Considering Earth ground vs floating ground in off grid solar

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Context: I'm building an off-grid, 100% solar powered home. I have a 12 kw solar array wired up to a Sol-Ark inverter and a Fortress 18.5 kwh lithium battery.

Question: What is the ideal grounding/earthing scheme?

Additional context: where I'm building, there is currently no requirement that my setup be exactly to code. I know that code is written under the assumption that one is connected to the electric grid, and so the recommendations don't always necessary apply logically to me, and so I don't want to just assume that code is best. I want to do what is actually most logical for my situation. With that said, if doing something to code doesn't result in a inferior design, then I'm all for it.

Lightning protection: Typically, lighting protection is one of the reasons cited for why homes usually have an Earth ground. If a lightning surge goes into your house wiring, then an Earth ground gives it somewhere to go so it's less likely to damage your electronics (or start a fire). On the other hand, adding an Earth ground actually makes your electrical system a more attractive target to lightning (by helping to create a low resistance path between the origin of the lighting and somewhere deep in the Earth). A much better protection from lightning is to create a separate lightning rod that provides a lower resistance pathway for the lightning than your house's electrical system. In addition, any nearby lightning strikes that miss your electrical system, could still cause damage to the electrical system by traveling through the ground in the nearby radius. For these reasons, I intend to install a separate, Earth grounded lighting rod...and as a result, it doesn't seem like there is no benefit to having an Earth ground on the house for lightning protection.

Shock protection: Typically, a house receives electricity from the mains power, which is grounded to Earth. If there is a short in an appliance whereby, say, one of the current carrying AC wires is shorted to the case of the appliance, and someone touches the appliance, then the person may form a lower resistance pathway from the case of the appliance to Earth ground, because the person is likely standing on Earth ground. This is often cited as a reason for having an Earth ground at the house: because if the appliance has a ground terminal, then the appliance case will be bonded to ground which goes all the way to Earth ground, and as a result, a person touching the case isn't going to be electrocuted. However, this protection is only necessary because the house power system is referenced to Earth ground in the first place. If the house had a floating ground, then the excess current wouldn't be searching for a pathway to Earth ground, and would have no reason to travel through the body of a person touching the electrified appliance case. It would be safe to touch the electrical wire, just as it is safe for a bird to land on an exposed electrical wire. So, when talking about an off-grid house, the desire for shock protection doesn't seem to motivate adding an Earth ground. In fact, it seems that a floating ground would be safer from a shock perspective standpoint, because that way a human standing on the ground will be unlikely to be on the lowest resistance pathway to complete a circuit.

All power to the house will come from the inverter -- either directly, as passed through from the battery system. Therefore, in the event of a broken appliance which has a short from a current carrying wire to the case, then the excess current will be searching for a path back to one of the inverter terminals. This seems like a decent reason to wire up my house's ground to one of the inverter terminals (say, negative)...as it would provide some circumstantial shock protection, although it seems the only possible circumstance would involve a person touching the inverter terminals while simultaneously touching a broken appliance case that is shorted out -- a pathologically unlikely circumstance.

Electrical interference: having the house grounded to Earth can introduce electrical disturbances into my own electronics, which at best is undesirable (eg, not great for HAM radio), and at worst, a danger, if there is a nearby lightning strike that energizes the house via the Earth ground.

Solar array: typically, the solar panel frames are all bonded together, and bonded to the house ground, which is of course bonded to the Earth ground. For better or worse, I did not run a ground through my buried conduit between the panels and the house, so it is not really possible for me to ground the panels to the house ground. However, I can't think of any reason to do so, other than by convention. The panels generate their own DC electricity, which is totally separate from the AC electricity generated by the inverter, so I don't see any reason why the two systems should want to share the same ground, especially if neither of them benefit from an Earth ground. The existence of the separate lightning protection system seems to eliminate any desire to bond the panels to Earth from a lighting protection perspective. From a shock protection perspective, I'm thinking perhaps it would be beneficial to bond the negative terminals of each panel to the solar panel frame assembly? But I don't think that is usually done.

Current plan: based on the above considerations, my current plan is to: a) bond the steel joists/I-beam of the house to the negative terminal of the inverter, to the ground wire in my main panel; b) not bother grounding the DC panel array; c) create an independent pathway to Earth ground in the form of a lightning rod, which is not connected to my AC or DC electrical systems.

