TRactors and lightning

[Anonymous Poster]

It is my understanding that high frequency currents (AC) tend to flow more on the conductor surface then center. Supposedly, low frequency A/C and of course D/C currents, are not that partial. This is one of the reasons, besides flex, that multi-strand conductors are used on hi-fi equipment.
Now, is lighting's EMF pushing an A/C or D/C current??
regards,
george

 

[W5FL]

Yeah, but those are mainly marketing hype. Nothing in the audio or ac voltage range applies to skin effect and flex is the only reason. RF voltages above 1 mhz starts to make a small difference, that is why hf connectors are silver plated (gold plating is for marketing purposes). At TV frequencies, skin effect is significant.

Lightning is DC but has a very fast rise time to the current waveform.

 

[Trvler]

If you've ever witnessed someone killed by lightning.. or understood that lightning can strike an object over 100 miles away from the storm, you'd never ask that question. When lightning strikes, get someplace safe. QUICKLY!

 

[TomG]

Yes, that's my understanding. The relative safety in a car is because you're in a steel box, not because the car is on rubber tires. At lightening voltages, everything is a conductor, and so everything is always grounded. Current always takes the path of least resistance. In this case, from the ground, arcing to the wheels, then through the body and off the roof.

An occupant in the car can be surrounded be extremely high voltage, but it's the same voltage all around. Within the car, there's little difference in potential. Still, within the car, advice is that metal parts should not be touched.

 

[KCB]

Lightening is a fast, complex AC pulse, not DC as is commonly assumed. Same for ESD, or electro-static discharge. Because of "skin effect" (lightening is thought to vary in frequency from 20khz to 500 khz) grounding cable should be stranded, to increase surface area, and have no sharp bends, to reduce inductance. A single point ground is best, to avoid ground loops.

 

[TomG]

If I understand the idea of lightening rods, it's that they provide low resistance paths around objects on the ground. Damage occurs when current travels through, for example, a tree (the sap boils) or jumps within a house.

My inexpert opinion is that the need for lightening rods is somewhat less important today, because the electrical service grounding is a fairly good lightening rod. The service ground, combined with codes that require bonding of metallic plumbing to the service ground buss, provide low resistance paths within a house. However, lightening rods still should improve safety. It is important to bond plumbing, well casing and metal drainage pipes to the service ground buss.

I use a more modest technique than a lightening rod system. I place grounding plates or rods on the opposite corners of a building. The second plate is a part of the service ground, or they can be a supplemental grounding system on buildings served by branch circuits.

The idea is that a strike to one side of a building draws current from a sizable ground area. The huge voltages can create differences in potential that result in jumps within the building. The idea of the second plate is to provide a low resistance path around the building and reduce potential differences within the building.

My electrical inspector did say that there is no research that proves this idea works. However, the technique is farily commonly held, and it isn't likely to hurt anything. Any actual research probably would be inconclusive since the number of instances would be small and the number of factors affecting each particular instance would be large.

 

[roysallis]

Do you know why lightning doesn't strike twice in the same place? It doesn't have to.
Seriously, some people think it doen't strike again in the same place but this belief is not true.

 

[TomG]

Definitely not. When I was dating my wife, we used to watch storms from her apartment in Toronto--just to see strikes on the CN tower. At the time the tower was the tallest freestanding structure in the world. I think it typically was struck in the hundreds of times a year.

 

[W5FL]

Since we are this far off subject anyway....

Polyphaser has done more research in the field of lightning protection for electronic equipment in a structure than anyone else that I am aware of. They produce an excellent book called Grounding Systems and sell everything from entry panels to lightning arrestors for all types of commercial communications equipment. They do not sell external structure points or cabling, but others do. It is very interesting reading. My interest is because my antenna tower gets struck a few times a year and it and the equipment are properly protected with over a 1000 feet of ground wire and ground rods and it always survives. There are many advantages to living on a hilltop, but weather is NOT one of them.

 

[KCB]

Sorry, I thought I had posted this information earlier. Anyway, I spoke too soon about lightning being AC or DC. Our FCC guy says it is a fast transient static discharge. I have attached addresses for some interesting info about the subject.

http://www.lightningsafety.com/

http://www.amasci.com/emotor/statelec.html <-- lots of links and leads to:

http://www.eskimo.com/~billb/miscon/curstat.html

http://wvlightning.com/index.html