I was just thinking about the two or three ground rods near my service entrance. Not sure if there is a code stating things other than so many feet deep. I never really thought about it, by my grounds must be poor. They are close to the foundation, on the dry side of the house, and they might not even go below foundation, because of the landscape. I got another ground rod on other side of house off my aluminum porch roof, and iron railing. I know the light box ground is also connected to the same porch metals. A strike somewhere might cause significant current right through house. Another ground rod off elevated deck attached to metal rods in the air, plus cb antenna.
Separate garage, wired from house. There is no ground rods at garage, and no boxes. I think there might be a code for a ground, not sure. I have no box in garage, except for junction boxes. There is a ground rod connected to sheds metal roof.
I showed why a plug-in protector with high ratings is not likely to fail right after your (HeyBub's) post.
I show how a service panel protector can survive very strong very near lightning strikes in another post and below.
But you don't get 50kA on the power service to your house.
An investigation, which is described somewhere in this thread, used a
100kA strike to a utility pole adjacent to the house. Only 5% of strikes are stronger, and the strike was about as close as you can get. It is the worst case that is likely. The surge current to the house was 10kA per service wire. 10kA per wire is the worst case with any probability of occurring. There is a reference to it in the IEEE surge guide.
Your 20kA protector can protect from 2 100kA strikes to the utility pole adjacent to your house. And that is with the minimum rating that you and the IEEE recommend.
Yes, exactly. There are multiple paths to earth lightning arestors on the distribution wire, and earthing at the transformer and other houses.
Not obvious what this refers to.
You can get follow-on currents in arc-type voltage limiters, where the surge initiates an arc and the normal voltage can continue the arc briefly. MOVs are not arc-type limiters.
Failing MOVs start to conduct at lower voltages and eventually conduct on normal voltage. Then they go into thermal runaway. UL listed protectors have thermal protectors to disconnect these failing MOVs.
Actually you can test them but it is quite impractical.
When MOVs fail UL requires (since 1998) they be disconnected. Decent protectors will tell you if that happens.
For plug-in protectors the IEEE surge guide explains the protected load can be connected across the MOVs, and be disconnected with them, or can be connected across the incoming wires. If connected across the MOVs the protected load is not exposed if MOVs fail. Connecting this way is one reason some manufacturers can have protected equipment warranties. Starting 2005 UL requires manufacturers notify buyers if disconnecting the MOVs does NOT disconnect the protected equipment.
With high ratings a plug-in protector is not likely to fail anyway (as covered in a post to Bub).
Also use high ratings for service panel protectors.
If using a rod the code wants it to be 25 ohms to earth, or you can install more than one with no resistance requirement. Easiest is to install 2 or more. I would guess the 3 rods are the power earthing system. I have occasionally driven a rod through the floor under the service. It avoids being close to the foundation and may be in moister soil. Rods are a crappy earthing electrode.
I would guess the rods off your metal roofs are crude lighting protection. It might prevent a fire at the point of the strike, but does not prevent major electrical damage.
For a lightning rod system, the earthing system must be bonded to the power earthing system. With high lightning currents to earth and resistance to earth plus impedance of down conductors there can be a very high voltage between down conductors and other metal. Metal within
6 feet of the down conductors may have to be bonded to them. Protection from a direct strike isn't simple.
The NEC wants metal support parts of an antenna connected to the power earthing system. And a protector(for coax just a ground block) that is connected to the power earthing system where the antenna lead enters the building. This will not protect from a direct lightning hit to the antenna.
The code wants a grounding electrode at a detached garage if fed by a feeder (with panel in garage), but it is not required if there is only a branch circuit to the garage.
FPL in conjunction with the University of Central Florida determined a typical lightning strike can penetrate the ground by more than a meter. They have fulgurites (sand turned to glass) that are almost 5 meters long tho.
In cases where the service runs mostly underground the profile for the possible lightning hits and magnitude would be substantially different I would think. If there is no utility pole and wires running down the street, no overhead service cable to the house, the targets for lightning are greatly reduced. At my house, all that is available is a transformer sitting on the ground that serves a few houses. Lightning could hit that, but it would be less likely than it hitting exposed overhead wires. And if it did, I would bet that a lot less than the
10K that Bud referenced would make it to the house via the service wires.
Lightning can of course hit the house itself. In which case, all bets are off. If it hits say the chimney, it could take various paths and there would seem to be a low probability that the surge protector at the panel would do much good. If the lightning made it's way to AC circuits in the house, the surge protector might help, but if the strike is at one end of the house, gets to circuits there, it's a long ground path back to the surge protector at the panel and hence I would not expect it to do much good.
