# rebar as a lighting rod **URL:** \ **Category:** Factual Questions\ **Created:** [August 23, 2005, 12:50am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201 "2005-08-23T00:50:39Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![The\_Long\_Road](https://avatars.discourse-cdn.com/v4/letter/t/b5a626/32.png) [@The\_Long\_Road](https://boards.straightdope.com/u/The_Long_Road)\ **Post date:** [August 23, 2005, 12:50am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/1 "2005-08-23T00:50:39Z")
In my backyard I have about 8 pieces of 10 foot long rebar stuck in the ground. The rebar goes down about 4 feet and is used as my tomato plant supports. I’ve always wondered what would happen if lightning struck the rebar. How would the electricity be directed during the strike? I met a guy who installed lightning rods(not exactly an authority) who said it would “protect the plants”(not my concern) and lighting wouldn’t hit it unless it was going to hit the area anyway. So I’m just wondering, a lighting strike in a 10 foot piece of rebar driven 4+ feet into the ground, what’s going to happen? ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [August 23, 2005, 2:35am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/2 "2005-08-23T02:35:21Z")
A typical lightning bolt has 1E9 J of energy, which is more than enough to vaporize a piece of rebar. The problem is figuring out how much of that energy gets dumped into the rebar, vs. heating the air, making noise, heating the ground, inducing currents in nearby objects, etc. ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [August 23, 2005, 2:53am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/3 "2005-08-23T02:53:30Z")
> [@mks57](#): > > A typical lightning bolt has 1E9 J of energy, which is more than enough to vaporize a piece of rebar. The problem is figuring out how much of that energy gets dumped into the rebar, vs. heating the air, making noise, heating the ground, inducing currents in nearby objects, etc. Vaporize? Cite, please. Based upon the lightning protection systems which I’ve seen in place which would hopefully conform to NFPA 780, the attractive electrodes and connective/grounding conductors were equal to or smaller than 5/8" rebar. How is it that they are still in place, absent physical damage? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 23, 2005, 3:13am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/4 "2005-08-23T03:13:26Z")
> [@danceswithcats](#): > > How is it that they are still in place, absent physical damage? Because, as stated, not all the energy was dumped into the conductors. Most of the energy is lost elsewhere. But, if all the energy of a typical lightning bolt DID dissipate entirely with a piece of, say, 5/8" rebar it WOULD vaporize it. A billion Joules is a lot of energy. ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [August 23, 2005, 3:32am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/5 "2005-08-23T03:32:41Z")
> [@danceswithcats](#): > > Vaporize? Cite, please. Based upon the lightning protection systems which I’ve seen in place which would hopefully conform to NFPA 780, the attractive electrodes and connective/grounding conductors were equal to or smaller than 5/8" rebar. How is it that they are still in place, absent physical damage? [The Power of Lightning](http://www.weatherwise.org/qr/qry.lightningpower.html) 1E9 to 10E9 J [Standard ASTM Reinforcing Bar sizes](http://xtronics.com/reference/rebar.htm) 10 feet of #4 rebar is approx. 3 kg [Chemical Elements - Fe](http://www.chemicalelements.com/elements/fe.html) Boiling point of Iron is 2750°C [Calculation of specific heat capacity with iron](http://www.allmeasures.com/Formulae/d1/d2/d3/Results2_2.asp?formula=5&material=Iron+%5BFe%5D) 3704250 J to heat 3 kg of iron by 2750°C ---
**Author:** ![The\_Long\_Road](https://avatars.discourse-cdn.com/v4/letter/t/b5a626/32.png) [@The\_Long\_Road](https://boards.straightdope.com/u/The_Long_Road)\ **Post date:** [August 23, 2005, 3:42am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/6 "2005-08-23T03:42:01Z")
So if I have a 10 foot piece of rebar stuck 4 feet into the ground, what’s usually going to happen if it takes a hit? ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [August 23, 2005, 4:48am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/7 "2005-08-23T04:48:23Z")
> [@The Long Road](#): > > So if I have a 10 foot piece of rebar stuck 4 feet into the ground, what’s usually going to happen if it takes a hit? Well, for one thing I think the point of the rebar will increase the electric field intensity in its vicinity and discharge the cloud, or whatever at a lower energy level than the typical lightning strike that takes place in the absence of a pointy object. So your rebar will most likely not be damaged any more than any other lightning rod. ---
**Author:** ![butler1850](https://avatars.discourse-cdn.com/v4/letter/b/779978/32.png) [@butler1850](https://boards.straightdope.com/u/butler1850)\ **Post date:** [August 23, 2005, 4:06pm UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/8 "2005-08-23T16:06:41Z")
The energy converted to heat in the rebar will only be that produced as a function of resistance. Since rebar is a pretty good conductor, it’ll certainly get hot, but not enough to vaporize the rebar. Most of the energy, in the form of electricity, will be conducted to ground, leaving an intact hunk of rebar. Otherwise, every grounding bar/lightning rod would need to be fully replaced after every strike, and would not be able to protect the structure on which it is placed to protect in the first place… “Martha, the steeple lightning rod has vaporized, I’m heading up to replace it before the next strike…” :rolleyes: Now, I’m not going to even attempt to refute the evidence above, that if ALL the energy was discharged, and converted to heat, ONLY in the rebar, without a discharge to ground, that it would vaporize. I’m sure it would. But in real life, it’d heat up some, and shunt nearly all that energy to ground. I’d also not put good odds on a bountiful harvest of tomatoes following a strike. -Butler ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [August 23, 2005, 4:12pm UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/9 "2005-08-23T16:12:57Z")
If all the energy that went into a structure were to be turned into heat, they’d always be repl;acing the TV antennae and the repeaters atop the Empire State Building and other takk city buildings. In most cases the current is routed harmlessly to ground (although I’ve seen some spectacular shots of shingles blown off the sides of houses by lightning bolts that passed through nearby lightning rod-to-ground cables) I’m not sure that there’s much point to a four foot high lightning rod meant to protect your garden. Unless your garden is out in the middle of a big flat area, it doesn’t seem very likely it’d be hit. And even if lightning strijkes a rebar rod near your tomatoes, I doubt if it’d do much harm. Dover’s got a couple of good books out on lightning. They’re worth a look. ---
**Author:** ![The\_Long\_Road](https://avatars.discourse-cdn.com/v4/letter/t/b5a626/32.png) [@The\_Long\_Road](https://boards.straightdope.com/u/The_Long_Road)\ **Post date:** [August 23, 2005, 5:43pm UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/10 "2005-08-23T17:43:09Z")
> [@CalMeacham](#): > > If all the energy that went into a structure were to be turned into heat, they’d always be repl;acing the TV antennae and the repeaters atop the Empire State Building and other takk city buildings. In most cases the current is routed harmlessly to ground (although I’ve seen some spectacular shots of shingles blown off the sides of houses by lightning bolts that passed through nearby lightning rod-to-ground cables) > I’m not sure that there’s much point to a four foot high lightning rod meant to protect your garden. Unless your garden is out in the middle of a big flat area, it doesn’t seem very likely it’d be hit. > > And even if lightning strijkes a rebar rod near your tomatoes, I doubt if it’d do much harm. > Dover’s got a couple of good books out on lightning. They’re worth a look. There is no point to having lighting rods protecting my garden and due to this I don’t have any. I use the rebar to support my plants (as stated in the OP). As stated, I don’t care about protecting my plants, a person who installed lighting rods said that is what would happen. Also, the rod is 6 feet out of the ground since it is 10 feet total with 4 feet in the ground. Thanks for the answers but… Everyone is misreading the question. I asked, "How would the electricity be directed during the strike?” not “what’s going to happen to the rebar?”. I can buy rebar for $5 a section so replacing it doesn’t matter. Where’s all that juice going if it hits the bar? If I’m 20 feet from it when it gets hit am I going to also be fried or will the 4-foot part in the ground be enough to protect anyone in the area? Assume it’s been raining heavily before the strike. I’m pretty sure they have taken hits already and they are still standing. New Orleans gets nasty electrical storms on hot summer afternoons. ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [August 23, 2005, 5:47pm UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/11 "2005-08-23T17:47:31Z")
