Isolated and bonded earth rods in PV systems

josealjimjosealjim Solar Expert Posts: 33
There are many opinions regarding grounding and earth equipotential rods in PV systems installations. I hope you could help me with this topic guys.
The installer and inverter company gave me some ideas about earthing pv systems but following Cariboocoo's advice and installing SPD protections right before entering into the building I though that 3 independent rodS in triangle right near the solar array to connect PV and SPDs would be a better idea that running any EGC throughout the building or bonding GEC to the building rods. Im a bit worried about voltage induction between isolated earth rods and also about closed loops if we equipotential them.

WHAT DO YOU THINK ABOUT THE CONFIGURATION? I NEED YOUR HELP.

Attachment not found.

The SPDs will be installed outside with IP65 box, they will be connected by EGC to the PV electrodes bar (as you guys can see on the pic). To ground that box, can I use the same EGC or should I run another wire for it?

Looking forward to hearing from you,

thank in advance

Comments

  • waynefromnscanadawaynefromnscanada ✭✭✭✭ Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    josealjim wrote: »
    I'm a bit worried about voltage induction between isolated earth rods.

    I believe you should indeed be worried about this. I can easily imagine a nearby lightening strike resulting in a huge voltage difference between the two separate, non-bonded grounding systems, resulting in current jumping from one ground system to the normal current carrying wires, then back to the other ground system - - - with lots of possible damage. It's definitely my opinion that the two grounding systems be bonded together.
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Isolated and bonded earth rods in PV systems
    I believe you should indeed be worried about this. I can easily imagine a nearby lightening strike resulting in a huge voltage difference between the two separate, non-bonded grounding systems, resulting in current jumping from one ground system to the normal current carrying wires, then back to the other ground system - - - with lots of possible damage. It's definitely my opinion that the two grounding systems be bonded together.

    Actually that would depend on the distances between the rods, which we don't know. Yes I see the d=20 d=40 but I'm not sure what that applies to. If it is 60 to 70 meters (200+ feet for Americans) between the PV rod and the nearest house rod there's not likely any issue with needing them connected.

    The shortcut here is that green wire from the SPD running to the array grounding rod. If that is large enough (6 AWG or better) it may as well continue to the house grounding rods which are probably outdoors too.
  • nsaspooknsaspook ✭✭✭ Solar Expert Posts: 396 ✭✭✭
    Re: Isolated and bonded earth rods in PV systems

    The shortcut here is that green wire from the SPD running to the array grounding rod. If that is large enough (6 AWG or better) it may as well continue to the house grounding rods which are probably outdoors too.

    and if the conduit is metallic it should be bond at both ends to the grounding conductor.
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Isolated and bonded earth rods in PV systems
    nsaspook wrote: »
    and if the conduit is metallic it should be bond at both ends to the grounding conductor.

    And the ground for the SPD should be as close to it as possible to reduce the resistance to Earth.

    Given the distance it might be a good idea to put an SPD at the array, tied to that ground rod.
    The underground wire run to the house should reduce the 'antenna' affect in the conductors to near zero, especially if the introduction to the house is below or near ground level.

    Even if we had all the details there would be disagreement on the best way to provide protection.
  • inetdoginetdog ✭✭✭✭ Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    And the ground for the SPD should be as close to it as possible to reduce the resistance to Earth.

    Given the distance it might be a good idea to put an SPD at the array, tied to that ground rod.
    The underground wire run to the house should reduce the 'antenna' affect in the conductors to near zero, especially if the introduction to the house is below or near ground level.

    Even if we had all the details there would be disagreement on the best way to provide protection.
    The resistance to the earth is mostly irrelevant to the functioning of the SPDs. What you need is minimum resistance to the neutral wire of the building power system and to the ground network of the building power system. As long as the AC lines do not move much relative to those two reference points, any equipment inside the building will not be subjected to damaging voltages.

    The ground rod resistance may well be on the order of 20 ohms or more, so the few milliohms of the connecting wire does not matter much.
    What does matter is the connection from the SPD to the ground or neutral bus of the panel. In particular uncoil the neutral pigtail lead and cut it to the minimum length necessary to reach the neutral bus bar.

    In the case of a PV array, you need to decide whether you are trying to protect the panels or the inverter when deciding where to put the SPD. Both ends of the DC wiring between array and inverter may actually be a good idea, protecting both.
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Isolated and bonded earth rods in PV systems
    inetdog wrote: »
    The resistance to the earth is mostly irrelevant to the functioning of the SPDs. What you need is minimum resistance to the neutral wire of the building power system and to the ground network of the building power system. As long as the AC lines do not move much relative to those two reference points, any equipment inside the building will not be subjected to damaging voltages.

