What is the ground rod for?

[kda3310]

OK, after taking Mike Holt?s cores on grounding versus bonding I still have one question stuck in my head. What is the point of the ground rod? If the system is bonded it seems to do more harm then good.

Jason

 

[shockme77]

To remove dangerous levels of voltage and current from metal parts caused by lightning strikes.

 

[jwelectric]

OK, after taking Mike Holt?s cores on grounding versus bonding I still have one question stuck in my head. What is the point of the ground rod? If the system is bonded it seems to do more harm then good.

Jason

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

 

[peter d]

To remove dangerous levels of voltage and current from metal parts caused by lightning strikes.

Does it really accomplish this? I don't see how a couple of 8' pieces of copper clad steel and a piece of #6 copper wire are going to be very effective in preventing lightning damage. We're talking about millions of volts at how many thousands of hertz. Any of the lighting strikes I have seen have trashed the electronics and other equipment in the home - all with ground rods and proper grounding of circuitry.

 

[celtic]

Any of the lighting strikes I have seen have trashed the electronics and other equipment in the home - all with ground rods and proper grounding of circuitry.

Did the house burn down...anyone killed?

 

[peter d]

Did the house burn down...anyone killed?

No. In one particular case, there was structural damage (roof shingles and sheathing blown off) but no fire.

I'm asking simply to satisfy my curiosity. Does a GES really accomplish much in the way of lightning protection.

 

[brian john]

Did you pull the ground rod out to inspect it?

 

[peter d]

No. Nothing on the service looked damaged (nothing blown up, no arc marks or anything else suspicious.)

 

[brian john]

Just wondering.....

http://www.fma-research.com/Q&A.htm

What is a fulgerite? OK, you're digging out in the back yard garden. You strike something hard. You keep digging...and unearth a 15 foot long, hard lumpy tubular "thing" that is white/green/gray in color. Quiz: What have you found? (1) Part of a UFO (2) A giant dinosaur leg bone? (3) fossilized lightning. Number 3 is the winner. Called a "fulgerite" it forms when a powerful lightning bolt melts the soil into a glass-like state. Such a record large specimen was found several years ago in Michigan. Visitors to the Great Sand Dunes National Park in Colorado get to see example of fulgerites extracted from the/ he nearby sand mountains. And in order to make point to the many hikers who take off for a days stroll through the dunes, the last sign they see warns them to take cover in thunderstorms and not to become fulgerites.

 

[bphgravity]

For those really interested in a good history of why we ground electrical systems, I would suggest getting a hold of the Soares Book on Grounding and read Appendix A.

I think it helps to understand what 250.4 really means. The bottom line is that the purpose of ground rod is to provide a code compliant method of grounding an electrical system. That is about it.

 

[George Stolz]

One item to note:

If the system is bonded it seems to do more harm then good.

Bonding does a lot of good. It ensures OCPDs open in the event of a ground fault.

Grounding is the ground rod's function, and skepticism is warranted of it's ability at that.

Check out this thread for an example of grounding that made a significantly positive impact on the system.

 

[bphgravity]

I just want to mention that George's thread and article is referring to a Lightning Protection System specifically designed and installed for that purpose. Though 250.4 implies grounding of the electrical system provides protection against lightning, it can't be looked at as lightning protection.

It seems like a minor point, but I feel it is necessary to keep hammering at this point. Over the last 100+ years the code has mixed methods and practices for protecting against outside surges and the practice of grounding one secondary conductor of an electrical system into all the requirements in Article 250. I would be willing to bet a telegraph lineman from 1890 would have a better understanding of Article 250 as it reads today than any of us.

Of all the changes in the NEC, Article 250 requirements are really the least changed in 100+ years. Though the article has been reformatted and moved around, many of the old requirements from when the electrical system included everything from the last outlet and back to the power source itself is still in there.

 

[tom baker]

A ground rod may protect the building from lighting.
If you want to protect the electronics and wiring inside, that can only be done with TVSS. Mike makes this point in his GvB classes.

 

[hillbilly]

"lightning damage. We're talking about millions of volts at how many thousands of hertz".

I've always thought that a lightning strike was direct current....
How is Hertz related?
Just a thought
steve

 

[al hildenbrand]

I've always thought that a lightning strike was direct current....
How is Hertz related?

The sudden "turn on" and "off" is where all the high frequency comes from.

Consider an ideal square wave. First its off, then its on at a DC value, then its off again.

The two state changes, in an ideal square wave are instantaneous. In the real world there is rise time, but it is still very fast.

The ideal, instantaneous "on" and "off" and flat on state can, mathematically, be created by adding together sine waves of varying magnitudes, starting with the fundemantal frequency matching the duration of the "on" state, and then running right on up to an infinitly high frequency.

 

[al hildenbrand]

With lightning, the bolt isn't just on, then off. The discharge goes through many steps up and down.

Each step change is extremely fast, so there are a lot of high frequency components in each lightning bolt step, even when the whole bolt is only a fraction of a second in duration and all that's left is the thunder clap.

 

[hillbilly]

That's interesting...I never thought of that.
How is that determined or measured?
steve

 

[tom baker]

II would be willing to bet a telegraph lineman from 1890 would have a better understanding of Article 250 as it reads today than any of us.

Of all the changes in the NEC, Article 250 requirements are really the least changed in 100+ years.

Agree. It seems to me early on the code came to grips with grounding, came up with the requirements:
Ground rod 25 ohms
Max length of external bonding jumper 6 ft
Etc
But we don't know the why or justification, its long gone.
Of course some of the language is confusing, but essentially we ground to protect from overvoltage, side flashing, and bond to open the circuit protective device

 

[zog]

Here are some fulgurites

 

[don_resqcapt19]

Steve,

How is that determined or measured?

I think that you connect the test instruments to a kite string:grin: