NFPA 780 Lighting protection method

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Hello, I'm an italian electrical engineer, my company builds cooling tower.
In rference of the title of this thread I need sono information about the calculation protection in accordance of NFPA 780. In the chapter
4.7.3 the NFPA standard shown th erolling sphere method. This method is necessary to calculate the covering of the structure throught rolling sphere method. The shere have a radius of 46m. After the chapter of the covering by rolling sphere method there is an other chapter that shown how and where in necessary to install the strike termination on the roof. In our case we have flat roof exceeding 15m in width and lenght.
Now My question is to evaluate the number of the strike termination is necessary to apply the rolling share method plus the Figure 4.8.2.4 (air terminal on Flat roof) or we can apply only the rolling sphere method?
If I apply only the rolling sphere method the number of strike termination on the roof is lowest than if I apply rolling sphere method + Figure 4.8.2.4 where are indicated these 3 rules

1 - 15m is the maximum spacing between air terminals
2 - 45m is the maximum lenght of cross run conductor permitted without a connection from the cross run conductor to the main perimeter or down conductor
3 - 6m maximum spacing between air terinal along edge

Thanks

bYE
 
See art.4.7 Zones of Protection.
The geometry of the structure shall determine the zone of protection.
4.7.1 Roof Types. The zone of protection for the following roof types shall include the roof and appurtenances where protected in accordance with Section 4.8:
(1) Flat or gently sloping roofs
(2) Dormers
(3) Domed roofs
(4) Roofs with ridges, wells, chimneys, or vents
If your roof type corresponds to the type indicated in this article, you have to follow Section 4.8. If not you may use only the Rolling Sphere rule-art.4.7.3 [or art.4.7.2 if it will be suitable].
 
Julius Right said:
See art.4.7 Zones of Protection.
The geometry of the structure shall determine the zone of protection.
4.7.1 Roof Types. The zone of protection for the following roof types shall include the roof and appurtenances where protected in accordance with Section 4.8:
(1) Flat or gently sloping roofs
(2) Dormers
(3) Domed roofs
(4) Roofs with ridges, wells, chimneys, or vents
If your roof type corresponds to the type indicated in this article, you have to follow Section 4.8. If not you may use only the Rolling Sphere rule-art.4.7.3 [or art.4.7.2 if it will be suitable].
Click to expand...

Thank you very much for your response.
In my case the cooling tower is equipped with a flat roof but I have 22 fan stacks of the type shown at this link (only for example):


http://midwesttowers.web13.hubspot.com/Portals/201965/docs/Stacks09.pdf

Each fan stack have a diameter of 11m

In my case I shall follow Section 4.8 or I can use only the Rolling Sphere method?

For the Lightning protection system I want to install some air terminal on the perimeter and 4 air terinal for each fan stack. In this case I have the coverage in according to the rolling sphere method without following the section 4.8.
Is correct what I say or for your opinion it isn't correct. For My opinion cooling tower don't have a roof with this characteristics:
(1) Flat or gently sloping roofs
(2) Dormers
(3) Domed roofs
(4) Roofs with ridges, wells, chimneys, or vents

thnks

bye bye
 
Julius Right said:
See art.4.7 Zones of Protection.
The geometry of the structure shall determine the zone of protection.
4.7.1 Roof Types. The zone of protection for the following roof types shall include the roof and appurtenances where protected in accordance with Section 4.8:
(1) Flat or gently sloping roofs
(2) Dormers
(3) Domed roofs
(4) Roofs with ridges, wells, chimneys, or vents
If your roof type corresponds to the type indicated in this article, you have to follow Section 4.8. If not you may use only the Rolling Sphere rule-art.4.7.3 [or art.4.7.2 if it will be suitable].
Click to expand...

In the section "4.8.8 Chimneys and vents" is written as follows:

Strike termination devices shall be required on all chimneys and vents
that are not located within a zone of protection, including metal chimneys having a metal thickness of
less than 4.8 mm ( in.).
4.8.8.1 Chimneys or vents with a metal thickness of4.8 mm ( in.) or more shall require only a
connection to the lightning protection system.

in my case I don't have chimneys but if you consider fan stack as chimneys they don't are made in metal but are made in fiberglass FRP. They are located in the zone of protection because on each fan stack I want to install 4 air terminal to ensure the covering with rolling sphere method
 
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