Ground rods for pole mount arrays

jtdiesel65jtdiesel65 Solar Expert Posts: 239 ✭✭✭
Hi,
What is the consensus on ground rods for pole mount arrays given that the pole is essentially a huge ground rod? In my case 8" pipe with 7 feet in concrete. The poles are >100 ft from charge controller/inverter/main ground.
If I'm looking at the right thing, looks like NEC now says the array should be tied to main panel ground at charge controller and installing a direct ground at the pole is optional. Is that correct?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    More or less, you are looking at "UFER" ground (using concrete and rebar to provide grounding). A couple of links to start:

    https://forum.solar-electric.com/discussion/2650/ufer-grounding
    https://forum.solar-electric.com/discussion/22466/solar-array-and-grounding/p1
    https://forum.solar-electric.com/discussion/4506/off-grid-grounding-technique/p1 (difficult lightning grounding--Almost no top soil)

    Unless you have 20 feet of 1/2 Rebar, each connected with cable to the other rods/central pipe/support--Not sure that really qualifies as an "UFER" ground.

    You probably would still be better off with a ground rod driven into the ground (or buried ground plate--Another solution).

    NEC (at least older versions) are not really strong on lightning grounding... More or less, DC and 60 Hz AC power, the current follows the copper cables. And the NEC uses "grounding" to pass enough current to pop the circuit breaker at the main panel (i.e., >15 amps for 14 AWG house circuit).

    For example, say you run a 120 VAC circuit from the house panel to a metal structure in the yard (lights, solar racking 120 VAC outlet, pump house, etc.)... You would (my take) run a 6 AWG ground cable from the main panel to the "metal things" in the yard. If there is a short from Hot to Frame, the return current will run down the 6 AWG back to the home where Neutral+Ground bonds are tide in main panel--And trip the 15 amp breaker.

    A ground rod would not do this... For example, the NEC says a "good ground" is 25 Ohms or less from soil to ground rod.. At 120 VAC, that would carry:
    • 120 VAC / 25 Ohms (worst case "good ground" = 4.8 Amps
    Not enough to pop a 15 amp breaker--And could "energize" a metal structure and electrocute somebody (120 VAC on structure, wet grass or puddles, wet feet, hand on structure, electrocution).

    That is why carry a 6 AWG (minimum) grounding cable from main panel to "remote metal thing".

    For lightning, it is not a DC or low frequency AC signal. Is is Radio Frequency current (very low frequency radio of something like 7,000 Hz or lower)--But because it is RF, current flow follows different rules... We talk about resistance (copper wire Ohms). But for RF we need to talk about "Impedance" or complex resistance... the DC resistance + Inductance + Capacitance of the cable. For a simple 6 AWG cable, the lightning may follow the cable for a maximum of 10-20 feet and then leave the cable to jump to ground (or something with lower impedance). Or if you have a sharp right angle bend in the copper wire, the impedance of the bend will cause the lightning to jump elsewhere.

    So that is why I suggest to do "both"... Ground wire back to main panel (DC and 120/240 VAC safety grounding) and local ground rods for remote structures... And to tie the local/remote ground rods with 6 AWG cable back to the main house ground rod (ground rods should always be "outside" the walls... You don't want to bring lightning energy into a home/building).

    If you have chances of nearby lightning strikes, then you should also look at using surge suppressors at various key points to direct lighting to ground (such as bringing in solar array wires to home, surge suppressors at wall entry point, suppressors on 120/240 VAC main panel, etc.).

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

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jtdiesel65jtdiesel65 Solar Expert Posts: 239 ✭✭✭
    edited September 14 #3
    Thanks!
    I find it confusing. IIRC, NEC used to want only one ground rod to avoid ground loops. But then I guess if the second ground rod is far enough away then loops may not happen.
    My neutral/ground bond is in the generator.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    I have not look at the NEC in detail for a couple of decades... But, the general requirement is to prevent ground loops "inside the home".

    And that is done by a single point grounding between Neutral and Safety Ground (in your generator--In this case).

    What this "single point" ground tie is trying to prevent is parallel current paths. For example, in your case the genset power all flows in L1/L2/N wires... And the only time "significant' current flows in the green wire safety ground is if there is a short circuit between Lx and safety ground (green wire ground, grounded chassis, grounded j-box, etc.). The green wire prevents the metal "thing" (box, drill motor chassis, etc.) from becoming energized and shocking/electrocuting somebody.

    If, for example, you have a N+G in the genset, and a N+G bond in your main breaker panel (in the house)... Then you have parallel current flow shared between Neutral and Green Wire safety ground. And we don't want load currents in Green Wire grounds (for example a failed neutral connection--And all current now flows in green wire ground--And no indication of failed Neutral Power wire). There are other secondary issues (such as if some AC current flows through knockout in electrical box Lx+N, and some current flows through Green wire/conduit--AC current flowing in "one hole", and out another "hole" creates eddy currents in the j-box sheet metal and can even overheat it). It can also trip (for example) a GFI outlet/breaker on the genset (shared current flow between Neutral--good current flow, and Green Wire--bad current flow).

