On Monday, I have to go to a house that has been struck by lightening. I am told that there is a switch and a light fixture that has been destroyed by this lightening strike. I am supposed to check the electrical system to see what all might be damaged, and repair/replace whatever I find damaged due to the lightening strike. Would a simple check with a multimeter reveal any damage that may have occured, or would I need to seperate all conductors and meg them? Anyone ever have to do this type of inspection after a lightening strike before? If so, any help would be greatly appreciated.
Lightening hit house
- Thread starter elvis_931
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mdshunk
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You don't have to seperate the conductors, but you do have to unplug all loads, remove all light bulbs, turn on all switches, and jumper out all GFCI's and dimmers. Then, meg all of the circuits from the panel. Naturally, you'll need to take the EGC's and neutrals loose from the bar. When lightning was involved, what you see visually is often only the tip of the damage iceburg. Make sure your GEC connections are present and still tight.
Pierre C Belarge
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- Location
- Westchester County, New York
Even though it is not electrical, take a peak at the water hoses at the clothes washer and the gas connections (if there is gas) to the dryer. If the dryer is electric, check the status of the cord. Look at all of the electronic equipment for scorching. Check the GEC connections. Don't forget transformers, such as heating - doorbells, etc...
As has already been mentioned, all loads need to be disconnected...that generally is the most time consuming part of meggering, as you will have to reconnect those same loads. All switch legs should be in the 'on' position, bypassing the electronic type of switches.
Type up a paper that you can record your results on so it is neat and professional.
Remember to price for all of this and Good Luck finding all of the electronic stuff/loads to be removed.
As has already been mentioned, all loads need to be disconnected...that generally is the most time consuming part of meggering, as you will have to reconnect those same loads. All switch legs should be in the 'on' position, bypassing the electronic type of switches.
Type up a paper that you can record your results on so it is neat and professional.
Remember to price for all of this and Good Luck finding all of the electronic stuff/loads to be removed.
mdshunk
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Well, most of the time you'll find isolated sections of cable that just megger bad, and you'll have to figure out a path to refeed whatever that cable does. This is a crazy labor intensive process to megger out an existing home. (not physical work, just very time consuming). You'll also find all the neutrals and grounds that have simply just been touching for years and years, as well as the outdoor receptacles that are full of spider's nests and dampness.elvis_931 said:
After you've disconnected all the loads (which you confirm with a standard ohm meter at the beginning of each circuit from the panel), just start meggerring. This will give you which circuits (if any) have a problem. Then, just divide and conquer. Try to figure out what all is on those nonconforming circuits and start meggerring it in smaller sections to isolate the bad part(s). Sometimes you'll even find switches and receptacles that visually look okay, but they megger bad. If you open them up, sometimes you'll see the smoke trail between the contacts that makes them megger bad.
Above all else, have fun.
catchtwentytwo
Senior Member
Anyone have any thoughts on how often the intensive inspection/meggering uncovers damages? I guess I'm asking is if there are any statistics that could justify that labor cost versus just replacing as much of the system as possible.
mdshunk
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Each and every time that I've been permitted to megger a home with lightning damage, I discover more nonconformity than was originally visually apparent. It is always much cheaper to do the testing than rewire the home, if that's what you're asking. Normally, its less than a day that we're talking about here for a comprehensive test and to pin down the areas for repair. The actual repair(s) may need to take place on following day(s), but at least you'll know what all is safe to turn back on and get back into service. Just leave the circuits off and disconnected that have troubles that need fixin'. That's my experience, anyhow...catchtwentytwo said:
catchtwentytwo
Senior Member
mdshunk said:
Marc, that is what I was curious about.
Thanks,
John
brian john
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- Location
- Kilmarnock, Va
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- Retired after 52 years in the trade.
I have been involved in numerous lightning hits and the location of damage compared to the location of the hit amazes me. I always tell our employees, test everything, and look at everything visual inspection is important.
Also keep records of your test conductors tested, test voltage, results of the test, time, date, technicians name
Also keep records of your test conductors tested, test voltage, results of the test, time, date, technicians name
catchtwentytwo
Senior Member
Brian,brian john said:
I've seen induced damage to communication and data processing equipment caused by indirect strikes. Haven't been involved in any "direct strike" situations but that could be because of where I live in Connecticut (or just plain luck).
A couple of questions if you don't mind:
- Have you seen more indirect or direct strike related damage? Or is it too difficult to tell?
- Did any of the buildings have TVSS protection and did it work or at least mitigate damage?
- Did any of the buildings have a lightning rods or similar exterior protection?
John
brian john
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- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
I think direct hits though damage was experienced in adjacent structures at the worst sites I have visited.
Did 2 visits for two different lightning hits to a tower on a cell site (have made sevral retrurn visits to sites that have expierenced multiple hits). At this one site they had basic cell spec installed grounding and TVSS and expierenced extensive damage. After the first strike they beefed up the protection. The halo ground (from the original installation) was supported off the metal walls with 4" nylon insulators at each insulator there was flash marks, equipment damage was extensive in the site and houses and stores around the site lost electronic equipment, TV's PCs and microwaves.
As noted above after the first strike they beefed up the site to include increasing the number of electrodes and extent of the electrode system, higher quality (? more expensive) TVSS, flat braid equipment ground straps, better cable protection.
I think you can design systems to minimize damage *1, but depending on the duration, magitude of the strike/strikes and location of the hit determines the damage and one cannot protect against all hits.
*1-I am NOT an engineer only someone that has been fortunante enough to have been involved with customers that have electrical issues. So this is just my opinion/guess.
