Lightning risk assessment- seems very biased

[greenspark1]

Hi,
I perform a lightning risk assessment about once a year using the NFPA 780 calc (annex L). It involves comparing the risk of lightning to the tolerable frequency. The problem is, every building I ever calc says it needs a lightning protection system (LPS). Literally a 50x50x15 office building gives a positive result. I am in the northeast and just don't believe that every building needs a LPS. Why haven't they all burned to the ground? I am having trouble using the calc since it seems to biased and never gives me a negative result. I checked the math twice and still have this issue.
Anyone run into the same issue or have a better calc they use?
Math below:

Building dimensions:
L (m)	16
W (m)	16
H (m)	5
Ae	1923
Ng (flash density)	2
Ae	1923
C1	0.5
Nd (lightning strike frequency)	0.00192262
C2	1
C3	1
C4	1
C5	1
C	1
Nc (tolerable lightning frequency)	0.00150
Nd	0.00192262
Nc	0.0015
Results:
Is Nd<=Nc	no
	=LPS is optional
if Nd>Nc	yes =LPS should be installed

Btw, very similar to the calc here: http://alltecglobal.com/resources/lightning-risk-assessment

 

[charlie b]

I don't do this type of calculation often. But I would say that I get the "LPS is optional" result about half the time. The key factor seems to be the occupancy of the building or the material it contains. A storage building that contains automobile repair parts and that is not occupied unless someone has to go there to pick up a part is probably going to get the "LPS is optional" result.

 

[junkhound]

And in Seattle, none?

Recall back in the '80s we had a once in 100 year lightning storm, and Seattle City Light lost 175 transformers that night as very few transformers had lightning arrestors.

LP seldom needed here except on skyscrapers and some church steeples.

To my knowledge, the local aircraft plant has never had a plane on the flight line damaged by lightning - when the company added an assembly plant in South Carolina, 3 planes were damaged in the first year. Location, location, location.

 

[greenspark1]

I don't do this type of calculation often. But I would say that I get the "LPS is optional" result about half the time. The key factor seems to be the occupancy of the building or the material it contains. A storage building that contains automobile repair parts and that is not occupied unless someone has to go there to pick up a part is probably going to get the "LPS is optional" result.

Appreciate the feedback.

Yea if I change the calc to a tire warehouse I might be able to get the LPS is optional result. But 90% of the time the calc says one is recommended.
If you run the calc for Seattle I bet you'd get the results.
I just don't buy it that almost everything needs a LPS. And a NFPA calc that almost always tells you LPS is recommended is next to worthless. It's like a gov agency recommending drivers in South Carolina that they need studded snow tires. Degrades trust in the system and is untrue.

 

[petersonra]

Degrades trust in the system and is untrue.

I think you are making a poor assumption. Just because there are few lightning strikes in an area does not mean a LPS is not recommended. You have no idea what the criteria is for making a recommendation of whether a LPS is appropriate or not.

No doubt the criteria used by the people who came up with it does not match the criteria you might come up with, but that does not make it "untrue". It is just that you are willing to accept a lower level of safety.

 

[greenspark1]

I think you are making a poor assumption. Just because there are few lightning strikes in an area does not mean a LPS is not recommended. You have no idea what the criteria is for making a recommendation of whether a LPS is appropriate or not.

No doubt the criteria used by the people who came up with it does not match the criteria you might come up with, but that does not make it "untrue". It is just that you are willing to accept a lower level of safety.

I agree there are a number of criteria used to determine if the LPS is recommended. The calc includes many risk factors. What is surprising is that when almost all of those risk factors are low, the result still says yes to LPS.

I think it is very unfortunate when governing bodies become too cautious (or lawsuit-averse) and make recommendations that are impractical, unfeasible, and/or cost prohibitive. I think the snow tires analogy is a good one. Yes, more ppl would be save with snow tires, but the cost/benefit doesn't make sense for most ppl.

 

[petersonra]

I think it is very unfortunate when governing bodies become too cautious (or lawsuit-averse) and make recommendations that are impractical, unfeasible, and/or cost prohibitive. I think the snow tires analogy is a good one. Yes, more ppl would be save with snow tires, but the cost/benefit doesn't make sense for most ppl.

First off, you are completely mistaken in suggesting NFPA is any kind of "governing body", other than governing itself. Nothing in any of its codes and standards has any force of law at all unless some governmental entity makes it so.

Just how much is someone's life worth to you?

The people who have came up with the calculation have made that determination and added in various factors that help them to determine how many lives will be lost over the normal life span of the building and its LPS, along with the cost of damages to the building that might be avoided if a LPS is installed. There are other costs that get put into these calculations such as the cost of having the fire department respond to the call that adds costs and risks on its own.

Incidentally, our own government has several different valuations of what a human life is worth ranging almost to $10 million. That is not all that far out of whack given that law suits often pay out amounts in that order of magnitude for negligent deaths.

Keep in mind that the way this calculation ends up being made you end up with a single lightning strike that might cost say $20 million. The cost of a LPS for the building might be $10,000. That would suggest it is cost effective to put LPS on 2000 buildings to avoid a single incident.