Grounding for a structure

HI

I cant remember too well the normal procedures for grounding structures. A large structure to mount 30 panels has been erected where we are to install. I believe if I remember correctly, that a separate ground rod should be provided for the structure. The main grounding rod will be for the inverters/generator/house. Is this correct?

Comments

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Grounding for a structure
    lazza wrote: »
    A large structure to mount 30 panels has been erected where we are to install. I believe if I remember correctly, that a separate ground rod should be provided for the structure. The main grounding rod will be for the inverters/generator/house. Is this correct?

    Yes, the structure needs its own ground rod.

    btw, that was an easy question. I'm sure glad you didn't ask whether (or how) the two ground rods should be connected to each other, or whether there should be neutral-ground bonding at each location.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • lazzalazza Solar Expert Posts: 334 ✭✭✭
    Re: Grounding for a structure

    Yes, that last time I asked about grounding, there were 200 odd posts in the thread :cry: .. which for a panda like me, was just too much to take in :cool:
  • mike95490mike95490 Solar Expert Posts: 8,817 ✭✭✭✭✭
    Re: Grounding for a structure

    does a 11" steel pole, in concrete, (24" di, 10' deep with rebar) need a separate ground rod ?
    (would a Ufer ground need a ground rod)
    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 ,

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding for a structure
    mike95490 wrote: »
    does a 11" steel pole, in concrete, (24" di, 10' deep with rebar) need a separate ground rod ?
    (would a User ground need a ground rod)

    No system actually needs a ground rod. Grounding is not required for function.
    Now, will it benefit from it or be required to have one by the AHJ?
    There's a question all right!

    You can test to see if the mount has sufficient ground contact itself by checking for Voltage potential between it and a good ground (i.e. measuring resistance).

    What you do about a difficult inspector ...
  • BilljustBillBilljustBill Solar Expert Posts: 213 ✭✭✭
    Re: Grounding for a structure

    This is some mental food for thought:

    Here is some of my "homework" I'm doing to get ready for a Spring/Summer 1800 watt ground array and a 1500 watt roof mount. It's a video talking about direct grounding of roof solar arrays and the hazard it causes by having two points of "different" groundings....

    http://www.youtube.com/watch?v=YuDqXFvRv94 (Preview)
    Bill
  • BB.BB. Super Moderators, Administrators Posts: 30,638 admin
    Re: Grounding for a structure

    Sort of mixing different issues (electrical grounding, static discharge grounding, and lightning grounding) all together.

    In the end, if a person decides they need multiple grounds (generally because of lightning), I do believe that multiple ground rods should be bonded together when connected to a common power system.

    With power system covering larger areas (larger homes, outbuildings, factory floor, etc.), some of the requirements become conflicted. And I am not sure there is an "ideal" solution short of using Isolation Transformers everywhere (expensive and inefficient).

    Having a machine grounded back at the main panel and a (for example) metal table/machine next to the machine bolted to the concrete foundation (or even set into its own foundation). You now have two large pieces of metal right next to each other that have different potentials. Those potentials can be driven by leakage current from the AC mains, or from lightning. Either can create a shock hazard.

    I measured >60 VAC on a salt water pumping system ground vs the ground rod next to a tank 50 feet away. Gave me quite a nip between two "grounds".

    Had treat wiring at the tank vs wiring at the outbuilding as if they were energized relative to each other (which they were). Used isolation transformers and optical isolation to minimize hazards and electrical noise issues.

    Each part of the system (AC and DC) needs to be reviewed in detail to understand what grounding/connections should be made.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazzalazza Solar Expert Posts: 334 ✭✭✭
    Re: Grounding for a structure

    I knew this would get complicated.