Any thoughts/feedback is appreciated
 

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  • #2
Averagesupernova
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My opinion is it needs to wired to meet code. Not being connected to commercial power does not mean much. You cannot expect a floating system to never give you a shock. Consider the capacitance alone. Not only that, as soon as you have a fault someplace (which has a good possibility of going undetected at first), now suddenly EVERY chassis that is connected to the third prong on an outlet will potentially be energized. There's a good reason why grounding exists the way it does.
 
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  • #3
anorlunda
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I second what @Averagesupernova said. Follow local electrical codes. Failure to do so may not only be dangerous, but it could render your insurance invalid if something goes wrong. You don't want to pay for fire insurance that won't pay out in case of a fire.
 
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berkeman
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Third.
 
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  • #5
Baluncore
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4'th.

Keep the lightning protection quite separate from the electrical ground and frame.

Electrically bond all panels together and to the support structure and the system ground stake.

Earth leakage of DC to the metal frame is a real problem with ageing PV panels, given half a chance it will kill you. You need to detect that leakage, which requires grounding of all metalwork.

Unlike AC that self extinguishes every half cycle, a DC arc will be sustained, it will start a fire.
 
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  • #6
berkeman
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Context: I'm building an off-grid, 100% solar powered home. I have a 12 kw solar array wired up to a Sol-Ark inverter and a Fortress 18.5 kwh lithium battery.
Do you have a local municpal or county building department where you could file your construction plans and get inspections and approvals? That would help a lot with the home insurance issues.
 
  • #7
Tom.G
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And don't forget about induced voltages from a nearby lightning strike. I once lived in a city apartment about 150-200 feet away from a 100 foot tall flag pole.

I would occassionally lose a computer printer when the flagpole was a lightning rod. The printers would always pass self-test but would not print anything. The 10ft. data cable was a long enough antenna to destroy the data input circuit of the printer. Fortunately, the computer was never damaged.

House wiring is a LOT longer than 10ft. How much voltage do you want on the wiring that will be desperately trying to reach Ground?

Ground the system, you will have a better chance to 'live long and prosper.'

Cheers,
Tom

p.s.(1) Once you have lightning protection (lightning rods) with solid grounding around disbursed equipment and the house, the best grounding for an interconnected system is 'single point' grounding; i.e. do not ground each part of the system individually, run a bonding wire between them. This avoids problematic differential voltages between equipment.

p.s.(2) A "Rule of Thumb" for lightning rods is the protected region is a cone, with the apex at the top of the rod, expanding to a circle on the ground with a radius 1/3 the height.

p.s.(3) Installing and Grounding lightning rods is somewhat of an art. Hiring an experienced, licensed installer is a very good idea, especially if you are in a high risk area. 'High risk' would include a prairie or desert for instance, also mountain tops, especially if there are not many trees close by (see the 1/3 rule above). Also, grounding rod requirements differ depending on the type of soil they are in.

Aww. I'm tired of typing... take care!
Tom
 
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@yahastu as others pointed out grounding is necessary. It helps in many occasions.
Now in theory if you only will use electrical items that are "double protected" having the square within a square symbol then without a proper ground you are just endangering the items and not yourself. Items like modern flat panel Tv's and other appliances that don't have a metal outer chassis.
But if you will also use other appliances like water heaters , kitchen tools, washing machine etc, then grounding is much more important as a fault can not only harm devices but harm you physically as when you will touch such an appliance with your wet hands or otherwise you will become the ground connection and a certain amount of current will pass through your body.

Even with grounding it is advised to use GFCI type breakers for kitchen appliances like electric ovens as well as bathroom devices like electrical water boilers.
This doubles the safety as ground current path takes away most of the current that could otherwise flow through you and GFCI cuts the current shortly minimizing risks.
 
  • #9
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Always by the code. It is not just about following rules, but about having a system 'everybody' knows and will be familiar with without decoding/understanding it first: and also about getting parts easily, designed straight for it.
Every component and device you can buy were designed with the local code in mind and can malfunction or possibly even became dangerous if used in a wantonly built system.
 
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  • #10
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My opinion is it needs to wired to meet code. Not being connected to commercial power does not mean much. You cannot expect a floating system to never give you a shock. Consider the capacitance alone. Not only that, as soon as you have a fault someplace (which has a good possibility of going undetected at first), now suddenly EVERY chassis that is connected to the third prong on an outlet will potentially be energized. There's a good reason why grounding exists the way it does.

