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Old 09-04-2008, 02:41 PM   #1
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Hypothetical Lightning/Grounding question


Ok, First off all, this is a purely hypothetical question (I am bored at work). I do not have a detached workshop (the Mrs said nothough there is some hope)

that being said:
Consider a generic regular house (with ground rods) plus a 4 wire sub panel in a detached building with ground rods all proper and done per code.

If lightning strikes a tree relatively near one of the ground rods, isn't there going to be a large voltage difference between the house ground rods and the detached building ground rods (assuming one set is closer to the tree than the other). And wouldn't this large voltage difference mean that you will have a pretty large current going through that ground conductor connecting the buildings?

Just one of the strange things I think of when incredibly bored... (no wonder my wife keeps me so busy at home)

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Old 09-04-2008, 05:00 PM   #2
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Hypothetical Lightning/Grounding question


That's a big question. When lightning strikes, potential differences go off the charts and just about everything everyone knows about electricity is on the rocks. Mother nature can deal some strange cards that we still don't quite understand and can only "guess" what will happen.

I'm close to saying that the "current" would be really high. I'm not a physicist, but, to answer your question your going to have to understand Amperes vs Coulombs/sec. and the speed of light. (without Einsteins help)

My opinion?, ain't nothin gunna happen to make any sparks fly.

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Old 09-04-2008, 05:40 PM   #3
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Hypothetical Lightning/Grounding question


You stated this was done to code. So there is no problem. All ground rods on the premises are required to be bonded together. So they would all remain at the same potential. And you just hit on the reason for that requirement.

Now get back to work.
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Old 09-04-2008, 08:13 PM   #4
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You stated this was done to code. So there is no problem. All ground rods on the premises are required to be bonded together. So they would all remain at the same potential. And you just hit on the reason for that requirement.

Now get back to work.

I would tend to believe that the "bonding", and being at the same potential would cease to exist with the amount of force behind a lightning strike. With between 100 million and 1 billion volts to ground and up to 200,000 amps I believe the resistance of a #6 copper conductor and the I squared R losses associated with that resistance would make the actual bonding between the two rods useless.

The whole grounding electrode system is only designed to give the lightning a path to ground so it hopefully doesn't have to burn itself a path to ground through your house after it gets into your wiring. I don't believe on a direct strike you could even hope for that much.

Maybe someone else has a code book handy and can do the math for the resistance per foot of a #6 copper conductor. I don't have the ambition tonight.
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Old 09-04-2008, 08:56 PM   #5
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Hypothetical Lightning/Grounding question


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You stated this was done to code. So there is no problem. All ground rods on the premises are required to be bonded together. So they would all remain at the same potential. And you just hit on the reason for that requirement.

Now get back to work.
I would agree that they will try to stay at the same potential, unfortunately the "ground" potential at the two locations will be much different. There is enoug resistance in earth that the voltage will vary considerably with distance from the strike. They are different enough between your feet that if lightning strikes a nearby tree it will actually come up out of the ground into your leg and back down out the other (which is why you should try to stand on one foot if you feel a lightning strike about to happen). Remember, current takes the path of least resistance, and that copper ground wire has a heck of a lot less resistance than the earth.

If I had access to a lightning prone area, I would sink two ground rods ~50' apart and connect them with a piece of wire. Then wait and see what happens in the next lightning storm... anyone feel like doing an experiment?
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Old 09-05-2008, 06:21 AM   #6
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Hypothetical Lightning/Grounding question


Lightning sometimes does what it wants to despite our best plans (much like the wife). Never heard that about standing on one foot. pete
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Old 09-05-2008, 09:12 AM   #7
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Hypothetical Lightning/Grounding question


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Originally Posted by daxinarian View Post
if lightning strikes a nearby tree it will actually come up out of the ground into your leg and back down out the other.

anyone feel like doing an experiment?
Sure. Connect a #6 copper wire from one big toe to your other big toe.
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Old 09-05-2008, 09:14 AM   #8
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Hypothetical Lightning/Grounding question


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Originally Posted by Silk;155124l
I believe the resistance of a #6 copper conductor and the I squared R losses associated with that resistance would make the actual bonding between the two rods useless.
I guess they put the requirement to bond all rods on the premises in the code just to sell more #6 copper wire.
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Old 09-05-2008, 09:22 AM   #9
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Hypothetical Lightning/Grounding question


Quote:
If lightning strikes a tree relatively near one of the ground rods, isn't there going to be a large voltage difference between the house ground rods and the detached building ground rods (assuming one set is closer to the tree than the other). And wouldn't this large voltage difference mean that you will have a pretty large current going through that ground conductor connecting the buildings?
Professional answer......YES


First current seeks all paths, but most will flow on the path of least resistance/impedance. Lightning being a rather high DC voltage event has little trouble pushing current into the ground/dirt. It will find the ground rod at the transformer, your ground rods at the house and detached building and water pipes, communication lines whatever. Given resistance over distance most will probably flow on the ground rods,grounding conductors and the equipment ground of the feeder to the detached building nearest the strike. It is likely that the equipment ground may melt (along with the other wiring)and damage to the electrical in the detached building will occur. But the house may be saved.. but not always. There likely will be a huge surge in voltage till that happens, so surge protection either at outlets or at panels can help prevent damage to appliances from surge voltage. Though with lightning there are times when nothing is going to help.
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Old 09-05-2008, 09:42 AM   #10
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Hypothetical Lightning/Grounding question


Here is a real life example of bonding. At my home. I live in the woods, in sand country, surrounded by trees which seem to be magnets for lighting strikes.