[The Master speaks](http://www.straightdope.com/columns/010824.html) ---
**Author:** ![Finagle](https://avatars.discourse-cdn.com/v4/letter/f/b19c9b/32.png) [@Finagle](https://boards.straightdope.com/u/Finagle)\ **Post date:** [August 23, 2005, 6:27pm UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/12 "2005-08-23T18:27:06Z")
I think the answer to “what would happen” ranges from[not much](http://answers.google.com/answers/threadview?id=17949) to “fresh tomato sauce”. First of all, there are many, many chain link fences whose metal posts are essentially the same as your embedded rebar. They don’t seem to attract excessive amounts of lightning. So unless your rebar is the tallest object for 50m around, you’re not increasing the probability of a hit. However, in the event of a direct strike, I doubt the tomato plants at ground zero would be protected. The ground is not a very good thermal conductor and all the energy in the bolt has to go somewhere. So I imagine that a small area around the rebar is going to get very hot, probably killing the roots of the tomato. Maybe even a small steam explosion might occur. If a person were standing nearby, they’d probably be in the same danger as if lightning had struck the ground nearby. The ground is also not a very good electrical conductor so voltage falls off rapidly with distance from the strike leading to steep voltage differentials. If you happen to be near your tomatos when lightning strikes, bring your feet close together and get on the balls of your feet to lessen the current across your body. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [August 24, 2005, 1:46am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/13 "2005-08-24T01:46:10Z")
> [@The Long Road](#): > > Everyone is misreading the question. I asked, "How would the electricity be directed during the strike?” not “what’s going to happen to the rebar?”. I can buy rebar for $5 a section so replacing it doesn’t matter. Where’s all that juice going if it hits the bar? If I’m 20 feet from it when it gets hit am I going to also be fried or will the 4-foot part in the ground be enough to protect anyone in the area? Assume it’s been raining heavily before the strike. I’m pretty sure they have taken hits already and they are still standing. New Orleans gets nasty electrical storms on hot summer afternoons. As I understand it the rod, being a point, raises the field intensity in its neighborhood and increases the chances that the first voltage breakdown of the air will be there. However I think people in the vicinity of the rod will be no more protected than people in the vicinity of a tree or other object that sticks up above the surroundings. The excess electrons on the ground, brought there by the falling rain, return to the cloud through the ionized air and I don’t believe that ionization occurs only at a point. ---
**Author:** ![UncleBeer](https://avatars.discourse-cdn.com/v4/letter/u/977dab/32.png) [@UncleBeer](https://boards.straightdope.com/u/UncleBeer)\ **Post date:** [August 24, 2005, 3:55pm UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/14 "2005-08-24T15:55:40Z")
> [@](#): > > The rebar goes down about 4 feet and is used as my tomato plant supports. I’ve always wondered what would happen if lightning struck the rebar. Fried green tomatoes, no doubt. ---
