Lightning protection system on building - increased risks

[hansforum]

Hi!

Does lightning protection system on building increase the chances that this building will be hit by a lightning compared to this same building without lightning protection system? Some people say that it does and some that it doesn't. What is correct?

If this is not correct forum to post this can someone recommend where to ask this question?

Thanks!

 

[jghrist]

No, the lightning protection system does not increase the risk of being struck, it just provides a safe path for the stroke current.

Charge builds up in objects below clouds, creating a voltage gradient. The charge build up doesn't have to have low resistance path because it is a slow process (relatively) and doesn't result in a large current until the air breaks down and the stroke occurs.

 

[hansforum]

I found an article about risk assessment for LPS. It uses type of the structure (metal with metal roof and metal with non metallic roof) in equation for risk assessment. If metal roof increases risk than we could conclude that metal LPS also increases risk of being struck.

Link to article:

http://www.interferencetechnology.com/uploads/media/Cole_DDG11.pdf

Now, who is right?

 

[jghrist]

The coefficient C2 for type of structure is a measure of how vulnerable is is to damage, not to how likely it is to be struck by lightning. A metal roof has C2 = 0.5 and non-metal roof has C2 = 1.0. The equation for tolerance risk has C2 in the denominator, so the risk tolerance is twice as high for a metal roof. Compare with the structure content coefficient C3 which is 0.5 for low value non-flammable contents (high risk tolerance) and 2.0 for high value flammable contents (low risk tolerance). Note that the collective area of the facility in the equation for environmental factors does not depend on the type of building.

 

[jimioy]

Putting an LPS (Lightning Protection System) on a structure probably does attract more strikes. However, since a strike can happen to any structure, the one with the LPS is far better protected. NFPA 780 can get you compliant if you elect to install one.

Jimi Young - civilian
184th CES/Electrical Shop
Ks Air Nat'l Guard
McConnell AFB, KS

 

[hansforum]

Let's say we have two equally high poles. One wooden and another metal. If we place them (first one then another) on the big flat empty field during thunderstorm, do they have the same probability of being hit by lightning?

 

[jghrist]

Let's say we have two equally high poles. One wooden and another metal. If we place them (first one then another) on the big flat empty field during thunderstorm, do they have the same probability of being hit by lightning?

Yes, but for safety, you should wait until the storm is over before replacing the poles.

 

[hansforum]

How is this possible when metal has greater ability of polarisation under electric field? On the top of the metal pole will be a lot more charge so the electric field will be stronger.

 

[PHovnanian]

The tops of your two hypothetical poles are both at ground potential. So the voltage gradients (barring one having some pointy protuberance that the other does not) around both will be the same. And its the voltage gradient prior to the strike that determines the lightning's path.

Following the strike, the metal structure may conduct a somewhat higher current. But the damage will depend on I^2R and the R of the wooden pole is much higher, so it will dissipate more energy. Even worse, once the current heats any moisture within the wood to the boiling point, it flashes to steam and breaks the structure. That's why lightning struck trees 'explode'. Metal just gets hot.

 

[2dye4]

The wooden pole has somewhat less probability of getting struck as it is much less conductive. As a charged cloud approaches induced charge builds up on the top of the wooden pole lowering the electric field gradient.

Now if the pole conducts the charge to ground then their is no lowering of the electric field gradient and the arc is more likely to start there.

same consideration for a wooden building with no lighting protection.
BUT the wooden building if struck is on fire, lighting protection is always recommended.

 

[jghrist]

From IEEE STd 998, E Guide for Direct Lighting Stroke Shielding of Substaitons

At best, the processes occurring within a cloud formation that cause charge separation are complicated. The important fact to the designing engineer is that a charge separation does occur in thunderstorm clouds. Experiments using balloons equipped with electric gradient measuring equipment have been performed to investigate typical charge distribution in thunderclouds, and these experiments have shown that, in general, the main body of a thundercloud is negatively charged and the upper part positively charged. A concentration of positive charge also frequently exists in the base of the cloud. Such charge distribution in a cloud causes an accumulation of charge of the opposite polarity on the earth's surface and on objects (e.g., trees, buildings, electric power lines, structures, etc.) beneath the cloud.

