Lightning rods -- how confusing can they be?

[Noka22]

TL;DR

I wanna create a discussion about how lightning rods work.

Two professors had a discussion one physician and the other eletrical engineer. The first one sad that during a experiment he observed a lightning rod avoiding any discharge, saying that lightning rods when well grounded should avoid discharges and not attract then. The engineer sad that complete opposite, who is right? I’m completely curious and didn’t find any answers that convinced me.

 

[phinds]

TL;DR Summary: I wanna create a discussion about how lightning rods work.

Two professors had a discussion one physician and the other eletrical engineer. The first one sad that during a experiment he observed a lightning rod avoiding any discharge, saying that lightning rods when well grounded should avoid discharges and not attract then. The engineer sad that complete opposite, who is right? I’m completely curious and didn’t find any answers that convinced me.

What did they each mean by "discharge" ? A lightening rod should discharge a lighting strike into the ground. Perhaps they were referring to not discharging into the air.

And by the way, please learn to pay attention to what you write.

 

[berkeman]

Welcome to PF.

Two professors had a discussion one physician and the other eletrical engineer.

By "physician" I'm guessing you meant to say "physicist"?

Also, what reading have you been doing about how lightning rods work?

https://en.wikipedia.org/wiki/Lightning_rod

 

[Tom.G]

It's been a heck of a long time since I looked into lightening rods, but here is what I recall:

They do both, although the 'avoidance' function is minor to the point of almost non-existent.

They work because they are mounted up high around the things they protect. Being high, they are closer to the highly charged clouds above them than the protected structure is.

Another characteristic is they typically have a sharp, pointed, tip (at least when new).

Electric charges concentrate at sharp edges/points, that is typically where you will find discharges of any high voltage system. That is why it is important that the grounding cable, if any ,does not have any bends in it.

The protected area around the lightening rod is a cone whose apex angle is roughly 30° to 60° from the top of the rod.

Further information at: https://apps.dtic.mil/sti/tr/pdf/ADA066293.pdf

Since that is a US military site, you may be better off doing a Google search:
http://www.google.com/search?hl=en&?=protected+region+of+lightening+rod

Cheers,
Tom

 

[Baluncore]

Lightning rods both attract and repel lightning, depending on the polarity of the charge buildup.

If the rod attracts charge, and is struck, then the conductor takes the current to ground without setting fire to your building. When the rod repels the lightning, it may strike your neighbour. Either way, you are safe.

 

[phinds]

Lightning rods both attract and repel lightning, depending on the polarity of the charge buildup.

If the rod attracts charge, and is struck, then the conductor takes the current to ground without setting fire to your building. When the rod repels the lightning, it may strike your neighbour. Either way, you are safe.

I've never heard of this. Can you provide a citation for repelling?

 

[Baluncore]

All things are relative, maybe for repulsion, read more attraction elsewhere.

All potentials are differential. Lightning discharges, break down the steepest voltage gradients locally, operating, ultimately, to reduce the most extreme potential difference regionally.

Lightning forks to neutralise local accumulated charge, be it in the clouds (parcels of air), or near the ground. An insulated neighbour, with no lightning rod, can preferentially attract the next discharge fork. In effect, you repel it by being grounded. It pays to ground yourself, in an attractive neighbourhood, as after the first fork or strike, you may be next for neutralisation. Does grounding make you less attractive, or more repulsive?

 

[phinds]

An insulated neighbour, with no lightning rod, can preferentially attract the next discharge fork

Sure, but that does not mean that a lightening rod repelled charge. Again, I ask, can you provide a citation for the repelling of charge from a lightening rod?

 

[Baluncore]

It seems the non-events are never analysed. All studies are of the strikes.

I cannot therefore produce a citation for "Building with lightning rod, NOT struck by lightning".

Sure, but that does not mean that a lightening rod repelled charge.

Corona discharge from pointed objects, reduce the chance of a direct strike. Is that not a repulsion?