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- Location
- Illinois
- Occupation
- retired electrician
This is the type of problem where I would not trust a megger reading to say that concealed wiring is ok. I have seen one example where the lighting blew some of the insulation completely off one side of a NM cable. The bare conductor was not touching anything and would have passes a megger test. If they would not let me replace all of the concealed wiring, I would walk.
Don
Don
mdshunk
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Don, I'm curious to know why you tend to generally poo-poo megger checking? Are you aware of a a particular accident associated with a formerly passed megger check? That is to say that your opinion is pretty radical, in that you'd only rewire a home that has suffered a lightning strike, and not rely on testing. You'd recommend rewiring a lot of homes, let me say. Any evidence of pyrolysis or fire that you've seen in locations that you're aware of that have passed a megger check?don_resqcapt19 said:
- Location
- Windsor, CO NEC: 2023
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- Hospital Master Electrician
Yeah, I had to look that up. Good word.mdshunk said:
mdshunk
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Here's an old article from the NFPA journal (short, quick read) about damaged wiring and long term pyrolysis. http://findarticles.com/p/articles/mi_qa3737/is_199809/ai_n8821450georgestolz said:Yeah, I had to look that up. Good word.
The damage described in this article would have been easily discovered with a megger check, in my opinion.
petersonra
Senior Member
- Location
- Northern illinois
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- Semi-retired engineer
I have had a few things I did get hit by lightning. There does not seem to be any real obvious reason for why some components get damaged and others don't.
All of my stuff is industrial, and it is often real hard to tell them they have to visually inspect and meg out every circuit and connection.
I had a case where a VFD acted "funny" after a close lightning strike. They replaced the VFD, the PLC and a bunch of other things, finally they went to meg out the motor and eventually they found one of the connections was mostly black crud inside layers of old tape. Somehow the lightning had destroyed the connection without damaging anything else in that circuit.
All of my stuff is industrial, and it is often real hard to tell them they have to visually inspect and meg out every circuit and connection.
I had a case where a VFD acted "funny" after a close lightning strike. They replaced the VFD, the PLC and a bunch of other things, finally they went to meg out the motor and eventually they found one of the connections was mostly black crud inside layers of old tape. Somehow the lightning had destroyed the connection without damaging anything else in that circuit.
- Location
- Illinois
- Occupation
- retired electrician
Marc,
I have only worked on two houses with lightning damage and that was a number of years ago. One them had part of the insulation blown off the NM in a visible area. This cable had passed a megger test. The damage was only found by accident. If this type of damage occurs on a concealed cable I am not aware of any testing that will find the problem.
I have also replaced feeder cables that were damaged when they were installed but still passed a megger test. You can have a lot of bare copper on a conductor within a metallic raceway and still pass a megger test, but only to have the conductor fail when it is exposed to water or when it moves slightly and the bare copper comes into contact with the raceway. Air is a good insulator and a very small air gap will let you pass the megger test. I am not aware of a better test, and am not saying that you shouldn't meg systems...all of my feeder and other large pulls are megged before being placed into service. In most cases the megger will find the problems...I just want people to be very aware that a conductor that has passed a megger test can still have significant problems that will come to light in the future.
Don
I have only worked on two houses with lightning damage and that was a number of years ago. One them had part of the insulation blown off the NM in a visible area. This cable had passed a megger test. The damage was only found by accident. If this type of damage occurs on a concealed cable I am not aware of any testing that will find the problem.
I have also replaced feeder cables that were damaged when they were installed but still passed a megger test. You can have a lot of bare copper on a conductor within a metallic raceway and still pass a megger test, but only to have the conductor fail when it is exposed to water or when it moves slightly and the bare copper comes into contact with the raceway. Air is a good insulator and a very small air gap will let you pass the megger test. I am not aware of a better test, and am not saying that you shouldn't meg systems...all of my feeder and other large pulls are megged before being placed into service. In most cases the megger will find the problems...I just want people to be very aware that a conductor that has passed a megger test can still have significant problems that will come to light in the future.
Don
mdshunk
Senior Member
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With that said, I suppose there's 3 approaches to remediating lightning damage.
- repair/replace whatever is visually damaged only
- megger all the circuits and make the repairs
- rewire the whole place
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
American Heritage Dictionary
py?rol?y?sis (pī-rŏl'ĭ-sĭs) Pronunciation Key
n. Decomposition or transformation of a compound caused by heat.
Brian's rule of testing.
1. Always test your tester (at a minimum I carry two of every small test equipment megger, amp clamp, multimeter, etc...).
2. If the results seem too good to be true, bad or strange in any way, check EVERYTHING.
3. Nothing is 100%.
4. Always use calibrated instruments.
5. It is easier to double check when on the site then from the office.
6. Document everything.
7. Visual inspection is about 80% of all jobs (this percentage seems to move depending upon my mood and memory, but never falls below 75% [I think]).
8. Most likely this should be number 1, be safe, use PPE when required.
py?rol?y?sis (pī-rŏl'ĭ-sĭs) Pronunciation Key
n. Decomposition or transformation of a compound caused by heat.
Brian's rule of testing.
1. Always test your tester (at a minimum I carry two of every small test equipment megger, amp clamp, multimeter, etc...).
2. If the results seem too good to be true, bad or strange in any way, check EVERYTHING.
3. Nothing is 100%.
4. Always use calibrated instruments.
5. It is easier to double check when on the site then from the office.
6. Document everything.
7. Visual inspection is about 80% of all jobs (this percentage seems to move depending upon my mood and memory, but never falls below 75% [I think]).
8. Most likely this should be number 1, be safe, use PPE when required.
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