Lightning Rod install?

[lynnbilodeau]

Had some friends whose house was recently set on fire from a lightning stroke.

Got me wondering if a lightning rod would be a smart investment. Our house is 105 feet long and about 30 feet tall.

Watched this video:

 

[LXCam]

We had an indirect hit about 20ft off the back of the house this summer. Originally I thought it hit the dish receiver but that wasn’t the case. Luckily the damage was minimal but it did get me off my *** to upgrade the bonding and add some tvss devices.

Personally if you live in a prone area I think it’s a good idea but in my opinion that determination is based on grounded pathway routings and the basic construction of the house. This place has everything under slab and only the dish riser and water/gas utility entrance points exposed.

The dish is now gone since we’re on cable and like I said I upgraded bonding the gas/water and power.

 

[rlitman]

Had some friends whose house was recently set on fire from a lightning stroke.

Got me wondering if a lightning rod would be a smart investment. Our house is 105 feet long and about 30 feet tall.

Smart investment? Probably not. I wouldn't imply that your home isn't "priceless", but that's not how your insurance company sees things. Residential insurance rates aren't significantly raised by lightning risk even in risky areas, and that should be a good indicator of your real risk. Also, here's a longer TOH segment I found in my watch history if you like them and want a little more on the topic:

In my neighborhood, we have power lines on poles and tall trees, so houses are generally under the shadow of the 150' radius rolling circle of protection offered by these tall objects. That lowers my relative risk to near zero, but I'm also in an area with low lightning density. I was going to include the FEMA map here, but it seems they only show lightning RISK and not DENSITY in their map. That's based on an insurance formula which includes regional values and repair costs, so in a spendy zip code, the monetary risk may be higher while the strike density is still low. NOAA points me to Vaisala data, which will show you the average number of events per square-km per year if you zoom in. Some parts of Oklahoma see a lot more strikes than others...

There's also the question of relative risk in relation to your structure's location. If your home is the only building on top of a hill in a strike dense area, LPS could make more sense. Is your home also a gasoline refinery with an attached daycare facility? If so, then yeah, you need a world class first rate LPS badly. Not everyone needs the same level of protection.

If you've got a day to kill and want a master class on lightning protection, watch everything from these guys:

DEHN Inc.

DEHN Inc. is a leading expert in surge and lightning protection, offering comprehensive solutions for a wide range of applications. Our adherence to international standards guarantees the suitability of our equipment and solutions for a wide spectrum of industrial needs, including but not...

www.youtube.com

Some things I made a mental note of:
1) 30' tall building is great news. The shorter the structure, the easier it is to protect.
2) Oklahoma has great ground conductivity. That makes it easier to drive rods to lengths that will actually protect you.

 

[lynnbilodeau]

Home is a long way from priceless. But, because it was built in 1902, because it is so tall, because we have three tall trees close to the property AND because of the unique construction, we have to insure it for about 3 times what I could likely sell it for. Unique construction in this case is "cavity brick", meaning there is no frame. Lower half of the house is four bricks thick with air pockets and "wall ties" throughout. Walls are 19 inches thick. The upper half is 3 bricks thick, same cavity brick constructions. Still very solid overall. It is likely built on a giant limestone field.

Given the 122 year old wood in the attic, I am thinking a lightning stroke could be serious damage. If I am going to put in a lightning rod system, I better do it before I get much older. Already getting difficult to climb around in the attic. It ain't gonna get any easier.

 

[Norcal]

Lightning protection is a specialty trade, if a structure is in a area prone to lightning strikes I would say it is prudent to have protection installed.

 

[kbuhagiar]

Not questioning the legitimacy of a well-thought-out lightening-protection plan, but if a lightning rod is installed on a building, doesn't that increase the likelihood of a strike?

And isn't that what you want to avoid? So in my mind, if a lightning protection installation isn't absolutely perfect, it could potentially invite more damage than prevent?

Last year I had lightning strike about 50 feet south of our house in Northern California, and it did almost $15,000 worth of damage (well pump, pump controller, modems, routers, and smart light switches).

We sold the house a few months ago so it is no longer our problem, but I'm still curious.

 

[wyliesdiesels]

Not questioning the legitimacy of a well-thought-out lightening-protection plan, but if a lightning rod is installed on a building, doesn't that increase the likelihood of a strike?

And isn't that what you want to avoid? So in my mind, if a lightning protection installation isn't absolutely perfect, it could potentially invite more damage than prevent?

Last year I had lightning strike about 50 feet south of our house in Northern California, and it did almost $15,000 worth of damage (well pump, pump controller, modems, routers, and smart light switches).

We sold the house a few months ago so it is no longer our problem, but I'm still curious.

no not necessarily. and it creates a pathway for the surge in an attempt to limit damage.

skyscrapper buildings often have lightning rods on the roof designed to direct the energy down a specific path limiting or eliminating damage to systems in the building

 

[rlitman]

Not questioning the legitimacy of a well-thought-out lightening-protection plan, but if a lightning rod is installed on a building, doesn't that increase the likelihood of a strike?...

No.

Ok, now let me explain how and why. Lightning strikes originate in the clouds with a downward "stepped" leader that hops down in steps typically stated to be around 150'. That number is not a coincidence, and it directly relates to the 150' radius rolling circle (part of the UL 96A and NFPA 780 specifications) I mentioned above. We'll get back to this.