 

[renormalize]

Corona discharge from pointed objects, reduce the chance of a direct strike. Is that not a repulsion?

Not necessarily: Reduction-of-attraction $\neq$ Repulsion.

 

[tech99]

The lightning rod will provide a safe route to earth for a strike in its vicinity. As Tom G mentions, the protection cone is 30 to 60 degrees. The shape of the rod has no effect on its operation, and it does not have to be pointed. When a thunder cloud passes overhead, the potential gradient between the cloud and earth becomes very great and corona discharges occur, sprouting out from all high points. These leaders extend to tens of metres in height. Then a downcoming leader starts zig zagging down from the cloud,. searching for the lowest resistance path and proceeding in legs of tens of metres. Eventually it nears an upward leader and makes contact. The path is now complete and a large upward (conventional) current passes in the form of a lightning strike. The lightning rod (or conductor) does not discharge the thunder cloud. There is a lot of information in the European Standard for the protection of buildings against lightning. Sometimes the discharge is from a positve part of the cloud and can then be a monster.

 

[dMohler]

Lightning rods are no mystery. They are simple electrodes connecting to earth ground and are raised above structures they are intended to protect so as to be the primary conductor to the ionized potential above the ground. The physician is mistaken thinking that the lightning rod is supposed to protect a structure by means of some opposing field that it magically produces. It may appear to him that because lightning does not strike a protected structure that the lightning has been repulsed. The electrical engineer is correct.

 

[sophiecentaur]

I've never heard of this. Can you provide a citation for repelling?

I read in an A Level Physics text book that a sharp point at the top of a high building readily ionises the air around it (corona) when there is a nearby build-up of charge under a cloud. The point can set up a flow of charges to form a spheroidal surface with a large radius. This reduces the local potential gradient (electric field) to suppress the formation of a serious strike in the vicinity. The conditions are very non-linear so the radius of the conducting spheroid can stabilise the situation; higher local fields can increase the radius, lower local fields will decrease the radius. A really sharp spike can cause this 'protective' corona in moderate fields. I have no idea just how big this spheroid would be but it would have to enclose most of the highest parts of the structure.

Someone else nearby may have an increased risk of a big strike of course. The effect is not a 'repulsion' but rather a slipping out by the back door and avoiding a problem.

In the event of an actual strike, the fat conducting strip to Earth can shunt the current from any path through the building and act as a fuse for just one strike, I guess.

 

[tech99]

Although we can demonstrate the ionising action of a point in the laboratory, practical experience says that the shape of the lightning conductor makes no difference. We can determine a cone of protection, beneath the lightning conductor, having a 45 degree semi angle. This is based on statistical information. The European standard for the protection of buildings against lightning provides this information.

 

[tech99]

Regarding the size of conductor, 50mm^2 copper strip will carry nearly all strikes without damage, and something like 10mm^2 will carry about half of strikes without damage.

 

[sophiecentaur]

Although we can demonstrate the ionising action of a point in the laboratory, practical experience says that the shape of the lightning conductor makes no difference. We can determine a cone of protection, beneath the lightning conductor, having a 45 degree semi angle. This is based on statistical information. The European standard for the protection of buildings against lightning provides this information.

So do the statistics reveal any significant change in strike numbers on a building, before and after a lightning conductor is fitted? I'm being lazy here but you know where to find that info quickly.

 

[tech99]

The text of the IEC Standard for the protection of buildings against lightning is reproduced here:-
https://cfpa-e.eu/app/uploads/2022/05/CFPA_E_Guideline_No_04_2013_N.pdf
"A lightning conductor protects the building from damage by direct strike by lightning, but does not
prevent the lightning striking the building. The lightning conductor has the function to lead the
lightning current down to earth in a controlled manner".

 

[sophiecentaur]

@tech99 Cheers; so that's the official opinion. Your point about 'in a Physics Lab" is well made.