lightning rods

[ROUSTABOUT]

Where I built the previous house was hit by lightening (and burnt) which supposedly never strikes twice so I'm OK.
Or am I?
OK, I have a TV tower and I did a good job of grounding it plus my hydro grounding is all up to par with 2 9 ft rods completely driven in.
Fingers are always crossed if that helps.

I've seen it strike twice I didn't stay to see if it happened a third or more times. It hit a pipe in a pond about 75 feet from me. Made my guts shake.

 

[Fallon]

I'm under the impression that very dry ground is a poor conductor and therefore a poor electrical ground (thus why electric fences don't work well when the ground gets really dry unless you're expecting the fence to zap between the hot wire and part of the fence instead of the ground).

My ground here is bone dry two feet down in the summer, but when you pound a rod down deeper you start getting a decent year-round ground.

I guess the ground ring or ground-in-a-trench works because there's just so much more ground contact that it makes up for the higher resistivity of the potentially drier soil?

Interesting related article: https://www.ecmweb.com/content/article/20892049/achieving-an-acceptable-ground-in-poor-soil

Correct. Of you have a 50 ohm resistor, you get 50 ohms of resistance. If you have 2 50 ohm resistors in parallel, you get 25 ohms of resistance. So basically you can make up for poor earth ground by increasing the surface area of the ground interface.

Generally you don't want ground rods any closer to each other than their length. So generally 8-10' or further apart. The usable area a ground rod can conduct to is kind of a cone. If rods are to close together they are kind of overlapping & decreased in effectiveness.

Adding bentonite around ground rods helps hold moisture & improves conductivity. There are some other things you can use to increase ground conductivity, but some are a bit toxic (salt).

 

[CobyRupert]

One does not install lighting rods to conduct direct strikes to the ground. Lightning rods work by dissipating static electricity. Stranded wires are only used for the convenience of the installers. Solid wires are more reliable.

It’s the grounding that dissipates static electricity.

How does a pointed metal rod sticking up in the air at the highest point dissipate static on the building underneath it?
Being directly grounded, lightning rods definitely decrease the distance of the “ground” to the cloud. Some would argue that this increases the chance of a strike, but lessons the damage.

 

[Frankenkubota]

I been close twice, both times on the golf course.

1st time with niece and nephew. hit a tree 30 yards away, all i remember is this huge explosion and then the sky full of debris, pine bark i guess.

2nd time, on pay phone in golf course clubhouse working. i saw a big ball of light and some guy asking me if i'm ok? I guess a big ball of light came out of the phone i was holding?

 

[grsthegreat]

I been close twice, both times on the golf course.

1st time with niece and nephew. hit a tree 30 yards away, all i remember is this huge explosion and then the sky full of debris, pine bark i guess.

2nd time, on pay phone in golf course clubhouse working. i saw a big ball of light and some guy asking me if i'm ok? I guess a big ball of light came out of the phone i was holding?

That’s what you get for poking electrical sockets with a knife…..
NOW YOUR A CONDUIT FOR LIGHTNING

 

[SPYDERLK]

On my place, we built our stick house. We knew that there were lightning hits all around us before that point. Saw evidence on the trees as well as the nearby noise level. Specified a lightning rod system on the house and for 15 years it has worked perfectly. [[ Such as system is designed to dissipate the electrical energy before the charge builds to a point where a strike occurs. Learned this in one of my engineering courses.]]

Next, we built a very large metal barn with concrete floor. It was not 3 months after building it that I found a big piece of sparred concrete around one of the frame columns. I knew it had to be due to a lightning hit. Nothing had been planned or implemented to deal with lightning, so I bought my own version of lightning protection - ground rods, and roof spikes. I planned to connect both to the steel frame so the frame was the conductor between. Remember that most homes are wood, so copper wiring must be added to their system. No so with a steel frame. I inserted 3 ground rods around the perimeter and wired them directly to the nearby frame. I still have the roof spikes; they were never installed. Turns out that is all that this building needed - never had any further issues in 10 years. What was missing was a connection between the metal and the ground - the concrete was a poor conductor, so that when the first lightning hit occurred, the concrete between steel and ground heated to the point that moisture in the concrete flashed - creating the sparring.

Buzzards - occasionally I still hear their toenails tap-tap-tapping on the steel roof. If I let out a few loud "barks", most times they will leave.

[[ ]] This truth is seldom shared. - - The mechanism? : High electrical potential at a surface that has great inflection. The point of a lightening rod is such a surface. For lightening the point is best blunted a little. The "point" promotes corona discharge, reducing very local earth/sky electrical differential. The lightening is thus a little less likely to strike there.

 

[SPYDERLK]

What you’re talking about is “skin effect”. It’s only a thing with AC currents. With DC currents, the current will be uniformly distributed across the cross section of the conductor. With 60 hz AC currents, skin depth is about 8.5mm.
One has to be careful with conductor ampacity charts if you’re using one for the other, or if you have 400hz as is common on air planes.

Lightening is a high frequency event. Skin effect and inductance are important. Strap copper instead of wire is about the best you can do to optimize so that its a low impedance path to a "bolt".

 

[Fuddy1952]

I been close twice, both times on the golf course.

1st time with niece and nephew. hit a tree 30 yards away, all i remember is this huge explosion and then the sky full of debris, pine bark i guess.

2nd time, on pay phone in golf course clubhouse working. i saw a big ball of light and some guy asking me if i'm ok? I guess a big ball of light came out of the phone i was holding?

God doesn't like golf.

 

[Grumpycat]

It’s the grounding that dissipates static electricity.

How does a pointed metal rod sticking up in the air at the highest point dissipate static on the building underneath it?
Being directly grounded, lightning rods definitely decrease the distance of the “ground” to the cloud. Some would argue that this increases the chance of a strike, but lessons the damage.

You clearly don't understand.

The pointed rod really does attract the local static charge. Just as scooting your stocking feet on carpet then spark jumps between your fingertip to the doorknob.

There is no way skinny lightning rods will withstand a direct strike. However if one disconnects the grounding wire leaving a gap up to 1" then watches when the wind blows one would see arcing across the gap. Perhaps you would call those "lightning strikes"? Not really coming from the sky any more than the spark from your finger.

By discharging the static on one's building one makes adjacent trees and structures more attractive than one's protected building. The difference in electric potential is greater, until there is sufficient voltage to break down the insulation of the distance between earth and sky. A bit of water in the air reduces the resistance.

Something we have been touching on but deserves much more understanding is "ground". Ground is not a firm fixed absolute. Ground potential (that is Electrical Engineering for "voltage") in one place is different than in other places.

 

[SPYDERLK]

The pointed rod really does attract the local static charge. Just as scooting your stocking feet on carpet then spark jumps between your fingertip to the doorknob.

[[There is no way skinny lightning rods will withstand a direct strike.]]

[[ ]] Careful. Its a very short event. The fractional second fusing current of 3/4" copper is bound to be extremely high - perhaps ~the Meg-Amp range.