The very last "step" happens backwards. As the stepped leader approaches the ground, streamers from the ground will reach up towards it, and when they meet in the air, a conductive channel to the sky opens up, carrying thousands of amps of current in what we would think of as a lightning strike. No one can predict exactly where the stepped leader from the sky will come down, but when it does, everything within 150' of it is fair game, and that's when a lightning rod can help, by providing the best upward streamer option. In its absence, every corner of every solid object in that area is going to compete to provide the best streamer, and whatever wins (usually a gutter or antenna or chimney) takes the hit.

What about that 150'? Who made that **** up? Well, this is based on long-term observation of lightning statistics, and the actual length of both the steps in the leader and the streamer length of any given strike is directly related to the charge imbalance and final strike current flow. 91% of lightning strikes have a 150' or longer streamer (and step) length, which correlates with a 10kA or greater current flow.

Note that the streamer length marks the zone of influence of the air terminal (lightning rod). Since lightning doesn't start in clouds merely 150' from the ground, it is impossible for a lightning rod to attract lightning from the sky.

And most lightning is actually more powerful than that. A >200' step (correlates with >16kA) would catch 84% of all strikes. On the smaller end of the spectrum, a >65' radius (>3kA) covers 99% of all strikes, and >90' (>5ka) covers 97%.

So, in your example, with a strike occurring within 50' of your structure, an air terminal almost certainly would have taken that specific hit. What happens from that point depends a lot on how well your lightning protection system is engineered. Because just connecting a rod in the air with one in the ground is NOT sufficient!

But just because it could have prevented your $15k loss doesn't necessarily make a LPS worthwhile to you. The lightning frequency data shows Escondido Ca to be hit by less than one strike per square km per year. I'm in the 4 events per year zone. Most of Oklahoma's population (OP's location) seems to be in the 64 strikes per square km per year zone, with the state ranging from 24 to 96, putting it squarely in lightning country. But in both regions, a site survey can tell you if local features put your structure at greater or lesser risk.

 

[kbuhagiar]

No.

Ok, now let me explain how and why. Lightning strikes originate in the clouds with a downward "stepped" leader that hops down in steps typically stated to be around 150'. That number is not a coincidence, and it directly relates to the 150' radius rolling circle (part of the UL 96A and NFPA 780 specifications) I mentioned above. We'll get back to this.

The very last "step" happens backwards. As the stepped leader approaches the ground, streamers from the ground will reach up towards it, and when they meet in the air, a conductive channel to the sky opens up, carrying thousands of amps of current in what we would think of as a lightning strike. No one can predict exactly where the stepped leader from the sky will come down, but when it does, everything within 150' of it is fair game, and that's when a lightning rod can help, by providing the best upward streamer option. In its absence, every corner of every solid object in that area is going to compete to provide the best streamer, and whatever wins (usually a gutter or antenna or chimney) takes the hit.

What about that 150'? Who made that **** up? Well, this is based on long-term observation of lightning statistics, and the actual length of both the steps in the leader and the streamer length of any given strike is directly related to the charge imbalance and final strike current flow. 91% of lightning strikes have a 150' or longer streamer (and step) length, which correlates with a 10kA or greater current flow.

Note that the streamer length marks the zone of influence of the air terminal (lightning rod). Since lightning doesn't start in clouds merely 150' from the ground, it is impossible for a lightning rod to attract lightning from the sky.

And most lightning is actually more powerful than that. A >200' step (correlates with >16kA) would catch 84% of all strikes. On the smaller end of the spectrum, a >65' radius (>3kA) covers 99% of all strikes, and >90' (>5ka) covers 97%.

So, in your example, with a strike occurring within 50' of your structure, an air terminal almost certainly would have taken that specific hit. What happens from that point depends a lot on how well your lightning protection system is engineered. Because just connecting a rod in the air with one in the ground is NOT sufficient!

But just because it could have prevented your $15k loss doesn't necessarily make a LPS worthwhile to you. The lightning frequency data shows Escondido Ca to be hit by less than one strike per square km per year. I'm in the 4 events per year zone. Most of Oklahoma's population (OP's location) seems to be in the 64 strikes per square km per year zone, with the state ranging from 24 to 96, putting it squarely in lightning country. But in both regions, a site survey can tell you if local features put your structure at greater or lesser risk.

Great info, thanks for that!

 

[engineer2]

We were taught in engineering school that lightning has fairly high frequencies, so it doesn't like to go through conductors with bends or corners. A lightning rod and conductors should be a straight line to the ground.

 

[rlitman]

We were taught in engineering school that lightning has fairly high frequencies, so it doesn't like to go through conductors with bends or corners. A lightning rod and conductors should be a straight line to the ground.

Code calls for a minimum 8" radius for all bends. The few rods I had to bend for "reasons", I wrapped around a scrap car rim to get the curve right. Also, you don't want inside corners tighter than 90 degrees, or else you may get arcing across the gap, and the wire itself must not be sleeved in anything metallic, because metallic conduit would add impedance just like a bend.

The bigger concern is when your lightning protection wire passes within the "separation" distance from something conductive. In that case, the induced current from a strike will lead to arcing between that metal and your wire, so you may find the need to bond lots of stuff if you plan your wire path too close to metal objects. The actual requisite separation distance increases with building height, so I can't specify a number, but it could be as little as a few inches, or as much as a few feet.