    No.
    SPD conducting high Voltage caused by nearby lightning strikes from PV wires to ground. AC Neutral doesn't enter into it.

    SPD conducting high Voltage caused by nearby lightning strikes from AC grid wires to ground. AC Neutral happens to be bonded to ground, but the SPD is still connected to ground not neutral. In some cases an additional SPD is needed between Neutral and ground.
  • inetdoginetdog ✭✭✭✭ Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    No.
    SPD conducting high Voltage caused by nearby lightning strikes from PV wires to ground. AC Neutral doesn't enter into it.

    SPD conducting high Voltage caused by nearby lightning strikes from AC grid wires to ground. AC Neutral happens to be bonded to ground, but the SPD is still connected to ground not neutral. In some cases an additional SPD is needed between Neutral and ground.
    Nope. The only thing that counts in terms of the SPD action is the ground wiringin the building system. Whether or not that is connected to earth itself is largely irrelevant if the wiring system is properly bonded on the way to the "ground" connection.
    Now if you are worried about touching the grounded shell of an appliance at the same time as you are standing in your bare feet outside the house, the ground electrode connection might actually be important. But at 20 ohms of resistance, it will still not protect you from a lighting induced surge. Staying either entirely inside or entirely outside the house will help. :)

    Note that lightning current conductors, such as attached to electrodes on the roof of the building, are a totally different story and have totally different design criteria from wiring ground systems. And the NEC does not really talk about direct lightning protection.
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Isolated and bonded earth rods in PV systems

    Again, no.

    The idea is to reduce high Voltage spikes to zero. Wire has resistance. So on the one hand we have you saying "cut it to the minimum length necessary to reach the neutral bus bar", which is correct, and then you turn around and say the resistance in the wiring to the Earth ground doesn't matter.

    Trust me, it does. As does the quality of the Earth contact. Which is why we have all these rules about grounding which, I must point out again, change now and then (sometimes back and forth) because even engineers can't agree on what's best.

    Just because you read it in the NEC once doesn't mean it's still there or will be there tomorrow or even that it is right. I'll take my feeble 8 years of education and 40 years experience over the cackling of theorists any day. I have never had anything go wrong with any system I've ever built or repaired. Ever.
  • inetdoginetdog ✭✭✭✭ Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    Again, no.

    The idea is to reduce high Voltage spikes to zero. Wire has resistance. So on the one hand we have you saying "cut it to the minimum length necessary to reach the neutral bus bar", which is correct, and then you turn around and say the resistance in the wiring to the Earth ground doesn't matter.

    Trust me, it does. As does the quality of the Earth contact. Which is why we have all these rules about grounding which, I must point out again, change now and then (sometimes back and forth) because even engineers can't agree on what's best.

    Just because you read it in the NEC once doesn't mean it's still there or will be there tomorrow or even that it is right. I'll take my feeble 8 years of education and 40 years experience over the cackling of theorists any day. I have never had anything go wrong with any system I've ever built or repaired. Ever.
    I do not doubt that you have built robust systems and have not had transient problems even though you are out in the great outdoors where rain and lightning are common.

    But what destroys electrical and electronic equipment is a voltage difference that they are actually exposed to. That means differences in voltage between the hot AC leads and the AC neutral or the AC ground which is used to protect the chassis from becoming energized. In some cases there are also external wires like TV antenna or cable and phone lines. But those external wires should be wire connected to a protective ground bus attached to the building wiring system ground.

    As long as you keep all of those wires close together in voltage, it does not matter in the least how far above earth ground the entire system moves. Makes no difference at all.
    Providing a good earth ground connection (as with a CEE/Ufer made from the building's concrete foundation) and providing an equipotential plane under the building (as comes along for free with a Ufer) will not hurt, but it will also not generally help.
    I believe in putting my effort into the places where it has the greatest payback and moving a ground electrode close to my SPD is not a high return action. Nor is putting my SPD anywhere other than the top of the main panel just to get it close to a ground rod.
    And I definitely do not recommend adding an additional ground electrode close to sensitive equipment unless it is very solidly connected to the rest of the building ground system.
    Reasonable people (on Mike Holt) differ about whether it would more likely be beneficial or harmful even then.

