Rolling Sphere Method, Mast Taller than Strike Distance

From what I hear, lightning doesn't have to play by the rules. It can and frequently does change direction which has a nasty habit of making all of those theoretical stipulations seem to be rather pointless. Some of the protection system installers don't bother with the high mast idea any more because you would really need a small forest of those masts to beat the statistics.

A certain Florida utility company has been known to force a discharge (lightning strike)where they want it, in order to protect high voltage switch gear, etc. Their approach seems to work better than the invisible field that such a system supposedly supplies. Its not something I would want to play with, if you know what I mean...

A tall mast that is grounded is pretty well accepted as having a lot of value but not being perfect. In this case the grounded mast takes the strike and conducts it down.

The "forest of spikes to discharge a storm" products are very different and probably can't be described as "well accepted". Some people have positive comments, some negative. I'm not well informed on them. I will not dispute any negative comments made about them.

http://hibp.ecse.rpi.edu/~connor/education/Surge/Presentations/Joe%20Crispino%20-%20Surge%20Presentation.pdf

A couple of commenets:

1) Would you please share the source of your reading. It would be helpful, especially if it is an internet link that others can review.

2) My guess is that your source was stating that as a guideline the radius of protection was equal to the height of the tower. I don't quite understand how you have a different radius of protection that is somehow being shown as a rolling sphere (I'm not saying you are wrong, I'm just saying that I don't understand it).

3) This "radius of protection" seems like a reasonable guideline for "protecting" equipment from damage (acceptable reduction of risk). If there is danger to life or risk of dangerous accident (release of toxic substances, ignition of large fuel tanks, etc.) then I would not recommend a safety solution based upon readings that require assistance from anonymous internet posters to understand.

4) Find a picture of the space shuttle on the pad. If the view is not too close you will be able to see a round thing sticking up past the top of the shuttle. In some views you can see wires going out to the side from the top of this tall thing. If I recall correctly this tall thing is a 185 ft tall by 6 ft dia. fiberglass pipe (warning - details may be off). The wire at the top is a 1/2" stainless steel cable that extends out to both sides of the shuttle and goes over a roller at the top of the fiberglass tube. This system works very well to protect the shuttle. This isn't exactly what you were asking about, but information on the performance of this system is probably available to the public.

Note that if you recall or read about a delay when they checked the shuttle after a strike it was probably not the shuttle that took the strike. Both ends of the 1/2" stainless steel cable have flashbulbs to ground. If the flashbulbs blow during a storm then the shuttle is inspected. I don't think that the shuttle ever actually took a strike.

"I don't think that the shuttle ever actually took a strike"
I don't think it has either, but the launch pad gets hit all the time...

http://www.space.com/6957-lightning-struck-shuttle-launch-pad.html

The strike is hitting the 1/2" diameter stainless steel cable that is "trapped" in a pulley at the top of the tall (possibly 185') fiberglass pipe (possibly 6' dia.) that is mounted on the top of the launch platform. The conducted energy is going down the wire and away from the pad. The radiated energy goes everywhere including toward the shuttle and therefore it is appropriate that the shuttle get inspected and/or tested.

In the picture the metal launch tower is not getting struck. If my memory is correct then the strike missed by about 185'.

Not exactly on topic, but Apollo 12 took a lightning strike about a half minute after lift off, then a second strike about 20 seconds later. Quick thinking, remembering one test in the simulator nearly a year earlier, kept the mission going.

http://en.wikipedia.org/wiki/Apollo_12

SolarEagle's picture is actually very good for this discussion.

Before lightning strikes (typically downward) there is usually a corona discharge from tall object(s). This corona discharge tends to go somewhat upward and this tends to happen many times in individual spikes of current.

When the lightning strikes it typically goes downward and typically follows the corona discharge path.

Here's one major problem. The time delay between the corona discharge and the strike can be dozens or hundreds of milli-seconds. In that time the corona discharge path can be blown around by the wind. In SolarEagle's picture the wind was probably flowing from far field right to near field left. As the strike came down it followed the "blown away" corona path down and then looped up to the more vertical and newer corona path. If you watch any of the NASA triggered lightning videos this can be seen very easily. If you think about the wind blowing around a charged path prior to the strike coming down then you can see why I select words such as "tend to" provide "acceptable level" of protection "to equipment only".

REMEMBER: be safe out there.

Yes, lightning is very unpredictable. The way that rolling sphere methods of design work is by providing a shorter path to ground for a lightning bolt (by way of a mast rather than a transformer or bus) that the bolt may "prefer" to take since there would be less resistance along that path. Yes it's theoretical, but there is no IEEE standard for lightning protection, and this is the preferred method for those who wish to take lightning protection into account in their designs.

1) I don't have a digital reference to the text.

2) That's not quite what they are stating. See SolarEagle's link if you'd like to see how they calculate the radius of protection based on the height of the mast and the radius of the sphere. Although it does not contain the answer to my question, it is a good reference about the rolling sphere method in general.