    We talk about only one N+G bond in a home... But the reality is that a typical neighborhood will have multiple N+G bonds... One at the pole transformer and one at each of the (for example) 5 homes attached to the that same pole transformer (6x N+G bonds total).

    In general, we see multiple Ohm resistance (up to 25 Ohms per ground rod)--Or may even see less resistance (everybody tied Neutral to copper water pipes). You will see some current flow because of voltage drop from pole drops and some sort of flow through ground and water pipes, etc... And we just don't worry about it.

    There was one example where somebody connect L1 (or L2) of their 120/240 VAC Hot wire to the ground rod (mistake). Nobody noticed any issues until one winter with snow on the ground--And noticed that the snow was melting around the ground rod for some reason. That was the first indication that they had messed up the N+G bonding connection. Note--Even in this case, GFI/Ground Fault breakers would not "see" a problem... All they care about is that the current on the L+N is equal--Which it was (no current on green wire to device).

    In the end--Ground Rods are (mostly) there for Lightning control (get lightning energy to earth as quickly as possible and away from the home).

    Don't even assume that "ground" is "ground"... I have seen large office/industrial buildings will multiple volts difference between grounds inside the building--Enough difference that the current flow between "grounds" would blow traces on terminal circuit boards (digital signal ground traces).

    And at a oceanarium... Had two ground rods 80 feet apart... One in an outbuilding, another near a saltwater tank. I got a hefty shock when I touched both grounds (>60 VAC between two ground rods). The large 3 phase 240/480 VAC pumps and the salt water in the earth--Made for some good size leakage current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    You need to have some rebar in the concrete.   My concrete guy built a rebar cage, and welded it to the Pole mount, then poured the concrete.   If there is more than a couple inches of concrete between the pole and the dirt, the concrete, while ion rich, can explode from steam build up from a lightning strike.  So you wan the rebar close to the edge of the concrete, but never poking though and touching dirt..  A rusting piece of rebar will destroy the concrete over a couple years, as the rebar rusts and the flakes expand.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Dave AngeliniDave Angelini Solar Expert Posts: 6,031 ✭✭✭✭✭
    edited September 15 #6
    And in Mikes concrete cage tour de force, the pole is ready to use after the strike. Probably only the CC, panels, SPD, and wiring needs to be replaced ;) 
     Long runs over 200 feet that I often use with a HV CC are #6 or better grounds at the pole and the minimum the inspector requires to the CC or the gage that the wire run is to the CC. Probably #10. 
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • jtdiesel65jtdiesel65 Solar Expert Posts: 239 ✭✭✭
    ok, thanks. I'll get some ground rods., The concrete has rebar, but not substantial amounts.

    I was also wondering if it's necessary to run a ground wire to every panel. The panels are bolted to aluminum rails that run the length of the mount. Is there any reason not to just run the ground from those rails rather than each panel?
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,031 ✭✭✭✭✭
    edited September 15 #8
    The reason is the panel will not be grounded unless it has a special means of fastening to the rail that gets a bite into both the panel and the rail. It needs to be reliably bonded over time.

     The weakest link will be the path into the system in a near miss. All of this is part of the reason we run the long ground to the house. We want everything at the same potential but not a big path to the house. In a hit, it is turn out the lights the party is over and is also another reason we do not want the ground wire to the house as good of a path as it is at the array.

    What make is the array above the 8" pole?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    So, this is what the 20' hole looked like just before the cement went in.
     
    facebook album   https://www.facebook.com/media/set/?vanity=120212794718849&set=a.613074248766032

    The pole is at least 30', with 50% underground.  Had to have local welding shop build an adapter from 15" to 8" so the pole top mount would fit ( it only went to 8" )  It's high enough that the array is safe if an 18 wheeler comes in and had trouble turning around. Buses and RV's are not even close..  Broke several drill bits trying to drill  & tap mounting holes for the combiner box.  It's an surplus elevator tube.




    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • jtdiesel65jtdiesel65 Solar Expert Posts: 239 ✭✭✭
    Its 8 250w suntech on a tpm10-h.

    There are two of them.

  • Dave AngeliniDave Angelini Solar Expert Posts: 6,031 ✭✭✭✭✭
    Fun on a hill is it not? 😉  That is a 8 or 6 inch pole?  
    DPW Solar
    Product Line:
    TPM Series

    Been some time since I used Direct Power and Water. Can the mount be turned east / west ?  Nice job BTH
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • jtdiesel65jtdiesel65 Solar Expert Posts: 239 ✭✭✭
    Thanks. Yeah, the hill is no fun. 8 inch pole. Don't quote me but I think the tpm8 and below can use a 6. They have a size matrix and a fairly nice design tool.

    It could be rotated but there are 4 set bolts in the cap that would have to be loosened first. IMO, it's not designed for rotation.

    My only complaint with the racking was the holes in the racking didn't match the holes in the panels. I had to drill new holes in the panels. I'm not sure if that is always the case, but I was under the impression that the mount was tailored to your panel specs. Anyway, doesn't really matter now. I was able to drill them on the ground and it just added a small amount of time to the install.
Sign In or Register to comment.