As I specifically explained in my post, meeting code is not an objective unto itself. I am not asking what needs to be done "to meet code." I am asking what makes the most sense from the perspective of electrical shock protection, lightning protection, etc. I have discussed all of these topics in my post, and my conclusion is that doing it "to code" increases the risk of electrical shock, increases the risk of lightning damage, increases the risk of electrical disturbance, etc. If you disagree with my analysis, please explain how you disagree.

I second what @Averagesupernova said. Follow local electrical codes. Failure to do so may not only be dangerous, but it could render your insurance invalid if something goes wrong. You don't want to pay for fire insurance that won't pay out in case of a fire.

Did you read my post? I explained all the reasons why building to code in this situation seems unsafe and illogical. I am looking to discuss this from an engineering perspective, NOT from a legal perspective. If you disagree with my analysis, please read my analysis and explain how you disagree.

4'th.

Keep the lightning protection quite separate from the electrical ground and frame.

Electrically bond all panels together and to the support structure and the system ground stake.

Earth leakage of DC to the metal frame is a real problem with ageing PV panels, given half a chance it will kill you. You need to detect that leakage, which requires grounding of all metalwork.

Unlike AC that self extinguishes every half cycle, a DC arc will be sustained, it will start a fire.

As explained in my post, I plan to keep my lightning protection quite separate (contrary to "code", which would have me merge the two together). Not quite sure what you mean by "Earth leakage of DC to the metal frame is a real problem with ageing PV panels, given half a chance it will kill you". Please read my discussion of the risk of DC shock in my original post and let me know if you agree or disagree with my conclusions

Do you have a local municpal or county building department where you could file your construction plans and get inspections and approvals? That would help a lot with the home insurance issues.

I am not asking about electrical codes or building inspections. This has no relevancy to my question

And don't forget about induced voltages from a nearby lightning strike. I once lived in a city apartment about 150-200 feet away from a 100 foot tall flag pole.

I would occassionally lose a computer printer when the flagpole was a lightning rod. The printers would always pass self-test but would not print anything. The 10ft. data cable was a long enough antenna to destroy the data input circuit of the printer. Fortunately, the computer was never damaged.

House wiring is a LOT longer than 10ft. How much voltage do you want on the wiring that will be desperately trying to reach Ground?

Ground the system, you will have a better chance to 'live long and prosper.'

Cheers,
Tom

p.s.(1) Once you have lightning protection (lightning rods) with solid grounding around disbursed equipment and the house, the best grounding for an interconnected system is 'single point' grounding; i.e. do not ground each part of the system individually, run a bonding wire between them. This avoids problematic differential voltages between equipment.

p.s.(2) A "Rule of Thumb" for lightning rods is the protected region is a cone, with the apex at the top of the rod, expanding to a circle on the ground with a radius 1/3 the height.

p.s.(3) Installing and Grounding lightning rods is somewhat of an art. Hiring an experienced, licensed installer is a very good idea, especially if you are in a high risk area. 'High risk' would include a prairie or desert for instance, also mountain tops, especially if there are not many trees close by (see the 1/3 rule above). Also, grounding rod requirements differ depending on the type of soil they are in.

Aww. I'm tired of typing... take care!
Tom

I haven't forgotten about induced voltages from lightning strikes...in fact, half of my discussion in my post was about that very issue! Did you read what I wrote?

@yahastu as others pointed out grounding is necessary. It helps in many occasions.
Now in theory if you only will use electrical items that are "double protected" having the square within a square symbol then without a proper ground you are just endangering the items and not yourself. Items like modern flat panel Tv's and other appliances that don't have a metal outer chassis.
But if you will also use other appliances like water heaters , kitchen tools, washing machine etc, then grounding is much more important as a fault can not only harm devices but harm you physically as when you will touch such an appliance with your wet hands or otherwise you will become the ground connection and a certain amount of current will pass through your body.

Even with grounding it is advised to use GFCI type breakers for kitchen appliances like electric ovens as well as bathroom devices like electrical water boilers.
This doubles the safety as ground current path takes away most of the current that could otherwise flow through you and GFCI cuts the current shortly minimizing risks.

