Solar PV earthing - cable size?

[bcroe]

I'm not arguing with the experienced. But it certainly is true, the safety ground is a different situation than lightning protection. My ground
array is out in the open, some distance from any building. It contains 48 concrete post grounds and 10 ground rods, well distributed to
tie the framework to ground. The resistance of each might be 25 ohms, but collectively they will at least reduce lightning damage.

Those are not a safety ground, which is an entirely different set of conductors with a very low resistance connection to the distribution box.
Bruce Roe

[somawheels]

In my case, I have no nearby utility lines and have a small PV (1.5kw) on a hillside.
It sounds like I am better off not grounding the solar panel frames at-all.
I have a Flexmax charge controller, and have grounded it because the manual seems to suggest I do this whatever my circumstances.

[sdold]

Thanks Dereck! I'm sorry if I implied that the ground rod plays a part in fault currents, I was trying to say the opposite, that it has nothing to do with them and that it's the job of the EBC to CAUSE a large fault current (back to the source) to open the OCPD. I thought you were trying to make the same point by asking "Why connect to earth ground" but I see now that wasn't what you meant. Sorry about that. There are at least a couple of Mike Holt videos describing what you talked about, with the ground rod resistance you mentioned. In one of them, he actually connects 120V to the rod and measures voltage at the rod (which is still 120V) and at various points outward from the rod.

[Sunking]

I don't mind if you're critical, it helps me to remember. Thanks for taking the time to talk about this. I'm having a lot of fun watching Mike Holt's videos and reading his papers about this.

Steve try to remember this and it may help. In Low Voltage applications (600 volts or less) NEC forbids earth (dirt and rock) to be used as a Conductor for the simple fact earth is a poor conductor despite what your 4th grade science teacher told you. Stop and think about that for a minute. You go out and drive a couple of rods into the ground, and you are lucky if it test 50 ohms. Now ask yourself this. What would the fault current be on a 120 volt circuit with a 50 Ohm fault return path.?

Did you come up with roughly 2 amps in your head? What is 2 amps on a 20, or 15 amp breaker? A small 250 watt load circuit right? It is not going to operate any 15 or 20 amp device now is it? 2-amps is load current, not fault current. Ideally we want Fault Current to be 6 times the OCPD rating so it operates any OCPD real dang fast.

Now when we get into high voltages Earth is used as a conductor and utilities use earth as a conductor. All the rules change when you go to high voltage. But that is another subject.

[emartin00]

I've seen a many websites say otherwise. some also say Lightning strikes some distance away can damage your equipment and this is a reason to earth the PV frame.

I hate to break it to you, but those many websites are incorrect. I'm an engineer and I work for a very large company that designs and manufactures electrical connectors, so it's my job to know these things.
Lightning protection is and entirely different thing, and requires MUCH bigger connectors and conductors.

[Mike90250]

I see, thank-you for that explanation.
........
I thought the main reason for earthing PV metal frames, is to prevent lightning strikes.

HA. Grounded metal will attract lightning like umbrellas at a golf course !
But the ground wire and rod direct the current around the outside of the house (if installed correctly) and you don't have a fire.
All your electronics and electrical gear will likely be fried, that's what insurance is for, Ground wires prevent fires.

[cebury]

Awesome job Sunking

[sdold]

I don't mind if you're critical, it helps me to remember. Thanks for taking the time to talk about this. I'm having a lot of fun watching Mike Holt's videos and reading his papers about this.

[Sunking]

I answered your question the way I did because to me, grounding describes a connection to the earth,

I understand and as I said you are making a common mistake. I am not trying to be critical of you, far from it. It is for that very reason that the last 2 NEC code changed a lot of the terms. Today there is only one cable known as Ground That would be the GEC. That cable originates at the Service connects where Neutral and Ground Buss. Usually at or near the Main Breaker Panel Box. The other end goes to the facility Ground Electrode System aka GES. All other conductor names do not have the word Ground. I gave one example EGC was changed to EBC. About everything thing else is just called a Bonding Jumpers.

Point here is Ground is a loosely used word. But if you were to have a test question to define Ground as it applies to electrical code means: Purposely electrically connected to Earth, or something else in place with Earth. Like I said the chassis of a car, hull of a ship, airplane frames, or even a Ham Sandwich

Perhaps this might help, think of ground as a Reference Point for Voltage. It is the ZERO Reference Point.

When you asked the question "Why ground?", I thought you meant "Why connect to (earth) ground?" Sorry if I misunderstood. Can I change my answer to "f"?

The right answer is F. The question did not have any specific sub system like a Bonding Jumper, it is all inclusive.

[sdold]

You are making a mistake even experienced Sparkies make. You assume the word Ground has something to do with Earth. It does not necessarily mean that

I answered your question the way I did because to me, grounding describes a connection to the earth, and bonding describes a connection back to the source for the sole purpose of carrying a fault current. When you asked the question "Why ground?", I thought you meant "Why connect to (earth) ground?" Sorry if I misunderstood. Can I change my answer to "f"? The ground tester is a modern version of the old "Megger", it's the 4-terminal type for 3-point FOP or 4-point tests. It also has a clamp-on feature that relies on the rod under test to be connected to a larger system with other electrodes.

