Ground connection in ohms

[Barba]

Hello people.
Can someone tell me how many ohms is good to go with the ground connection for the off grid inverter?
Thx so much

[inetdog]

Hello people.
Can someone tell me how many ohms is good to go with the ground connection for the off grid inverter?
Thx so much

infinite is fine.

1. There is no strong need for an earth ground at all for the AC output of an off grid inverter. But the manufacturer's instructions may require an earth ground anyway.
You do need a path by which any leakage or fault current in the AC wiring or attached devices can find its way back to the inverter so that fuses or breakers can clear the fault.
Usually an earth connection is not a useful part of such a path.
2. It is not at all easy or straightforward to measure the resistance of a ground connection when you use one. How are you planning to test this?
3. The NEC tells you how you are allowed to make an earth connection (Ground Electrode) but not what resistance is allowed.
4. The value of 25 ohms appears in the NEC for obscure reasons, but is not a performance requirement.

[Mike90250]

Back when copper or steel pipe was use as an :Earthing connection, it was pretty straight forward. But now, you have to be sure your "reference point" Ground is much less than the official ground rod, because when you measure, you are adding both resistances - the Goes-inta and Comes-outa.

[Sunking]

Anywhere from 0 Ohms to infinity will work just fine. As long as you have at least two rods buried is all that matters.

Earth does nothing except provides a path for lightning.

[Barba]

I have no idea how I am going to do this test yet.
But the instructions I have says below 2 ohms.
I was planning to install the rods near the inverter around 10/20 ft.
The cable section to be used has a section of half inch!!!!
It seams quite serious! But still afraid not to be able to reach that value...
What about using more than 2 rods?

[Sunking]

I have no idea how I am going to do this test yet.
But the instructions I have says below 2 ohms.

Throw the Instructions away they are useless mumbo jumbo written by tech writers who have no clue what they are writing or why.

To obtain a 2 Ohm earth Resistance would require a copper mine buried in the ground. Even if it were possible to obtain 2 Ohms buys you nothing, That is way to high of a resistance to operat eeven a small breaker at voltages under 600 volts. Think about it 2 Ohms x 10 amps is 20 volts. On a low voltage system is completely useless. By low voltage I mean 600 volts and less. That is why Electrical Codes forbid ground to be used as a circuit, it is useless.

All the code, any installer, electrician, technician, engineer, and inspectors want to see is 2 ground rods separated by 6 feet between them at the point of entrance. Screw the instructions.

Trust me there is a lot of things you can say about me, but SAFETY is goal one on my book. No one would ever say I tell anyone to do anything that tis not safe, sound practice. That is what I am paid to do.

Here is what the codes actually says.

Part 250.53 (A.2), Supplemental Electrode Required

A single rod, pipe or plate shall be supplemented by an additional electrode of the type listed in 250.52 A, 1-8 (above). NEC provides an exception: If a single rod, pipe or plate electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.

Translated means drive two rods, call it a day, and collect a check your done with it.

Catch is it is almost virtually impossible to obtain a 25 Ohm ground from just one single rod in the ground. No Inspector is going to accept it. Solution, drive a second rod and call it done. There was a reason long ago for the 25 Ohm requirement. Most engineers and electricians do not know where the requirement came from. It came from the days of Telegraph and Early Telephone service. Single Wire Telegraph required a low resistance connection to earth and used earth as a conductor. It takes two wires to have a circuit and Telegraphs only ran 1-wire the cheap bastards.

Early day telephones were Party Lines, and the Telephone company used Earth as part of the Ringer circuit on Party lines. Takes two wires to make a circuit. They can apply ring voltages in 3 modes. Tip to Ring, Tip to Ground, and Ring to Ground. At your house you would be connected Tip to Ground while your two neighbors were Tip to Ring and Tip to Ground. That allowed your phone to ring and not your neighbors.

Do you have an old Party Line or Telegraph Line? If not throw the dam instructions away. They do not have a clue what they are saying. Just repeating what the other Monkey said.

Only the privileged and educated get to use Earth as a conductor. They are the only ones that can meet the requirements to make it safe. First requirment is the operating voltage must be high in what is called Medium Voltage and above. Medium voltage is 4160 volts and higher. Only utilities and the big boys get to play with that. Public is not welcome to the club and will be shot if caught trespassing.

