Direct lightning strike, what will the damages be?

jonr · May 2017

So the panel frames are grounded. My understanding (numerous, consistent sources) is that any system with multiple earth ground points should run wire between them to help make them equipotential grounds. For example "The final product must include the bonding together of separate grounding electrodes of different systems."

NAWS says "If you have a panel array that is more than 50 to 75 feet or so from the rest of the system, it should have it's own frame/mount ground (not electrical ground)." It's not clear if it means there should also be a wire between panel and building grounds. IMO, there should be.

BB. · May 2017

The 6 AWG copper wire between frames and pieces of metal is really an "electrical safety ground". In case you have, for example, a power line cross (blown down utility lines) or an electric drill that snags a cord in sharp metal, or even one of your solar panel wires gets shorted to metal--The 6 AWG cable between the components is to "ground" the power so that it does not shock you--And will trip a breaker/fuse at the power source.

That "trip a breaker" means that the 6 AWG grounding wire has to go back to the "system main ground water pipe/master ground rod) (i.e., your main AC panel safety ground). Two ground rods 50 feet apart will only carry a (rough maximum) of ~2-5 amperes between them at best for 120 VAC. Not enough to "trip" a 15 amp breaker in your main panel (a "good ground rod installation" can have as high as 25 Ohms resistance).

Lightning wise, the 6 AWG connection from your array down to a ground rod at the base of the array is all that is needed to conduct the lightning away from "things you care about". And not (for example) through your bolts set in concrete footing (with or without rebar) and blowing up your concrete footing.

Personally, I would like a 6 awg minimum direct buried ground wire from the remote rod (array) to the master rod/ground point in the home/power room. I believe it can help reduce differential voltages between the various points in your system. However, lightning itself will (guesstimate) only follow a 6 awg 10-20 feet or so before it finds another path (lightning is a "Radio Frequency" event--These currents do not follow copper wire like DC or low frequency AC (60 Hz power) does. A different/enhanced set of design rules are required for lightning.

-Bill

mvas · May 2017

mcgivor said:

Attract lightning? From all research done, by no means am I disputing your claim, in fact it seems that is exactly what happened, experts in the field claim, lightning is either going to happen or not, offering a path to ground therefore is not attracting, it is diverting what would have hit something else close by. It's a controversial subject and there is no clear consensus and by no means do I claim to have the answers. Thanks for the input.
Edit, did you mean diverting the strike to the tower?

When lightning is going to strike, the path will be "The tallest object, with the lowest resistance to ground".
"The tallest object, with the lowest resistance to ground" is now your tower.
If your tower was not there, then the lightning strike would have been elsewhere, example: a tall tree.

At the Lightning Lab in Florida, they actively attract lighting strikes to their specific location by launching a small rocket, at a very specific time, within a fine wire attached = "The tallest object, with the lowest resistance to ground". They very successfully attract lightning strikes to their exact location.

My question was ... "When your tower gets struck by lightning, how are you going to properly discharge 28,000+ amps into the ground with destroying nearby electronics?"

mvas · May 2017

mcgivor said:

The panels are bolted to a welded steel frame, supported by six legs encased in concrete footings 50 cm. In diameter burried 50 cm., the grounding would be whatever the conact and resistance values are, no intentional grounding or bonding. The rational being, why make the array a target by intentionally offering a good ground path to something you don't want to take a strike in the first place and was the prime reason for adding a mast with a good ground, to offer a better target. There is much controversy regarding what is the best method including some who claim the NEC is wrong in their recommendations when it comes to PV systems, the internet is flooded with information, by "experts " in the field whose recommendations are so polarized it's impossible to come to a conclusion, not to mention non experts who base their ideas on what the believe to be correct. What I have observed is lightning hitting the two cellular and one satalite communication towers 800 meters away on numerous occasions, their lightning protection must work as the network stays up and running, each one has a single air terminal the same I use only larger and more than likely a more elaborate grounding system, to protect there more valuable equipment. Wish I knew all the answers, but based on the general consensus it would appear there is no, one answer, every strike is different and unpredictable.

There are some real experts at the Lightning Research Lab in Florida.
I agree with you, that the NEC group has really confused the grounding fault & lightning grounding issues regarding PV Systems.
One of the NEC members made a video stating that some of the NEC Grounding Code was actually incorrect.
And then the Local Inspector has his own ideas of what is proper grounding.
Grounding to prevent accidental electrical shock of a human vs grounding for lightning are really two different issues.

BB. · May 2017

Technically, people and systems do it all the time. Takes lots of properly designed and installed copper to do it. Also, you want to direct the energy away from what is inside the building.