Several people have replied telling me what they think I should do, but as of yet, nobody has actually given a reason why it would be helpful. I wrote a rather detailed post explaining my reasoning on this. If you disagree with my analysis, please let me know how you disagree
 
  • #11
Baluncore
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Please read my discussion of the risk of DC shock in my original post and let me know if you agree or disagree with my conclusions
In fact, it seems that a floating ground would be safer from a shock perspective standpoint, because that way a human standing on the ground will be unlikely to be on the lowest resistance pathway to complete a circuit.
You are completely ignoring the differential DC voltage between the aluminium PV panel frames. When two panels develop DC leakage, then they will take the voltage of their position in the string.
That is why the mounting rail systems used are designed to bond all panels together, and to ground, so the leakage detection will trip. You ground all panels together to protect the fireman who must get up on your roof to where the DC arc is running between un-bonded mounting hardware.
 
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@yahastu What you wrote is not exactly an analysis , it's a bit of what you know together with some mistakes and wishes.

Lightning protection: Typically, lighting protection is one of the reasons cited for why homes usually have an Earth ground. If a lightning surge goes into your house wiring, then an Earth ground gives it somewhere to go so it's less likely to damage your electronics (or start a fire). On the other hand, adding an Earth ground actually makes your electrical system a more attractive target to lightning (by helping to create a low resistance path between the origin of the lighting and somewhere deep in the Earth). A much better protection from lightning is to create a separate lightning rod that provides a lower resistance pathway for the lightning than your house's electrical system. In addition, any nearby lightning strikes that miss your electrical system, could still cause damage to the electrical system by traveling through the ground in the nearby radius. For these reasons, I intend to install a separate, Earth grounded lighting rod...and as a result, it doesn't seem like there is no benefit to having an Earth ground on the house for lightning protection.
Lightning rarely strikes electronics inside houses if ever, lighting can strike a powerline but that's another issue. Earth ground doesn't make your house appliances a bigger target for lightning. Current flows only between potential differences, if that PD is across the sky and ground it's between that.
But I would agree if you have a completely off grid system maybe the better option would indeed be to have a separate lightning rod.


Shock protection: Typically, a house receives electricity from the mains power, which is grounded to Earth. If there is a short in an appliance whereby, say, one of the current carrying AC wires is shorted to the case of the appliance, and someone touches the appliance, then the person may form a lower resistance pathway from the case of the appliance to Earth ground, because the person is likely standing on Earth ground. This is often cited as a reason for having an Earth ground at the house: because if the appliance has a ground terminal, then the appliance case will be bonded to ground which goes all the way to Earth ground, and as a result, a person touching the case isn't going to be electrocuted. However, this protection is only necessary because the house power system is referenced to Earth ground in the first place. If the house had a floating ground, then the excess current wouldn't be searching for a pathway to Earth ground, and would have no reason to travel through the body of a person touching the electrified appliance case. It would be safe to touch the electrical wire, just as it is safe for a bird to land on an exposed electrical wire. So, when talking about an off-grid house, the desire for shock protection doesn't seem to motivate adding an Earth ground. In fact, it seems that a floating ground would be safer from a shock perspective standpoint, because that way a human standing on the ground will be unlikely to be on the lowest resistance pathway to complete a circuit.
You are making a crucial error here assuming that it is only the real current that flows from point A to B that counts as in DC , but remember we are talking about AC , AC for example flows through capacitors , and capacitors have a non conducting dielectric between the plates. AC has a component called reactive power, it is made of inductance and capacitance, everything has capacitance including your body.
Have you ever seen those screwdrivers with a small lamp , you put that screwdriver to a phase/live while holding it in your hand and it glows, yet you have rubber soles and most likely are standing on a wooden floor.
You body has capacitance , and AC charges and discharges capacitances every half cycle. Even with a plastic floor etc touching a chassis that is live due to fault will give you a tickling sensation or a mild shock depending on many parameters that can vary like humidity, dry or wet skin, your body size etc.
So even though the floor type and conductivity and shoe type and conductivity and skin etc matter in terms of how strong the electric shock will be it will still be there simply because of capacitance and so it is not safe either way.
But current through a capacitance is determined by the source voltage , adding a earth ground makes most of the fault current to bleed away into ground so the fault voltage is low if any given good GFCI breakers will trip quickly. This limits the shock severity and possibility due to limiting the fault chassis voltage to a low level and tripping the supply.
I'd say either way with grid or off grid you need to have earth ground and it's beneficial to have GFCI breakers for exactly this reason.
If you have an AC of say for example 230VAC in your outlets and those 230V for whatever reason appear on your washing machine chassis then i really doesn't matter whether it is grid or off grid, all that matters is that there are 230VAC on the chassis and in a situation without earth you will get a nasty hit irrespective of what is your house connection, whether grid or off grid. Simply because of capacitance.
If that happens when you are wet and just out of a shower it will be just made worse that's it.

