lightning protection NFPA 780

[jcole]

Good day guys and gals.

I have been asked if we should install lightning protection like is governed by NFPA 780. We are a waste water facility and the structures I was asked about was a holding tank approximately 30' in diameter and 30' deep with about 20' above ground. The other structure is a small metal building containing an odor control system for the tank.

What do you guys think about these systems? Do they really work? Some engineers think they may attract lightning.

Would this system need to be connected to the grounding electrode system per NEC? It seems to me it would be a violation since code says all grounding electrodes present should be connected together.

Per a google search, there seems to be a lot of confusion on these systems.

Thanks for the replies.

 

[Ingenieur]

what is the tank made of?
how is it constructed?
steel on concrete pad with rebar?
what damage can it sustain?

what is the building ht relative to its surrounding structures?

 

[jcole]

what is the tank made of?
how is it constructed?
steel on concrete pad with rebar?
what damage can it sustain?

what is the building ht relative to its surrounding structures?

The tank is concrete and the building is metal. Concrete pad with rebar. Its about the same heighth as the surrounding buildings.

 

[Ingenieur]

The tank is concrete and the building is metal. Concrete pad with rebar. Its about the same heighth as the surrounding buildings.

imo no need to the tank

the bldg you need a risk analysis
value
haz mat/fire...chlorine, etc
consequential damage if out of operation, health hazards
lightning probability map, chance of a strike
cost of protection

 

[jcole]

Thanks for the replies.

The building will have a nonflammable caustic solution for odor control but not really anything that may create a catastrophe if struck by lightning. If the building were to burn down due to a strike than our insurance would replace the building and equipment so I don't really see a reason for it.

But....Do these type systems really work. It doesn't make since to me because the lightning is seeking the earth. The building is connected to earth. So essentially, part of the earth. So, how does a little pointed rod detour a lightning strike to ground instead of using the structure? Just does not make since to me. I feel like the people that sale this stuff are like snake oil salesmen. And others online feel the same way.

 

[Ingenieur]

you use these systems on bldgs prone to strike (tallest around), ones that a hit may cause big damage or outtages (phone switching center), etc

it does not really increase the chance of a strike measureably
air is the highest z and limiting factor
say you have
100' tree at 50 Ohm
70' bldg at 5 Ohm
you have 30' more air
the resistivity of air is on the order of e16 Ohm-ft
so even though the bldg is lower gnd R the 30' of air adds many, many thousands of ohms to the path

when the bldg is hit the current spills ALL over the bldg
but will prportionally go thru the lowest R path, the lightning system
like a drainage system
so 90% may go thru the lightning system, but 10% over/thru the bldg
this mitigates damage
also since the lightning rod is higher (less air) AND lower R than the bldg it 'attracts' most of the strikes current

 

[Timbert]

So, how does a little pointed rod detour a lightning strike to ground instead of using the structure?

In the simplest terms, the air terminals ("lightning rods") are designed to 'attract' a strike (rather than the lightning hitting some random point on the structure) because they are a low resistance path to ground. Since the air terminals are connected to a well-designed system of down conductors which are properly grounded, current will flow harmless through the down conductors.

An analogy would be a roof doesn't prevent rain from the reaching the ground, rather it controls how and where it reaches the ground. You gather the water that hits the roof and funnel it down through a system of gutters and downspouts rather than letting it flow where it wants to.

 

[jaggedben]

First, I would say that whether a lightning protection system is called for a is function of the frequency of buildings being damaged by lightning in that area and the financial risk involved if the building were to be damaged by lightning. If a waste-water treatment system is put out of commission by a lightning strike, what are the consequences for the community, and is the cost of a lightning protection system therefore justified? That is the question.

Second, to the extent that the physical details of the building matter, which is not a great extent, it matters far, far less what the building is made of than how it compares to the topology around it. You say it's 20ft tall. Is it in the middle of a wide open field with nothing else nearly that tall around it? Or is it in a forest of many trees that are taller than the tank? If the latter, it is most likely that lightning will strike a tree and not the tank. To the extent that the location of lighting strikes is not truly a function of the cloud locations, which is not a great extent, lightning tends to strike the tallest object around because a smaller distance traveled through the very-electrically-resistant air is more important than the electrical resistance properties of the object that is struck. (Think about the number of metal-roofed barns in the midwest; if they were all more prone to lightning, nobody would be putting metal roofs on them.)