground bonding" takes place on the Battery Bank negative (traditional) connection. If you ground it at the negative battery terminal of the PT-100... I guess that could work fine. I like to make the battery the heart/center of the systems. Placing a ground elsewhere can be confusing for others working on the system.

    Lightning wants the most direct path to the earth. If you home rod is close and the wiring is very short run, then it should be OK.

    What you want is all the stuff in the home to be at the same voltage ideally (think Faraday Cage--Inside the cage, all surfaces are at the same voltage--Inside a Faraday Cage, you cannot tell if the cage is at zero volts or a million volts.

    Of course, if you put two probes in the ground a 100' apart--A lightning strike will produce a voltage gradient. And there is nothing you can really do about that.

    Where there has been issues with voltage gradients (besides lightning)--A power line short to the pole/tower (buried cables, etc.). This creates a gradient around the tower... And you get an animal such as a cow--With their legs "at the corners" of a cow, that can direct stray current right through the cow's heart and kill it.

    https://stopthesethings.com/2019/05/03/french-farmers-re-volt-cattle-killed-by-stray-current-latest-victims-of-wind-power-onslaught/
    https://www.progressivedairy.com/topics/management/stray-voltage-affects-cow-behavior-milk-production
    So I had the bright idea of disconnecting the mast bond to the adjacent ground bar and measuring the resistance between the two. About 3.5K, and the ground is damp from recent snow. So, I'm thinking the mast anchor really isn't a Ufer. I'll run an Equipment Bonding Conductor (formerly EGC) in the raceway to the Combiner Box at the array.  I'll run a down rod from the array to the premises ground rod.  My major question at this point is whether the EBC (EGC) to the combiner should be tied to the array down rod at the array.  This applies to the roof array as well.  I think not as this would provide a path for lightning into the house.  Your Thoughts?
    Measuring ground resistance is sort of difficult... Dry vs wet, rock vs soil, etc... But lightning that just traveled 1-2+ miles through the sky is not going to be deterred by a few feet of concrete.

    Lightning will not travel more than 10-20 feet or so down a 6 AWG cable before it finds a different path to ground (AC impedance vs DC resistance).

    Tying the two ground rods (and grounding systems together) between the home and the array (or other outbuildings) is to prevent voltage gradient issues for DC (solar/battery power), and AC (60/50 Hz house power).

    I have added some links below for further light reading:

    -Bill

    How to measure soil resistance (in 50 pages):


    Multipoint grounding solution for Florida AM radio stations and lightning:


    HAM Radio Grounding presentation:


    Lightning and lightning control:



    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BillDBillD Registered Users Posts: 4
    Hi Bill,

    All very interesting.  By following the Magnum manuals exactly someone else working on the system should have good documentation of the installation.  After reading your links, my #4 solid down rod seems a little puny.  The #6 recommendation found many places must come from the NEC minimum GEC which is really not the same as a down rod.  The Fla references use 4/0 stranded!

    So I'm convinced it is better to use the ground rod at the tracker array and connect it to the house ground rod with my #4.  I'll also connect the combiner EBC (EGC) to the down rod at both arrays.  The EBC is #10 stranded in EMT so the lighting is not going to want to get in there very far.  Plus there's surge protectors.

    So I'm under the happy illusion that I can proceed.  As to whether I know what I'm doing, that's another question entirely.  The variability of information on this stuff is surprising.  The good news is the site is relatively protected from lightning, having higher ridges nearby (a few hundred yards) to the East and West.  There hasn't been a very close strike in the 20 years or so the house had been up, that we know of anyway.

    There are a few peculiarities in the current 20 year old system.  The tracker array is cabled into the house mech room with 3-10 SJ, ground surface mounted, no raceway.  The ground wire in the SJ is disconnected.  The disconnect at the tracker array had apparently failed and was wired around (I put in a new one temporarily and it will be rewired with jacketed MC).  The tracker ground rod was connected with #6 solid only to another ground rod another 100 feet from the house in a different direction, near a shed that originally held the inverter when the house was being built.  My partner built the house and all this work was done well before I was on the scene.  I've wanted to clean it up since we met.

    FYI I'm a computer engineer, so am used to fooling with electrons, just not this many at the same time!

    Thanks Again.
  • BB.BB. Super Moderators, Administrators Posts: 31,452 admin
    Yep, you got it... There is minimum, and built like a tank. 4/0 stranded is more tank-like.

    Lighting is RF (radio frequency) energy... Granted is is VLF (very low frequency) of around 7,000 to 10,000 Hz maximum frequency.

    It is not that Lightning is AC (although, I guess that there can be current going back and forth like an inductor+capacitor oscillator), but it is the "impulse" of the lightning that make it have lots of higher frequency content.

    If you want the "Math":


    For lightning control wiring--You need to use the "complex impedance" model of electrical circuits, skin effect, etc. plus just a lot of copper to carry the current.


    It is pretty amazing, but the skin effect becomes a limiting effect even at 60 Hz and 4/0 cable (roughly).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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