[Sunking]

You see this a lot though. This is from a Fluke document that came with our new 1625 ground tester: "If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people."

You are making a mistake even experienced Sparkies make. You assume the word Ground has something to do with Earth. It does not necessarily mean that. .

Some Sparkies even have trouble understanding what a Grounded System is. All it means is one circuit conductor is intentionally bonded to Ground or somethin gin place of ground like a car frame. What we are discussing is a Grounded System. One circuit conductor is Grounded aka Neutral. It makes Over Current Circuit Protection real easy by just using one OCPD on the undergrounded hot circuit conductor aka Line.

I know how your Ground Tester Works, it requires no connection to earth. All they are doing is impossing a current on L-N and measuring the voltage between N-G. If they see a voltage, the meter knows there is not a proper EBC in place or the third Green Wire.

The Planned Fault Path is the third green wire. Both the Grounded Circuit Conductor (improperly used as Neutral), and Green Ground Conductors are bonded together at a common Buss Bar in your panel box. Look at the diagram. At that Ground buss is where the GEC terminates to reference both N & G to Earth Potential. No current flows through the GEC to earth. The green ground wire never carries any current unless you have a fault as shown above. The fault current then uses the green G conductor to return the current to the source.

That third wire or Green wire use to be called Equipment Ground Conductor (EGC) . There is that damn word Ground again. So to clarify we changed the name to Equipment Bonding Conductor or EBC. The Planned Path is the Green Wire, not earth or ground.

[sdold]

You see this a lot though. This is from a Fluke document that came with our new 1625 ground tester: "If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people."

[Sunking]

I'll stick my neck out and say "e". I hope I'm right, the reason is that I don't see how a ground connection helps trip overcurrent devices.

Excellent question. Now you get a shocking answer.

No ground is required or needed to operate a fuse or breaker, just a path. Examine the pix below.



What you see is an accurate representation of any house hold circuit. It is a properly Earth Grounded System where one of the circuit conductors is bonded to a Reference aka Earth. That is done at the service entrance, Going to your receptacles are 3-wires called Line (Black), Neutral (White), and Ground (Green) . Note the fuse installed on the Line conductor aka hot. Under normal operation load current flows from Line to Neutral conductors only. Ground is doing nothing.

Chit happens and inside your gizmo the Line Conductor becomes detached and touches the equipment case energizing your gizmo you are holding in your hands. Sparks and fire fly, then quits. When the Line Conductor in second pix makes contact with the grounded chassis via Green Ground wire bonded to the chassis. You now have a dead Fault of L to G. That results in a 100 or more amps flowing in the Line and Ground conductor. In the Line conductor you have a 20 amp breaker with over 100 amps flowing. So the breaker operates within 6 cycles and opens the circuit removing power.

Now let's take out our imaginary eraser and erase that Earth Ground and let it Float. We repeat the same thing above and it still works with no Earth Ground. Why?

Because the N & G Conductors are bonded together at the Main Bonding Jumper. Power does not flow through ground. It comes in and leaves on the utility conductors, not earth. In other words current returns to the Source. Does not matter if it is the utility, battery, or generator. Only one power source uses earth as a conductor, lightning and static.

[sdold]

I'll stick my neck out and say "e". I hope I'm right, because I don't see how a connection to ground helps trip overcurrent devices.

Edit: If you meant "Why bond", I think it's "f".

[Sunking]

How does Utility power get to the solar panel frames?

Two ways.

1. Overhead lines tend to fall.

2. Utility Transformer or distribution Primary to Secondary fault.

The way to look at this is earth grounding is NOT to protect what happens inside your home as it cannot possible do that. It is to protect your home from what is outside trying to come in. I have taught quite a few Grounding and Earthing classes to contractors and through Mike Holt Higher Education and had a test and told every student if they did not get this question right they would fail. Did not matter if they got all the other 49 right, fail question one and you fail the test and course. No one ever missed it, now your turn.

Q Why Ground?

a. Provide an effective low impedance Planned Path for external Faults (example utility services like power, CATV, and Telco) to be routed to earth safely.

b. In larger commercial and industrial installation short out capacitive coupling between circuit conductors capable of developing 25 amperes of steady-state AC leakage endangering personal. Secondary is leaking current deteriorates cable insulation.

c. Minimize touch potential differences.

d. Provide a reference point for signals, transients, and return point of low impedance circuit conductors to safely and quickly operate over current protection protection devices through their equipment bonding conductors.

e. Minimize static charges

f. All the above

One last point. Under normal operating conditions Ground Does Nothing, at least not in your homes and electrical systems. Just because it does not normally do anything does NOT mean it is not needed because it is. It is there to minimize your risk of FIRE. Notice I did not say electrocute you? It certainly aids in minimizing the risk of electrocution, the code is there to prevent fire and explosions. Face it most home have two power sources that do not play well together. Bad things happen when you play with electricity and natural gas or fuel oil.

So like it or not, we guys who make the code conspired to make one size fits all approach for Article 250 which covers Grounding and Bonding. In some application the minimum requirements may exceed a particular application and be some over kill. So what it is safe. It would be impossible to have different requirements for all the possible applications.