They only thing the public can do is bury a single wire in the ground to give lightning a path home. Otherwise electrically you are forbidden to use earth under the penalty of death. One mistake and you are dead. No questions asked.

[Barba]

I have 2 rods of pure copper 6 ft long. I will dig a hole and put water also before stick them in.
Can I use this ground connection for the SPDs?
I also got a new battery bank project of 280 Vdc where I have to ground either the + pole or - pole to the ground.
Can I use the same rods for all them?
Thx

[inetdog]

I have 2 rods of pure copper 6 ft long. I will dig a hole and put water also before stick them in.
Can I use this ground connection for the SPDs?
I also got a new battery bank project of 280 Vdc where I have to ground either the + pole or - pole to the ground.
Can I use the same rods for all them?
Thx

You not only can use the same ground electrode system for all of them, you are pretty much required to.
If you only use 6 foot copper rods you will not have enough rod in the ground to meet NEC requirements.
If you dig a hole and fill it instead of driving the rods into the ground your resistance will be much higher and digging for the ground rods will not meet NEC requirements either.
Finally, it is extremely unlikely that two rods of any length inserted into the ground in any way will get you to your 2 ohm goal. You are more likely to see something above 20 ohms.

If you want to get a one time measurement, the digging a hole and filling it with salt water (without backfilling any dirt at all) will probably do the job. And a few days later the resistance will be higher then if you just drove the rod in.

Bottom lines:
1. There is no reason to need or justify a 2 ohm ground resistance.
2. Which is good, since it may be prohibitively expensive to get it. Certainly not going to get it with anything you have suggested so far.

[Mike90250]

That's why UL listed ground rods are steel, with a copper clad coating on the outside, needs the steel to be stiff enough to pound in (use a rotary hammer)

[Sunking]

I have 2 rods of pure copper 6 ft long. I will dig a hole and put water also before stick them in.
Can I use this ground connection for the SPDs?

Rods must be 10 feet long

I also got a new battery bank project of 280 Vdc where I have to ground either the + pole or - pole to the ground.
Can I use the same rods for all them?

Required by code, rods have to be bonded together to form a common Ground Electrode System.

[inetdog]

Rods must be 10 feet long

Yes and no....
Eight feet of the rod must be buried in the earth after the rod has been driven.
The simplest way to do that is to use a ten foot rod, but an eight foot rod with the top end just below ground level will also work.

[Sunking]

Yes and no.

I stand corrected, minimum length is 8-foot length and 5/8" diam sold in every Box Store. However mos pros use 10-foot rods 1-inch diameter sunk 18 to 24 inches below grade. Problem is most folks and even some sparkies really do not know what earth ground does or how to use it.

Avoid 5/8-inch diameter rods if possible. They just are not rigid enough to take the pounding required to sink them in the ground unless you use the WATER TRICK. Think Vaseline and spanking the monkey.

[Barba]

In Italy, where I have this newsystem , the law requires less than 5 ohms. The company is have contacted for doing the job said 2 ohms can be done but requires a special treatment of the soil.in the same time they told me 2 ohms can be achieveable in time after the treatment.
I ask them what's the procedure like and I was answered they drive 3 rods of 3,5 meters(11ft circa) 2 meters below the surface.
The drive speed must be such that it must be buried with 3/4 poundings.
I ll let you know what i achieve afterwards.

[Sunking]

Barb are you in a rural area that uses SWER power (Single Wire Earth Return)? There is no reason to have a low resistance ground unless your country uses earth as a conductor which is extremely dangerous.

Whoever told you all it takes is 3-rods and chemicals to achieve 2-Ohms is taking to many chemicals and is delusional. Sure 3 rods might work in a Salt Water Marsh, but not most normal soils. They are talking up to hundreds of pounds of copper buried in the ground. Sure I can build you a 2-Ohm ground today, but has no guarantee as in a month of so can easily go up to 100 Ohms as soil conditions change throughout the year.

SWER Power is used in extreme remote areas of Alaska and Australia, but only as a last result.

[Barba]

I don't live in a rural aerea, but 5 ohms is required by law when DC voltage is above 80 Vdc.
The inverter instructions is talking about 2 ohms.
I ask the company and they analyse the soil before doing the job.based on the soil type they will operate accordingly.
The contract states 2 ohms in achieved after 2 weeks and it is guaranteed for 5 years.
I don't know the price yet as it all depends on the soil.