    PS: If you can give an example of how any device would be harmed by the line/neutral/EGC combination moving together with respect to earth ground, I would be happy to look at it.
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Isolated and bonded earth rods in PV systems

    I think you are confusing using an SPD to suppress a transient Voltage spike generated within a system and using it to suppress a spike generated outside a system (lightning). As far as the latter is concerned your neutral line is just another path it can take on its way to completing its own circuit, which is to ground. You want to aid it in this endeavour by providing it with the shortest, lowest resistance path possible - and one that doesn't go near your electrical equipment.

    If you put an SPD on the main service panel of most houses then connect its ground wire to either the neutral bus or the ground bus you will accomplish both ends as the two bus bars are bonded. Unless that bond fails; then you have the curious situation where it will work for one purpose but not for the other. Some off-grid applications do not have any such bond. Fortunately they usually don't have any significant outside AC line "antennas" so it isn't an issue; rarely do inverters produce the kind of power system spikes that grid power does.

    Which brings us to the scenario of long, exposed AC lines to outbuildings that can have sufficient resistance between beginning and end that the N-G bond and any nearby SPD is of little or no value for protecting any equipment in the outbuilding against stray Voltage from lightning. This is where it is sometimes necessary to put in an additional ground rod used only to sink any transient spikes which may arise due to lightning (power system spikes would be clipped at the source regardless of wire length).
  • inetdoginetdog ✭✭✭✭ Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Isolated and bonded earth rods in PV systems

    No, I am not.
    Getting the lightning induced transients on the power line safely to ground is an admirable goal, but as long as you do not let them cause a voltage differential between the house hot wires and the house neutral and ground you have done all that is necessary to protect the inside of your house. You might protect your neighbors who do not have SPDs some with a good ground electrode, but it will not help you much.

    Not having a ground may prevent the SPD is participating in dissipating all of that induced energy, but all I am asking of it is to prevent the resulting voltages from appearing across the house wires (including ground wires).
    And, just to broaden the subject a bit, a lighting induced voltage event is quite likely to cause large earth currents which can result in a high voltage differential between rod electrodes at different points on the property. You do not want those ground voltage gradients to induce currents in the building EGCs, so in my umble hopinion (shared with Mike) the best system is a robust Ground Electrode System with direct heavy wires connecting all of the electrodesand a robust Equipment Grounding Conductor (EGC) system inside the house and only one point of interconnection between the two, at the service entrance.
    SMA SB 3000, old BP panels.
  • josealjimjosealjim Solar Expert Posts: 33
    Re: Isolated and bonded earth rods in PV systems

    The distance between PV and building rods: about 60 m
    The distance between SPDs right before the building and PV rods: 20m.
    Distance between SPDs right before the building and building rods: 40m
  • josealjimjosealjim Solar Expert Posts: 33
    Re: Isolated and bonded earth rods in PV systems
    Actually that would depend on the distances between the rods, which we don't know. Yes I see the d=20 d=40 but I'm not sure what that applies to. If it is 60 to 70 meters (200+ feet for Americans) between the PV rod and the nearest house rod there's not likely any issue with needing them connected.

    The shortcut here is that green wire from the SPD running to the array grounding rod. If that is large enough (6 AWG or better) it may as well continue to the house grounding rods which are probably outdoors too.

    The distance between PV and building rods: about 60 m
    The distance between SPDs right before the building and PV rods: 20m.
    Distance between SPDs right before the building and building rods: 40m
  • nsaspooknsaspook ✭✭✭ Solar Expert Posts: 396 ✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    josealjim wrote: »
    The distance between PV and building rods: about 60 m
    The distance between SPDs right before the building and PV rods: 20m.
    Distance between SPDs right before the building and building rods: 40m

    I would normally would place the shunt protection devices as close as possible to a power panel or combiner box so it can be connected directly to the lowest impedance terminal to short the power lines at that 'ground' potential point. There should be no ground loop currents present on the hot lines (in a near strike) if the shunt devices short, only voltage differentials from the power wires to ground potential due to GPR and induction voltage from the strike magnetic field currents. If your SPD connection box (combiner?) is 20m from the house connecting a rod there seems a unnessary complication with few benefits if you also have SPD protection (to handle the same effects on that 20m run) on AC and DC at the power panel and it's bonded directly to the grounding system there.
  • CariboocootCariboocoot ✭✭ Banned Posts: 17,615 ✭✭
    Re: Isolated and bonded earth rods in PV systems
    inetdog wrote: »
    No, I am not.
    Getting the lightning induced transients on the power line safely to ground is an admirable goal, but as long as you do not let them cause a voltage differential between the house hot wires and the house neutral and ground you have done all that is necessary to protect the inside of your house. You might protect your neighbors who do not have SPDs some with a good ground electrode, but it will not help you much.