**Author:** ![TokyoBayer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tokyobayer/32/13989_2.png) [@TokyoBayer](https://boards.straightdope.com/u/TokyoBayer)\ **Post date:** [August 25, 2005, 3:11am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/15 "2005-08-25T03:11:20Z")
IANAGFE (I am not a ground fault expert) but my BIL is. Since he’s written papers on the effects of lightning, I asked him the question and his reply follows. > [@BIL](#): > > Relative to the custom made tomato plant stakes, that is one way to acquire fried green tomatoes on the vine. What the fellow has inadvertently installed in his backyard with the sections of rebar is an non-bonded and poorly installed “clustered lightning abatement array”. Contrary to popular myth and folklore, lightning strikes do not away strike the tallest objects. The installer of lightning rod mentioned below was partial correct when he commented that the sections of rebar are not likely to be struck by lightning unless there was some type of atmospheric disturbance in the area. However, lightning very rarely strikes without some type of atmospheric disturbance in the local area. > > To be specific, if lightning were to strike at or near the steel rods, depending on the actual arrangement and spacing of the rods there is a strong probably that more than one of the rods would be struck simultaneously. Depending on the specific geographical area where the fellow backyard is located, the magnitude of current discharged could easily range between 10,000 amperes to 60,000 amperes. The magnitude of current discharged from a cloud-to-ground strike will flow regardless of the item(s) struck. The inherenent resistance of the item is not a variable. The magnitude of current discharged from the cloud through the steel rods would be distributed in the soil in 360 degrees along an XYZ axis until all of the energy was dissipated and there was a equal potential difference between the cloud and the local earth. As the current dissipated there would be a dramatic voltage rise between every surrounding item, thing, or person that had a difference of resistance relative to the earth plan. The voltage rise is often in 1000’s to 10’s of 1000s voltage range. It is the differences of potential and the subsequent current flow that causes people to be seriously injured or killed during lightning strikes. If a one or more subterranean paths conducted a significantly higher magnitude of current through the soil then that soil could turn to glass or calcify. The current being distributed in the soil would likely turn any moisture in the soil very rapidly to high pressure steam. The expansion of the steam pressure is what often causes the soil to erupt, the roots and bark of a tree to explode, or the fracturing of concrete foundations and bricks. All of these events occur in microseconds. > > The fellow with the 10’ rebar as tomato stakes would be well advised to avoid being around his tomatoes on overcast days. ---
**Author:** ![The\_Long\_Road](https://avatars.discourse-cdn.com/v4/letter/t/b5a626/32.png) [@The\_Long\_Road](https://boards.straightdope.com/u/The_Long_Road)\ **Post date:** [August 25, 2005, 3:58am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/16 "2005-08-25T03:58:18Z")
> [@TokyoPlayer](#): > > IANAGFE (I am not a ground fault expert) but my BIL is. Since he’s written papers on the effects of lightning, I asked him the question and his reply follows. Wow. A real answer. Thanks! ---
**Author:** ![TokyoBayer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tokyobayer/32/13989_2.png) [@TokyoBayer](https://boards.straightdope.com/u/TokyoBayer)\ **Post date:** [August 25, 2005, 4:22am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/17 "2005-08-25T04:22:40Z")
No problem! 🙂 BIL and I don’t agree politically, but since this is \*General Questions \* and not \*GD \* then I’ll quote him as an expert. 😃 Thought about replacing them with wooden stakes? ---
**Author:** ![Random](https://avatars.discourse-cdn.com/v4/letter/r/57b2e6/32.png) [@Random](https://boards.straightdope.com/u/Random)\ **Post date:** [August 25, 2005, 5:07am UTC](https://boards.straightdope.com/t/rebar-as-a-lighting-rod/318201/18 "2005-08-25T05:07:43Z")
> [@TokyoPlayer](#): > > Thought about replacing them with wooden stakes? Tomato supports? I’ll see your 10’ iron rebar and raise you 12’ copper pipes. As I understand it, unless I were to install something made of silver, nothing’s a better conductor than copper. (And as much as I enjoy my tomatoes, spending money on silver has to involve something a bit higher on the Random Heirarchy of Needs.) The copper’s been there 4 years. I’m in the Midwest. We get the occasional electrical storm. Nothing vaporized or killed yet. (Mind you, the idea of a lightning bolt frying one of those GD squirrels that are eating my Brandywines is pretty damn appealing.)