Over the years we seemed to loose telephones to lightning strikes. The phone would make a loud ding during a strike and would never work again.

Then we got satellite tv. The box has a modem inside. The lightning fried two of the boxes the first summer.

Then we got the interent, dial up connection. Lightning fried 4 modems and 1 whole computer during that summer.

The telephone company came out and drove another rod to cure the problem.

Then we bought surge protectors. The next summer lightning fried 7 surge protectors, 2 modems, and the satellite box. These were separate strikes, not all one.

I asked the telephone company about bonding their rods to the electric service rods per code. They told me not to dare do that or they would torture and kill me. Or take away my phone service. They were adamant.

In a continuing education code course I asked the instructor about the bonding. He told me it was a requirement to bond and to tell the telephone company to take a leap.

So I snuck outside and ran a #6 copper wire from the telephone rod around the back of the house to the electric service rods. The distance is about 50 feet.

I did that about 8 years ago and have lost absolutely nothing since. Except some trees.

Anyone want to tell me bonding the rods together doesn't work? And that the requirement in the code is for nothing?
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Old 09-05-2008, 03:29 PM   #11
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Hypothetical Lightning/Grounding question


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Anyone want to tell me bonding the rods together doesn't work? And that the requirement in the code is for nothing?
Yeah, Iíll take a stab at it.

I believe the resistance of #6 copper is around 0.491 ohms/kft. Correct me if Iím wrong

Lets say that the detached shed/garage whatever it is, is 50 feet away

The formula for single phase voltage drop is: Vd = (2*L*I*R)/1000

Now we should be able to agree that the available current of a lightning strike is around 200,000 amps. If you disagree, just google it.

Vd = (2*50*200,000*0.491)/1000
Vd = 9820 Volts

Now Iíll be generous and say that about Ĺ of the current will go through the earth ground instead of your bonding wire (Much more of the current will go through the earth than that, but weíre working off of your assumption). That would leave 4910 volts difference between the 2 ground rods.

Let me be even more generous and say that 90% of the current would go straight to earth instead of through the bonding wire. What do we get? 982 Volts!

Letís convert that to Watts 982 * 20,000 = 19,640,000 watts or 19.64 Megawatts

Thatís one hell of a big toaster you just buried in your walls! I believe the wire would just vaporize!

Now you tell me, how much do you think that #6 bonding wire is going to help you on a direct or near lightning strike (as the OP was talking about)ÖÖÖÖ PLEASE TELL ME!

Now I donít believe anything I just said because I think the current (on a near/direct strike) would go almost 100% straight to earth and not travel over 50 feet to a second ground rod and this scenario would not happen. But if Iím mistake by only 1% you just vaporized your conductors. I was just trying to illustrate the ridiculousness of believing a bonding wire will keep 2 points at the same potential in the case of a direct lightning strike.

The whole point behind the Grounding Electrode System is to establish a zero potential to ground for your electrical system and to give lightning a path to ground.

The purpose of bonding is to keep all your conductive metals at the same potential which is hopefully zero volts to ground, but we all should know by now that doesnít really work to well either ( try researching stray voltage).

In answer to your post, Yes, bonding does serve the purpose it was intended for, which is not..... I will say again........Not to keep ground rods the same potential during a lightning strike

I need a drink after that one
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Old 09-05-2008, 03:31 PM   #12
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Hypothetical Lightning/Grounding question


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Remember, current takes the path of least resistance, and that copper ground wire has a heck of a lot less resistance than the earth.
If you think that a wire is a better conductor than the earth, you are sadly mistaken. You must first understand that resistors connected in parallel reduce the total resistance. So if you can imagine the earth as a conductor with a resistance (as all conductors do), then you must also see that there are virtually an unlimited number of resistors in parallel, the entire planet! That is what makes it such a good conductor. That is why utilities use it as a return path for their own power system.

P.S. By the way, the Minnesota public utilities commission found that 59% of current returns through the earth. The Michigan attorney general says as much as 75% of consumers energy returns through the earth. Now mind you, this is the current that is supposed to be returning through the neutral connection. Does that convince you of what a good conductor the earth is?
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Old 09-06-2008, 08:38 AM   #13
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If you think that a wire is a better conductor than the earth, you are sadly mistaken.
I don't believe I just read that.
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Old 09-06-2008, 08:47 AM   #14
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Hypothetical Lightning/Grounding question


Believe it. Do you understand the concept behind parallel paths? All this nonsense about voltage gradiants and jumping with your legs together to get away from a downed wire was taught many years ago. Today we mostly realize that appr. 90% of the voltage will be absorbed within the first 6 inches of contact. The reason behind not getting a good earth ground is the connection point to the earth not the earth itself. Once you are withing the earth you have unlimited pathways for the current to travel. What happens when you put 2-50 ohms resistors in parallel? You get 25 ohms. Now what happens when you put trillions upon trillions ( or as Carl Sagan would say "billions and billions") of resostors in parallel?

Think about it, and then get back to me.

Oh, and by the way, do you understand the difference between bonding and the grounding electrode system yet?

Last edited by Silk; 09-06-2008 at 08:50 AM.
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Old 09-06-2008, 08:54 AM   #15
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Think about it, and then get back to me.

Oh, and by the way, do you understand the difference between bonding and the grounding electrode system yet?
I am not even going to respond except to say this was completely uncalled for. I am not on this board to be personally attacked and bullied by you Silk. Have a great life. I am off this board.

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