All power to the house will come from the inverter -- either directly, as passed through from the battery system. Therefore, in the event of a broken appliance which has a short from a current carrying wire to the case, then the excess current will be searching for a path back to one of the inverter terminals. This seems like a decent reason to wire up my house's ground to one of the inverter terminals (say, negative)...as it would provide some circumstantial shock protection, although it seems the only possible circumstance would involve a person touching the inverter terminals while simultaneously touching a broken appliance case that is shorted out -- a pathologically unlikely circumstance.
If the inverter is not grounded then it's just an inverter with two inputs (from solar panels) and 2 or 3 outputs, the inverter itself is "floating" if it's output neutral or specified ground/chassis pin is not earthed.
In fact I believe all commercial and approved inverters must have a grounding option, please search google it shows countless examples of inverter grounding in a solar panel scheme.
Now since in a proper grounding scheme all points are grounded then yes your wall sockets grounds and inverter ground and other ones will be met at some point through grounding wires. But I suggest do not just connect them together and then leave floating , and even more, don't connect your grounding wires to the inverter power outputs, neutral for example. In this case the current from phase will be able to flow both in the normal return as well as in the fault return without any real consequences.

Electrical interference: having the house grounded to Earth can introduce electrical disturbances into my own electronics, which at best is undesirable (eg, not great for HAM radio), and at worst, a danger, if there is a nearby lightning strike that energizes the house via the Earth ground.
It's the current path that is crucial, a nearby lightning strike disposes the energy into ground, if you are within your house when this happens , then this poses no risk to you because there is no PD across any element. That is why ground rods are not made in many different places connected by different wires instead they are made at a single spot and house wires are tied to that spot.
Solar array: typically, the solar panel frames are all bonded together, and bonded to the house ground, which is of course bonded to the Earth ground. For better or worse, I did not run a ground through my buried conduit between the panels and the house, so it is not really possible for me to ground the panels to the house ground. However, I can't think of any reason to do so, other than by convention. The panels generate their own DC electricity, which is totally separate from the AC electricity generated by the inverter, so I don't see any reason why the two systems should want to share the same ground, especially if neither of them benefit from an Earth ground. The existence of the separate lightning protection system seems to eliminate any
Current plan: based on the above considerations, my current plan is to: a) bond the steel joists/I-beam of the house to the negative terminal of the inverter, to the ground wire in my main panel; b) not bother grounding the DC panel array; c) create an independent pathway to Earth ground in the form of a lightning rod, which is not connected to my AC or DC electrical systems.
You can still run a grounding cable to the solar panel chassis and metalwork and then connect that cable to your common ground point.
I'm not sure avoiding grounding and running your grounds to inverter negative is a wise idea. I would caution against doing that.
I would suggest doing as I and others have advised.

Let me show you a reason why capacitance matters in AC.
Yes birds do sit on power lines but their bodies are small and their legs are closely spaced so the total leakage current running through them is small to the point they are not endangered but see what happens when a helicopter which is also in air approaches a power line.
This is the reason the worker first attaches a wire to bring the helicopter and himself to the same potential as that of the line and so that the line charges and discharges the helicopter frame through that wire , if that wire was not there , the arc you see would travel through the body of the worker.
Sure household AC is much lower voltage but capacitance is still there and it will cause some current to run through you if you touch a fault device. Think about that.

 
  • #13
berkeman
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Thread closed for Moderation...
 
  • #14
berkeman
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Did you read my post? I explained all the reasons why building to code in this situation seems unsafe and illogical. I am looking to discuss this from an engineering perspective, NOT from a legal perspective. If you disagree with my analysis, please read my analysis and explain how you disagree.
Please consult your local building department. We are not going to debate the NEC here on PF. Thread will remain closed.
 
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