Sweet. That is a really imaginative design - sorta steampunk in fact. Thanks for the link. I will try to get a picture or two of mine and post them somewhere (got a good photo sharing site?). I've seen lots of 2 or 3 foot lightning rods, but this is the only one I've seen that is so tall. The cable that was used to connect the rods to the ground was often a thing of beauty as well. There is a good example in a park near Wilmington, DE that protects a beautiful oak tree; it is a very thick braided copper cable that divides off to follow each major limb. The park formerly was the private garden of one of the DuPont women, so she had the werewithal to provide the care that such a specimen deserved.
Then you assumed that ESD meant 'Electro Static Discharge' instead of 'Electro Static Damage'
It is never a shield 'from cumulonimbus discharge'. It is, however a safety issue. Grin all you want. A bonded and grounded roof can mean the difference in a minor problem, or a major fire.
Well, this is getting overly long, but I gotta say...
I spent 4 years as the electronics tech for the EMC/ESD Lab at 2 different facilities for NCR. EMC stood for Electromagnetic Compatability and ESD stood for Electrostatic Discharge. Those were the terms in the 1970s and early 1980s, used by everyone with whom I came in contact, including the guys involved in the military's TEMPEST program. Maybe the initials have changed in meaning over the years, but back then, when everything was being driven by the FCC part 15, subpart J rules, those were the terms we used.
Cumulonimbus clouds shed a lot of moisture - we call it precipitation. My roof is an effective shield from that discharge. My general apathy is my effective shield towards pedantic humorless replies. Please lighten up a bit.
I'll get some shots this weekend; it can't rain forever, can it?
OK, both you and Mike (Michael?) have posted rather dire commentary about the lack of grounding on metal roofs. That has made me wonder if there are any real guidelines for such. Metal roofs (here I'm referring to galvanized steel types, from standing-seam to v-crimp and the more modern variations, applied to wood-frame structures) have been around for at least a century. I question that they would be more prone to attracting strikes than any other roof covering, but I readily admit that I've not looked very deeply into the issue.
Bonding and grounding would be a significant and problematic matter, though. The burst of current from a lightning strike would require an assload of bonding points, as I doubt that the roof itself would be sufficient to drain away all the juice from the point of the discharge to a single cable (no matter how thick the cable might be). Another problem with most lightning rod schemes is that ground (soil) conductivity is not always good enough to conduct the full current of the discharge from any single point, so there is a lot of ancillary discharge through all sorts of unplanned pathways (viz, lots of your house structure). A direct strike on any house would necessitate a distributed grounding "grid" to carry away the current before it could do real damage. That's a lot more bucks than most people are willing to spend.
The military used to (still does?) protect sensitive areas, such as ammo dumps, by erecting a tall, well-grounded tower nearby. There is a cone of protection (don't remember the angle) that extends around such a structure.
There is also the matter of whether it is best to ground a high point on a house if there are other tall objects nearby. "Well, I was going to go to that tree, but the spike on the top of that house looks so much more attractive to me; I think I'll hit it" (Apologies for anthropomorphizing an electrical discharge.)
Anyway, I'd like to hear thoughts on the matter of grounding a metal roof (on a wood frame building) as opposed to treatment of other types of roofing material. Lightning protection is a very interesting, complex, and poorly understood subject. Maybe a separate thread?
Metal roofs are no more a problem than metal plumbing, like the drain stack vent that must protrude above the roof ridgeline. Or a TV antenna attached to the chimney.
The top of the tower is called an air terminal. Tall thin metal poles are often used. Such poles are usually seen in electric power switchyards in substations.
The half-angle of the cone is 45 degrees. In other words, the protection footprint is a circle on the ground of radius equal to the tower height.
All of this is well understood. There is a MIL Standard (I don't recall the number, but it's publicly available) that is widely used for figuring out how best to protect structures from lightning.
'Electro Static Damage' was used in electronics manufacturing in the '90s and early part of this millennium. failed parts are damaged by multiple causes. Electro static is just one, but it was controllable in a proper work environment. Conductive mats, wrist straps, heel grounds and properly grounded soldering irons. A continuous audit of these systems brought 'Electro Static Damage' down to an acceptable level, of near zero. :)
Have you ever seen what happens when lightning strikes an ungrounded roof? If you are in the wrong spot between it and ground, the discharge continues through your body. In that case, you might as well be outdoors, naked and holding a metal rod as high as you can.
A properly grounded metal framed & grounded building can still be damaged. One was an office area. The strike magnetized the entire skeleton of the building, making all their color video monitors unusable.
I think it is Mil-hdbk- 419a that you can't remember . I also think they revised the method used to calculate the lightning protection from a grounded pole.
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