    Not having a ground may prevent the SPD is participating in dissipating all of that induced energy, but all I am asking of it is to prevent the resulting voltages from appearing across the house wires (including ground wires).
    And, just to broaden the subject a bit, a lighting induced voltage event is quite likely to cause large earth currents which can result in a high voltage differential between rod electrodes at different points on the property. You do not want those ground voltage gradients to induce currents in the building EGCs, so in my umble hopinion (shared with Mike) the best system is a robust Ground Electrode System with direct heavy wires connecting all of the electrodesand a robust Equipment Grounding Conductor (EGC) system inside the house and only one point of interconnection between the two, at the service entrance.


    "As long as you do not let them ..."

    Calling Zuess & Odin!
    No one else can bloody control lightning or the fields it generates; it does what it bloody well wants.
    It doesn't travel across conductors the way standard electric power does, for example; even the stray Voltage can operate with skin effect and traverse a supposedly grounded conductor as though it were not.
    It is trying to Earth itself and uses any handy man-made conductors it happens to find along the way to achieve that goal. One should help it along as much as possible, particularly in bypassing any potential routes through your house.

    I've seen enough of its effects over the years that I'll stick with my methods known to work. You should "get out in the vans" more. Electrical Safety Ground and lightning protection aren't the same thing, although people constantly confuse them.
  • josealjimjosealjim Solar Expert Posts: 33
    Re: Isolated and bonded earth rods in PV systems

    Following your indications I have made another config.
    - SPDs right near the PV earth rods
    - Metallic conduit grounded at both ends
    - Bare cooper grounding electrode bonding jumper

    Attachment not found.

    - What do you all guys think about this one?
    - The distance between array and building is about 20m. I worried about lighting strikes induced voltages into the dc wires along the metallic conduit, I dont want any high voltage entering into the building. By grounding both ends of the conduit will they be avoid?
    - What kind of EGC and GEC wire should I use (section,etc)

    Attachment not found.

    We dont want to break the asphalt to much in order to set the wires. Could i place the bare cooper bonding jumper as indicated in the picture?

    Thanks
  • nsaspooknsaspook ✭✭✭ Solar Expert Posts: 396 ✭✭✭
    Re: Isolated and bonded earth rods in PV systems

    At the Panels you now have effective shunt protection but I still think a SPD very near the inverter/DC switch is a good idea to protect it from line transients. Running two conduits is not a good idea for circuit feed wires as you want any fields to effect all wires (including the ground wire inside the conduit bonded at the switch/inverter NOT at the end of the conduit, that conduit end IMO should be connected directly to the earth electrode grounding wiring outside the house) equally so the voltage transients are common-mode to earth ground with the SPD in the circuit to handle any large voltage imbalance at the electronics.
    Attachment not found.
  • inetdoginetdog ✭✭✭✭ Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    nsaspook wrote: »
    At the Panels you now have effective shunt protection but I still think a SPD very near the inverter/DC switch is a good idea to protect it from line transients. Running two conduits is not a good idea for circuit feed wires as you want any fields to effect all wires (including the ground wire inside the conduit bonded at the switch/inverter NOT at the end of the conduit, that conduit end IMO should be connected directly to the earth electrode grounding wiring outside the house) equally so the voltage transients are common-mode to earth ground with the SPD in the circuit to handle any large voltage imbalance at the electronics.
    Attachment not found.

    An interesting effect that is important at lightning induced current frequencies is that a ground wire running inside a ferrous metal conduit or protective sheath becomes, in effect, an RF choke and greatly increases the resistance of that ground wire to high frequency transients.
    The solution, required by the NEC for GEC wiring but not for EGCs, is to bond the ground wire to the conduit at both ends. That makes the conduit, which has no iron core choke effect from its own magnetic field, the effective ground conductor for high frequency transients.
    Nothing in the NEC prohibits you from doing the same with a wire EGC, but it is not required.
    SMA SB 3000, old BP panels.
  • nsaspooknsaspook ✭✭✭ Solar Expert Posts: 396 ✭✭✭
    Re: Isolated and bonded earth rods in PV systems

    One of the complications of using conductive conduit for wiring that's often overlooked is that the 'pipe' becomes two conductors at RF. The inner surface and outer surfaces are seperated by skin effect and are only electrically joined at the ends. Currents that flow on the ouside are isolated from inner currents for most of the distance in a long section causing unbalanced field energy from one end to the other that causes the outer surface to act as a antenna. For real antennas a coax RF choke is commonly used to stop outer shield surface current flow, for power feeds good earth grounding over the length and at the ends helps to shunt the current into the earth so it won't induce a voltage into the conductors.
  • josealjimjosealjim Solar Expert Posts: 33
    Re: Isolated and bonded earth rods in PV systems

    Attachment not found.

