Lightning & conductor impedance

[mclain]

I have been having a discussion on lightning strikes. Does anyone know what the impedance would be in a 50? run of 535-mcm cable that was used as a conductor between a air terminal and grounding point during a lightning strike. I am of the opinion that with lightning being a high frequency event the resulting impedance would negate most of the value of having a deep earth grounding point.

Don McLain

 

[dereckbc]

Re: Lightning & conductor impedance

Don I have that information at home, if you can wait until I get there I will post, just cannot remember of the top of my head. I can tell you the size of the conductor has very little to do with the impedance @ lightning. The formula is based on number of amps rise vs time, the most common number is 10,000/8 microseconds which comes out to about 1000 volts per linear foot of cable provided there is no encirclement.

 

[wirenut1980]

Re: Lightning & conductor impedance

I agree that with a high frequency of a lightning strike, the impedance will be higher, but would the impedance not be higher on any metallic path? IMO the idea is to have as low of an impedance as possible (or required), so that if lightning does strike the terminal, the majority of it's destructive voltage and current will choose your LPS system path to ground, and not some other undesired path.

 

[mclain]

Re: Lightning & conductor impedance

David I agree you want to achieve as low as possible within reason. My point is if you can obtain 4 or 5 ohms ground resistance with traditional grounding and 1 to 2 ohms with deep earth grounding which could cost four to five thousand dollars more. Then you add in the impedance of the conductor that might be 100 ohms (or greater) and you have 105 ohms with traditional grounding and 102 with deep earth grounding. Is it worth the additional cost?

Don McLain

 

[dereckbc]

Re: Lightning & conductor impedance

Don, I design and write lightning, grounding, and protection engineering practices for telephone companies. If you want to stick a label on it its called a Power Protection Engineer, or Central Office Power Engineer.

You have a point in one since, but might be overlooking some mechanics. You are correct, the impedance of a conductor of say 10 feet does exhibits an impedance of around 1000 ohm?s, the size of the conductor is insignificant. When you talk about a 5-ohm GES, you are taking reference at say136 hertz, which has nothing to do with lightning frequencies. So you are trying to add 1K-ohm to 5 and come out with 1K, doesn?t work that way. You are trying to add apples to oranges.

OK, enough confusion. When I specify say a 5-ohm system, I want a lot of copper in the ground. The reason is ground saturation and step potential. I want my site to be surrounded by a ring with radials extending away from critical areas. The large amount of copper in the ground gives it the ability to discharge lightning quickly without saturation and minimizes step potential differences to acceptable levels. A single or two-rod system cannot do that. If it were not specified, you can imagine what you would get.

 

[tom baker]

Re: Lightning & conductor impedance

mclain
This won't help you, but I had a slide rule from Polyphaser that calulated the impedance of a wire type for lightning. I gave it to Mike Holt and have yet to get another. Perhaps you could get one from Polyphaser

 

[mclain]

Re: Lightning & conductor impedance

Dereckbc
I will give you the real world situation I am looking at. This is for portable structures that move every two to six days. We have thousands of square feet of metal in contact with the earth. We have one ground rod at the generator skid and main grounding point is a 12? pipe set approximately 30? in the earth. 400 mcm bounding cables in a double loop fashion connect all skids so if one cable is cut the bonding loop will still be attached. We normally have a ground reading of around 1.6 ohms. It has been suggested that we install a deep earth grounding point and run a cable approximately 50? to this point and bond it to our bounding loop to increase lightning protection. I do not see how it would help due to the high impedance that would be caused by a lighting strike. It has been my experience that the lightning will be discharged by the metal structure more so that the GEC.

Don McLain

 

[dereckbc]

Re: Lightning & conductor impedance

Originally posted by mclain:
I do not see how it would help due to the high impedance that would be caused by a lighting strike. It has been my experience that the lightning will be discharged by the metal structure more so that the GEC.

I agree since you have a portable building. Does this building building have a metal skin, and what is it used for if you do not mind me asking?

 

[dereckbc]

Re: Lightning & conductor impedance

Don, I cannot locate the equation but here are some numbers from IEEE std 1100 (aka Emerald Book) Table 4-1 for 4 AWG and 4/0 @ 40 feet.

4 AWG @ 40-feet = 20 micro-henries
4/0 @ 40-feet = 18 micro-henries

 

[mclain]

Re: Lightning & conductor impedance

Dereckbc
The structure is an oil-drilling rig with 150? mast. It is constructed of I-beams and large steel columns. There is no outside skin. The base is of 20? I-beams. The load on the base can be over one million pounds. We recently had a lightning strike were one person received enough of a shock to require medical attention. No one on the structure was shocked. The people who was shocked was working in a concrete lined pit about 8? deep directly below the rig. When the lightning struck the charge was discharged into the earth through the steel structure and GEC. The man who required medical treatment had one hand on a steel pipe that ran 1100? into the earth. We are now in the process of installing lightning detectors, and a lightning prevention system to try and forestall lightning strikes in the future. You were right about the apples and oranges. I wasn?t think very clearly on that one.

Don McLain

 

[bob]

Re: Lightning & conductor impedance

Mclain

It has been suggested that we install a deep earth grounding point and run a cable approximately 50? to this point and bond it to our bounding loop to increase lightning protection

I would agree with you. I think just laying the 50 ft of conductor in a 24" ditch would provide an improvement without the deep driven rod. I think your guy who was injured was a victim of a difference in step potential.

We are now in the process of installing lightning detectors, and a lightning prevention system to try and forestall lightning strikes in the future. You were right about the apples and oranges.

What kind of system are you considering? I have heard of several methods that attempt to do what you want. I am not sure how good they are in preventing strikes.

 

[mclain]

Re: Lightning & conductor impedance

Bob
We are looking at systems from two different companies. Lightning Eliminators and Lightning Master. Our technical department has received independent test data from both companies that show their systems do lessen the number of strikes. I think the testing was carried out at the University of Alabama high voltage lab. The type of system is Charge Transfer Technology. The theory is that free ions are release to reduce the electrical build up between the structure and sky. I had my doubts about the reduction but I guess if they have independent data from a university you have to give it a try.

Don

 

[bob]

Re: Lightning & conductor impedance

Mclain
I've seen a presentation on Lightning Eliminators
system. Our 911 station has their system installed on the antenna. I do not know if it works or not. This area of the 911 is a high
strike location in this part of Alabama. In fact we have the highest number of srtikes, second only to central Flordia.