Turns out, that is not all that difficult to do. Since lightning is a "radio frequency" event--There is something called the "skin effect". Basically, with RF (and AC) current, the changing current causes a changing magnetic field which forces the current to the outside surface of the conductor.

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

So a copper pipe (or braided copper cable) conducts lightning as well (and depending on shape) better than a solid copper rod.

When you have a building--If you have (for example) a lightning rod in the center of a roof, and 4x copper down leads at the 4 corners of the building, then the lightning current wants to flow down the 4 corners and not down through the middle of the building where your electronics may be.

Obviously there are difficult installations. A radio antenna--That also attracts lightning tries to bring the lightning energy right into the radio gear itself. Look up HAM Radio guides for lightning control.

Here is an FM radio station and the fixed they needed to their grounding systems:

https://www.copper.org/applications/electrical/pq/casestudy/nebraska.html

You are correct that a direct strike will need lots of copper... A Class one cable is around 3 AWG and a class 2 cable is around 2/0 (based on weight of copper--not physical diameter):

http://www.kuefler-lightning.com/conductors.htm
http://colonialwire.com/wp-content/uploads/2013/09/WIRE-WEIGHTS1.pdf

I was suggesting 6 AWG as both the minimum grounding wire for NEC (physical strength and good for grounding something like a 200 Amp AC service--Check your NEC book). 6 AWG will "fuse" around 600 amps of continuous current (which lightning is not).

For short pulse of current, a 6 AWG solid copper wire will fuse at ~5,000 Amperes (0.5 seconds). And 4/0 awg (class 2 lightning control) is rated around 42,000 amps at 0.5 seconds.

http://www.commscope.com/Docs/Fusing_Currents_CCS_40_TP.pdf

The other was base on an article that looked at the effects of lightning strikes (I believe in Europe) on bonding cables in older buildings. They had found that, in some cases, 8 AWG and smaller wiring had vaporized/failed as a result of a strike. And that no 6 AWG or heavier cabling had failed. Of course, I cannot find the article now...

-Bill

jonr · May 2017

I have no doubt that if you are building an isolated tower to divert lightning, more distance is better (ie, it reduces ground potential rise in the areas that you care about). Maybe even use a buried horizontal ground wire leading away from the areas of interest.

BB. · May 2017

Some things just are not meant to be be in lightning prone regions:

https://www.nachi.org/wind-turbines-lightning.htm

According to a German study, lightning strikes accounted for 80% of wind turbine insurance claims.
During its first full year of operation, 85% of the down time experienced by one southwestern (United States? -Bill) commercial wind farm was lightning-related. Total lightning-related damage exceeded $250,000.
The German electric power company Energieerzeugungswerke Helgoland GmbH shut down and dismantled their Helgoland Island wind power plant after being denied insurance against further lightning losses. They had been in operation three years and suffered more than $540,000 (USD) in lightning-related damage.

Lumisol · May 2017

LOL The planet Earth is a lightning prone region.

mcgivor · May 2017

@mvas asked:My question was ... "When your tower gets struck by lightning, how are you going to properly discharge 28,000+ amps into the ground with destroying nearby electronics?"

The mast is a 2" galvanized 1.5 mm wall steel pipe, an eight foot 5/8 copper clad ground rod installed 3 feet below grade! with a #2 uninsulated copper cable from rod to pipe, clamped to the pipe using stainless hardware , 3× 5/8 rebar welded to the pipe driven into the ground ~4 feet, concrete encases the pipe from 3 feet below grade and 4 feet above grade, the soil is moist dense clay. So basically whatever current that will discharge, it is intentionally located as far as possible 35 feet from underground cables and equipment room but close enough to the array to be within the 45° theoretical coverage. The overhead cable that was too close was planed to be removed, it more than likely was the cause of the inverter failure, EMP, just got hit sooner than expected, 4 days after installation.

mcgivor · May 2017

jonr said:

I have no doubt that if you are building an isolated tower to divert lightning, more distance is better (ie, it reduces ground potential rise in the areas that you care about). Maybe even use a buried horizontal ground wire leading away from the areas of interest.

More towers are planed, further away 100 feet, much higher ~60 feet in a perimeter formation to offer more points of discharge, hopefully averting a close hit. From what I understand, once in the ground it doesn't travel too far, being damp clay should help and the water table at this time of year is about 12 feet below grade.

jonr · May 2017

This source says a tower should be a minimum of 30 feet away. So your 35' sounds good.

http://lightning-protection-institute.com/lightning-protect.htm

It seems to me that it might also be useful if underground wires (or plastic conduit) had an insulation rating much greater than the normal voltage on the wire - in case of high ground potential.

Direct lightning strike, what will the damages be?

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