    Is Mike talking about the problems of installing a separate grounding the PV structure as a risk in cases of lightning? or is he talking about grounding the -ve side? I figure that as the structure is not connected to the PV +/- circuit that it's not an issue
  • BB.BB. Super Moderators, Administrators Posts: 30,638 admin
    Re: Grounding for a structure

    The basic idea is simple--If you put one ground rod (cold water pipe) into the ground and run all ground wires from that rod to all metal/electric devices (as requiered in the home)--All is OK. You touch any piece of metal (plumbing, grounded electrical conduit, grounded appliance with three prong cord, etc.)--then you cannot get a shock between "grounds".

    For example, you have a ground rod at the home/main panel--Then run it out to the shed 100' away--And have a sink out there--You have two ground points--One at the house and a second at the shed.

    If everything is "correct"--It should not matter--Ground is Ground everywhere... But if you put a "bias" into the dirt itself (a miswired 120/240 VAC "Hot" grounded instead of the neutral), then you will have a voltage difference between the home ground (and even water pipe) vs the ground rod at the pole transformer neutral. Your ground rod may be at 120 VAC at the house, and near zero volts at the shed--So your "green wire" and Neutral at the panel in the shed is 120 VAC with respect to the sink/plumbing in the shed.

    And a miss-wired neutral/earth bond does not have to be done at your home--It could be a misswire at the neighbor's home (shared transformer between several homes). It it a common mistake--probably not. But I have seen some pretty strange wiring in older homes.

    Similar issue with lightning--Have a strike at the shed and its ground is different than the ground at the house (thousands of volts).

    In the end--I am tending towards distributed ground rods in a system... If you have a ground rod at the home, put another one at the shed and run a 6 awg wire between the home and shed ground rod. If I had lightning in the area--I would even think about placing a neutral to earth bond (to the ground rod) in the shed too). Basically, keep the ground "integrity" at each home/workshop location.

    We talk about the idea of having only one neutral/earth bond in a home--But the reality with typical residential wiring, we have the Neutral/Earth bond in the main panel of the house, a N/E bond at the pole transformer, and several more N/E bonds at the next door homes.

    In theory, doing multiple earth/neutral bonds can induce lightning current into the AC wiring--But in reality, lightning energy does not travel very far (maybe a few 10's of feet) in "small diameter" wiring (at least that is what we are told when running lightning grounds).

    As Mike Holt says, the NEC is not about lightning grounding... Which is a shame since one would think that a "unified" code would try to avoid making things worse in other respects (such as lightning and solar arrays).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Grounding for a structure
    lazza wrote: »
    Is Mike talking about the problems of installing a separate grounding the PV structure as a risk in cases of lightning? or is he talking about grounding the -ve side? I figure that as the structure is not connected to the PV +/- circuit that it's not an issue

    Mike wrote:
    does a 11" steel pole, in concrete, (24" di, 10' deep with rebar) need a separate ground rod ?
    (would a User ground need a ground rod)

    lazza, there's a typo in Mike's response... "user" should be "ufer". Read about ufer grounds.

    I think that Mike is talking about lightning and static grounding, not electrical grounding.

    However, in the event of nearby lightning a transient electrical connection may develop between your PV circuit and ground. That "connection" may be an arc between ground and your PV wiring (very bad situation), or the "connection" may be where you want it to be: through a lightning arrestor.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • mike95490mike95490 Solar Expert Posts: 8,817 ✭✭✭✭✭
    Re: Grounding for a structure
    ....
    What you do about a difficult inspector ...
    Backhoe ? (wrong) Install $5 ground rod & $8 #6 wire.
    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 ,

  • Joe94Joe94 Solar Expert Posts: 42
    Re: Grounding for a structure

    For what it's worth I just had a electrical inspection on my house and the inspector told me that a Ufer ground is the only ground connection that meets the new NEC regulations. I haven't done any research to verify if that is true or not.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding for a structure

    In order for an inspector to truly understand the code he'd have to be both an electrical engineer and a lawyer (that's not going to happen) and even then they'd still only be rendering an opinion. This is the same as any of us, and indeed the NEC itself. The main difference being the inspector's opinion is the one you have to comply with unless you like arguing in court.

    Considering the large number of installs done without UFER it obviously is not the only connection that meets NEC.
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