    What do you think about this new config? Im working in thailand and i dont trust installers here so Id like arrange everything following NEC

    I will check any possibility of electrical loop at the inverter anyway.


    The two conduits config was given to me by the installer, so lets make it one:
    Attachment not found.

    Should the connection between conduit and array earth be made by bonding the array end of the conduit by EGC to the array bus bar and the house end of the conduit by bonding to the electrodes bonding jumper as attached?
    The house end of the conduit should be bonded by GEC or EGC?

    THANKS A LOT
  • vtmapsvtmaps ✭✭✭✭ Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    josealjim wrote: »
    The two conduits config was given to me by the installer, so lets make it one

    Why did the installer want two conduits? There are very good reasons to have two conduits in some systems.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • josealjimjosealjim Solar Expert Posts: 33
    Re: Isolated and bonded earth rods in PV systems
    vtmaps wrote: »
    Why did the installer want two conduits? There are very good reasons to have two conduits in some systems.

    --vtMaps

    I have a meeting tomorrow with him and ill ask him.

    What do think about bonding the conduit to the electrodes bonding jumper? How could i make it?

    Attachment not found.
    Attachment not found.
  • ggunnggunn ✭✭✭ Solar Expert Posts: 1,973 ✭✭✭
    Re: Isolated and bonded earth rods in PV systems
    Even if we had all the details there would be disagreement on the best way to provide protection.
    Foreshadowing? :D
  • josealjimjosealjim Solar Expert Posts: 33
    Re: Isolated and bonded earth rods in PV systems

    Thanks everybody for your help.

    Id like to give a brief explanation about my situation in order to make u understand my ignorance on these issues. I have 2 years of working experience, 1 year as internship working on solar thermal plants simulations and troubleshooting on site, on my second year (already with an engineer contract) ive been developing the automation of biogas generation and biogas upgrading plants, the executives told me that they wanted to introduce photovoltaic systems in our portfolio, so i started to study theory and simulation making simulations. There is no technical supervisor for pv projects here and we dont have any consultant company for advices, and the electrical engineers here (Thailand) dont care about how installers do. Thanks to this forum and information given by suppliers Im having a better idea about how a good installation should be made, so I really appreciate every suggestion given.

    The engineers where I work told me that the lighting ground system and and main service ground system were not but after digging a bit I saw that we have one earth ring system for the building where LPS and MDB busbar are connected. There is a lot of controversy about the best PV grounding system when you dont rely on any code and I would like to have the PV parking we are building grounded with the best configuration possible.

    After knowing that we have a common earth ring for LPS and building services and with all the information provided by this forum i made two configurations:

    Attachment not found.
    On this configuration, all the modules and structure are bonded and connected to a busbar at the array. From the busbar there is an PVC 16sqmm cable going to a 18 m far handhole, from there to the building ring and connected to the SPDs, the same PVC 16sqmm continues to the inverters for ground fault protection.
    Although i dont need to rely on NEC code, it says that we can not use EGC as GEC. I dont know if this config I accomplish that

    Is this config correct? Would it compromise the groundfault protection of the inverter?

    What about this other one?
    Attachment not found.
    Here there is a GEC to the handhole and from there to the building ring. SPSs are attached to the GEC at the handhole. An EGC goes from the array busbar directly to the inverter for the groundfault protection. Is this config better?
    There are no junction box for parallel connection outside the inverters, each string to its inverter dc input but the thai legislation makes you include a circuit breaker right before the inverter. If I size the EGC according to NEC 250.122. it would be an AWG14 wire but the installers already ran a pvc 16sqmm (they havent connected yet, we are waiting the inverters).

    On this second config, the connection from the handhole to the earth ring of the building, should it be by bare copper with the same size than the ring?
    Is correct to use both GEC and EGC with the same size 16sqmm although with a much smaller one for EGC would be ok?


    In brief,
    Which of both would you choose?
    What would you change to make it better?

    BR and THANKS
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