3) I'm not looking for someone to propose a concrete solution. Rather I've noticed an oversight on the topic (in every discussion/manual/book about the rolling sphere method that I have seen), and am trying to get other engineering professionals, who may have noticed the discrepancy as well, to present their thoughts on the issue. Since strike distances are generally large I don't see my original question being one that has been thought on that much.

4) That's pretty interesting.

and yes, IEEE 998, is the standard.

"So I've read that for the maximum radius of protection your lightning mast should be equal to your strike distance (a.k.a. the radius of the rolling sphere), which I fully agree with. My question is what happens to the radius of protection if your mast height exceeds the strike distance."

This is correct for side strokes. To answer your question, nothing happens to your radius of protection. It is not predicated on the height of the mast. Anything higher and it is just a waste of steel as it affords no better area of protection, for side strokes. It still provides the SAME protection - the rolling sphere creates a "zone" or an area as it passes through its roll...it is not just in one location at one time...the concept is an artifice to allow human understanding. The radius is independent of heights of steel, as the IEEE 998 calculations show.

What the sphere really represents is the entrance of a stepped leader into the center, as it touches grounded, and only grounded objects, the leader will continue, statistically speaking, to the ground path as the radius is equidistant from the center.

For direct stroke protection, if you have, say, four masts that your sphere can rest on, you can increase the height to provide for better (higher off of the ground) protection to the interior objects. But outside, with side stroke protection, you are provided with no better, yet no worse protection, as a result of higher steel. The stepped leader will just be intercepted at a higher point in the sky, to your point about it "resting on top". So your calc will either tell you to decrease the spacing between the masts, or intuitively, raise their height. You can raise the height of each a few feet instead of add two masts, perhaps.

The area of protection (with a radius equal to the radius of protection) absolutely IS based on the height of the mast. If you have a five ft tall mast I don't think you will be protecting much. I think we are confusing terms. The strike distance (radius of the rolling sphere) is independent of the height of the mast, but the radius of protection (to protect equipment of height he) is dependent on both the height of the mast as well as the equipment and follows the equation r=(S^2-(S-hm)^2)^0.5-(S^2-(S-he)^2)^0.5.

And I agree in the thought that if the mast is greater than S (the strike distance/radius of the rolling sphere) it will not alter the radius of protection at all, it will be at its maximum. That being said, if you use the equation above where S=70 and he=some constant, the radius of protection is a maximum where hm=S (as it should be), but is the same for a hm=S-5 and hm=S+5 (where as I believe that hm=S+5 should be equal to the maximum as well).

This is where my question about whether or not the lightning would strike the side of the mast rather than just the point of it came in to play. Since as the radius of protection mathematically decreased as the mast height surpassed the strike distance, I was searching for a reason why and thought I'd ask some others that may have ran in to this before. I know it is because if you square (S +/- hm) they will be equivalent, but I wanted clarification so that I knew to always round hm down to S if that was accurate.

Another thought is what about a shield wire? Say a shield wire is 500 ft off the ground and no supporting structures are close enough to have any effect, and S=70. How low does the area of protection go? I'm picturing a sphere of strike distance S being centered along the conductor and any stepped leader entering that sphere will strike the conductor (theoretically)... That is where the thought of maybe the sphere being centered about the point of the mast came from.

Thanks for the reference to IEEE 998.

Yeah I got some terms confused, sorry. But anyway, I hope I answered your question. You can also check out NFPA 780, but 998 is the defacto standard. Regarding the shield, and all of this stuff for that matter, you really need to visualize. I always draw this stuff in plan and profile view, to scale, especially when I was teaching myself this stuff (oh you mean they taught none of this in school?). ;). In the case of the shield, the sphere would simply "roll under". The same is true with mast that are spaced farther than the strike radius, it will "roll between". And you are afforded no side protection if the separation is just barely greater than the strike distance. Again, try drawing some ofthis, it really helped me at least.

It is very important that you have realistic values of strike distance S. The utility for which I do a lot of work uses S = 248.7', which was calculated based on empirical lightning strike data for our region. There's no way we are going to put up a 250' mast - our standard height is 60'. With that hm and S, we get a radius of about 44' within which 30-foot high equipment is protected (based on the rolling sphere method). You need to determine what S is for your location.

A shield wire works too, and acts like a bar running between the two end points. The sphere rolls off each side of the bar the same as off a mast point. Thus, instead of a circle of protection, you get an oblong formed of a protected rectangle centered on the length of the wire with a hemisphere of protected radius r at each end (kind of a sausage shape). You can also make larger shapes of multiple masts with wires running between.

Yes. The value for S I generally use is around 77.98', and the standard masts we use are 55' or 70'.

Another question I haven't seen addressed thoroughly is how to calculate radius' when two masts (or a mast and a shield wire) are of differing heights? And what about shield wires that aren't parallel?

A 15-foot black walnut tree in grove of tall oak will get hit by lighting more times than the oaks. Lighting protection can decrease but not elimanate the chance of a damaging lighting strike.

"A 15-foot black walnut tree in grove of tall oak will get hit by lighting more times than the oaks."

Really? Why? Chemistry of the wood, maybe? Length of taproot and availability of path to ground(water)?

The protection is statistical. Of course.