Lightning Protection for Circular Roofs

[EE_Intern_2]

Hi,

I'm trying to figure out the correct spacing for air terminals on a circular shaped flat roof (no slope).


I'm thinking that the part of the roof between two air terminals may protrude out of the zone of protection.

How would you guys go about calculating the correct number of air terminals?

Say that the perimeter of the roof is 165 feet.

NFPA 780 clearly shows applications for flat roofs but not for circular shaped roofs.

 

[copper chopper]

this is why engineers get paid way more than we do... and if you do the calculations and your not an engineer and that building gets hit by lightning and damages stuff can you provide insurance that your specs are correct. what ever system your installing there engineers are supposed to do this for yur and provide a detailed print and book on how they want it installed.

 

[bob]

They are spaced about 20 ft apart. Check here for added information.

http://www.google.com/search?source...rminals&gs_upl=0l0l0l3650lllllllllll0&aqi=s57


http://www.elec-toolbox.com/usefulinfo/lightprot.htm

 

[EE_Intern_2]

@ Bob. I think the code's idea for 20 feet spacing only applies to straight edged roofs. I'm not sure it applies to circular roofs where the edges of the roof that is between terminals protrudes out.

If anyone else has any additional information, please share.

we received a drawing from a lightning protection company but they don't include any calculations. they just placed 10 air terminals around the perimiter of the roof. Without seeing any calculations, I am very skeptical about if this is providing adequate protection . I have google searched this to death and can't find anything on protecting circular shaped buildings/roofs or any formulas that apply to other than straight edged roofs.

 

[ron]

I would sugegst you review NFPA 780: Standard for the Installation of Lightning Protection Systems on the NFPA website and use the rolling sphere method of determining coverage and review other spacing criteria.

 

[bob]

EE
I think you are not understanding the function of the lightning terminal. You said that the perimeter is 165 ft and 10 rods. That's about 16.5 ft spacing. The protection area is not a straight line between the rods. I have included some information taken form the Lightning Protection Institute web site. The fact that the area is circular does not affect the coverage area. Do you have rods install in the interior of the circle or just on the edge?



Zone of protection in NFPA 780 is described by a 150 ft. radius sphere model. The air terminals or lightning rods are designed to support this "ball" off the insulating material of the structure, and therefore take the lightning strikes rather than the structure. If you consider this "attracting" lightning for a 10 ft. radius in certain instances, then I guess the strike termination devices do attract slightly. Many people have the idea that lightning can be attracted over some great distance, thinking that lightning protection "attracts" a large number of strikes to your property or causes lightning to strike your house rather than the neighbors. This concept of attraction is not true. Lightning protection generally can be thought of as protecting against lightning strikes that were going to occur to your house anyway.

 

[ZoneOfProtection]

The 20 foot spacing is correct, however, after locating the points you have to draw a straight line between two adjacent points and determine the mid point of that line. Since air terminals must be located within 24" of the outside edge of the roof, the distance from the mid point to the roof edge must be less than 24". If it is greater additional points must be added until it is less than 24" . UNLESS the outside edge is covered by a zone of protection by sitting the rolling ball on the two air terminals and rolling it to grade or lower roof area protection.
Something else to note - if your circumference is 165' then your diameter is 52'. depending on this height of the air terminals used you may require a Midroof point. Midroof point are spaced on 50' unless the area is covered by a zone of protection created by the air terminals.

 

[EE_Intern_2]

Thanks ZoneOfProtection, that simplifies it. I actually did that midpoint calculation that you described and then tried to plug it into the square root formula in NFPA 780. But I was overdoing it since as you pointed out it, as long as it is with 24" from the edge it is OK. Good Job.