Even a nearby lightning strike to say a tree could have effects by raising the ground potential near the house, etc. But I think realistically, the most frequent kind of surge is one that winds up coming in on the service conductors. And with underground service, IMO that is probably at least an order of magnitude less likely. At least for big surges, the kind that would make it to the house if the utility wires close to the house were hit. You could still see more modest surges that make it to the house from an exposed utility getting hit blocks away, etc.
And also factor in many manufacturers count each protection mode seperately to arrive at that number, eg if it uses MOVS rated at 20KA and there is one from L1 to ground, one from L2 to ground, one L1 to L2, that makes it a 60KA rated device. Meaning that 100KA one is likely really 33.3KA.
It also shows that on the granger site. I think what happened, the earlier model was superseded with the 150k unit, but they got the data sheet wrong or not updated. I had the lesser rated unit, and it blew. These things are warranted, so keep all records.
Couple of years ago, I got his with a surge from lightning. I have no idea where the actual strike was, but. . .
I have a detached garage. On the outside of the garage away from the house I have a two lamp spotlight mounted under the eave. There was a hole in the downspout that runs by it . Inside, the wire to the light plugs into a receptacle. I found pieces of the plug and receptacle five feet away from the box in the garage. One of the circuit breakers in the house on that buss was fried as well as my TV, receiver, and doorbell that are on that circuit.
The good news is, I was able to replace all the electrical parts for about $15. The better news is, the next night I had a new 47" flat screen HD TV.
Years ago, there a loud crack nearby. Didn't notice anything until next day, my telephone modem would not work. After that I installed mov's on tele line.
Oh, yeah, there is one of those two, 8 townhouses away (the other end of this building). It's only 2 feet high. Failed once on July 4th weekend, 29 years ago, but from overuse. Water heater and water failed the same weekend, when I had 3 guests from out of town.
Since I have two metal chimneys, I thought it would be rather easy to ground one of them (although just now I realize I'd have to ground both of them, since the grounded one would stop attracting lightning.)
But I called some national Lighning Hotline number and he kept saying if I didn't live in Florida, I didn't have much of a problem. Huh? But then I got busy with other things. .
That makes sense.
After about 15 years, my burglar alarm keypad/control panel was smoking a little one day, and failed, and might have been damaged by a surge, I guess (even though I had connected it correctly to a 5 foot earth rod) , but so far nothing else.
I bought a double-D surge supprrssor but haven't installed it yet.
As I understand things, lightning seeks earth ground. By grounding your chimney, you make it an easier path, and therefore more likely to be hit.
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Since I have two metal chimneys, I thought it would be rather easy to ground one of them (although just now I realize I'd have to ground both of them, since the grounded one would stop attracting lightning.)
Grounding something will not make it stop attracting lightning. Lightning is seeking a path to ground. By grounding a chimney you are providing a safer path. Hopefully if lightning strikes the chimney, most or all of the energy will follow the ground wire you have provided. But, it all depends. The more sharp turns, the longer the wire, etc, the more likely some of the lightning could choose to go another way too.
FL is certainly a very high activity area, possibly the highest in the USA. But it's all relative. Just because you live somewhere with 25% of that activity doesn't mean you can't be hit too.
I thought it was the build-up of a positive charge on some high point that attracted the lightning, and the ground let the positive charge be neutralized from spare electrons in the earth. Hence the lightning wouldn't strike. ??
I think what Micky may be referring to are some of the systems that claim to reduce lightning strikes through various means. Whether any of that has been actually proven to work, I don't know. But I think these systems are more than the typical lightning rods that have been installed on buildings for hundreds of years. The purpose of those conventional systems, as gfretw states is to provide a safe path for lightning if it does strike. And my guess would be that if one looked at two identical buildings, one with and one without lightning rods, the one with might have somewhat more strikes, because it has a mighty fine ground point high up in the air. Hence a bolt that might have instead gone to a nearby tree, winds up hitting the rod. So, you could have more strikes, but no damage, because the system conducts the energy to the earth. Whether any studies have been done on that, IDK.
As I remember, there are 2 companies with other technologies than simple rods.
One presumably is more 'attractive' to lightning so you need fewer protection points.
The other allegedly discharges the cloud.
There are some good reasons the second approach will not work.
NASA tried one and it just worked like a conventional lightning rod.
One of the copmanies sued the NFPA (over NFPA780, the installation standard, or it might be UL). The lightning company lost the case.
Far as I know, the magic rods are not recognized by the industry.
There is disagreement over whether a sharp rod point is better than a dull one. The research that has been done shows little, if any, difference - with something like a 1/2" radius being slightly preferred.
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