The accumulation of opposite charge in a pole is not a fast accumulation where a large amount of current (dq/dt) would be affected by the resistance of the pole. It is a slow buildup that occurs as the cloud passes over the earth. The amount of charge accumulation does not depend on the pole material. The charge in the earth and the pole, being of opposite polarity to that in the cloud, increases the voltage gradient, it does not lower it.

 

[hansforum]

If material doesn't change probability of getting struck, what about the shape of the object? Does sharp objects have greater chances of getting struck compared to round objects?

 

[samundra24E]

Ofcourse sharper objects have more chance of getting struck. Thumb rule is a lightning mast will protect radius of twice of its height provided that shielding angle is 30 degrees.

 

[jghrist]

Tall objects have more chance of getting struck. Some experts believe that the increased local voltge gradient around a sharp object will increase the probability of a strike. Others don't think it makes a significant difference. Traditional lightning protection design does not differentiate between a sharp or dull mast in any calculations.

 

[cyberix]

If you draw electtric field lines from the cloud to the pole it does make very little difference whether the pole has a sharp point or a dull one. The field lines converge to the tip of the pole (assuming the pole is conductive). The example of the wood pole is misleading as there is a resistance between the tip of the pole and ground, thus the tip of a wood pole can never be at ground potential except when there are no charges flowing. The concentation of field lines at the tip of a pole will accelerate free electrons which will hit oxygen or some other atoms and dislodge more electrons thus creating an avalanche which will ionize the air around the tip which can lead (but not always) to a flashover (strike).

 

[dpc]

I think your original question is really the wrong question. The purpose of lighting protection is to protect not to prevent lightning strikes. Lightning hitting a metal pole or metal building generally does not cause major damage. Lightning hitting a wood structure can cause significant damage and fire. The lighting rod and down conductors help prevent this. Whether or not they are struck more often than an unprotected structure is really beside the point. If a protected building is struck 10 times and suffers no damage, that is better than an unprotected building that is struck once and burns to the ground.

 

[PaulSaul]

The effectiveness of an LPS is dependent on the earthing resistance of its base. where the resistance is bad, wouldnt an LPS rather make the protected premises more vulnerable to lightning strikes by failing to dissipate the electric field?

 

[jghrist]

The effectiveness of an LPS is dependent on the earthing resistance of its base. where the resistance is bad, wouldnt an LPS rather make the protected premises more vulnerable to lightning strikes by failing to dissipate the electric field?

A stroke that hits an air terminal and flows through a copper down conductor into a high resistance ground electrode will cause less damage than a stroke that hits a roof and flows through the building to earth.

 

[hansforum]

So where are we now? Cyberix says that the top of the wooden pole is not at the ground potential and that it has less chance of being struck and Jghrist says opposite.


One more question, when large current flows through down conductor in LPS on the building it makes large voltage drop, so we have very high voltage at the down conductor. Why don't we have little strikes between down conductor and wall or roof? Or why doesn't current flow from down conductor through installation elements to wall, because current would flow through the wall if there was a strike and no LPS?

 

[dpc]

There can current in a lot of places if the building gets a direct strike. The majority of current will usually flow in the down conductors, and prevent major building damage. That is about the best that can be hoped for.

But you have to keep in mind that you are trying to funnel a lot of electrons into the grounding system and sometimes some of the electrons get away. A bolt of lightning that traveled 20,000 feet **through the air** can find a lot of ways to get to ground.

You might do some Google searches for NASA research papers regarding lightning protection. At the Cape, they get a tremendous amount of lightning; they have a lot of valuable things to protect, including spacecraft, and they seem to do a really good job of protecting it.