lightning

As a kid, we had lightning strike a 40ft walnut immediately adjacent to the garage. Blew the bark off the tree on one side from top to bottom! Believe me, you *know* when the strike is that close -- things *shake*!

Many years later, a near-by strike took out our phone service (fried the protection circuit placing the station set "off hook" permanently), "magnetized" the (CRT) TV, etc.

Here, our utilities are below grade so rarely suffer from

*local* outages.

EMC is important. Of course, it's often difficult to define what A/B/C failure conditions are, and so are at the tester or manufacturer's discretion... or the customer's, if they complain enough.

The rampant and poor quality computerization of everything doesn't help, either. Needless to say, an NTSC analog set would've, at worst, just blinked... of course, decoding NTSC is a far cry from HDMI and such, too.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com 

"John Larkin"  wrote in  
message news:9gq6ma9ggir269htrhjjq3mgf22snl05dj@4ax.com... 
> 
> 
> There's a big thunderbumper moving in, and we just had a lightning 
> strike somewhere nearby. My computers were fine, but my LG monitor 
> crashed and reset. Weird. 
> 
> 
>

I wonder what the electric and magnetic fields peak at, a block away from a good strike.

Hardly any B,

(enter e.g. 100000 A and 30m, for 0.66 mT)

but the dB/dt and therefore EMF will be pretty good. EMP and all that.

Within that kind of distance, you also have to worry about spreading resistance of the Earth. Which is probably the predominant voltage to worry about (with indirect strikes), especially for buried or underground utilities that happen to be running through (and directly or incidentally grounded to) the vicinity of the strike. The effect will be considerably pronounced in rocky areas where heavy (granite?) bedrock is near the surface.

Probably not something we have to worry much about, here in Wisconsin, seeing as above-ground poles are common (so, who even gets indirect strikes anyway, right?..), and where the bedrock is fairly deep (porous, moist, electrolyte-rich limestone, usually buried under 100+ ft of glacial till).

The main exception being further north, where the ancient (~2e9 yr) granite basement rock comes near the surface, and conductivity is poor. As I recall, there used to be a military ELF antenna there. The relatively low surface conductivity makes the ground return current take a deeper loop on average, hence radiating a couple microwatts more at 70Hz or whatever it was. When your bitrate can be measured by stopwatch, pad and paper, every bit of efficiency counts!

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com 

"John Larkin"  wrote in  
message news:od27math3st3ufp6dj2j014p8ksj07h0es@4ax.com... 
> On Mon, 25 May 2015 15:41:47 -0500, "Tim Williams" 
>  wrote: 
> 
>>EMC is important.  Of course, it's often difficult to define what A/B/C 
>>failure conditions are, and so are at the tester or manufacturer's 
>>discretion... or the customer's, if they complain enough. 
>> 
>>The rampant and poor quality computerization of everything doesn't help, 
>>either.  Needless to say, an NTSC analog set would've, at worst, just 
>>blinked... of course, decoding NTSC is a far cry from HDMI and such,  
>>too. 
>> 
>>Tim 
> 
> I wonder what the electric and magnetic fields peak at, a block away 
> from a good strike. 
> 
>

I live in the tropics and powerful lightning strikes are common here. But I have reasons to remember two incidents in particular:

The nearest strike I've personally experienced was almost exactly

30 years ago, in June 1985 IIRC. It struck a large tree at the rear perimeter of my compound about 50m from my house. It was LOUD! My live-in servant happened to be looking out the kitchen's rear-facing window and he said that he was thrown staggering back by the blast. I don't know if it was really a reflex action on his part.

But the biggest strike in terms of apparent energy was a year earlier, in September 1984. I was in a friend's house when the lightning struck at around 8 pm. It felt like it was really close. The TV died instantly. Half an hour later, her sister arrived home from night college and she said that she thought the bolt had struck just outside her classroom. My folks said the same thing when I got home, 2 km away.

The next day, I received damage reports from power stations all over my state (This was a time when I spent a lot of time helping out the government-run power company). They all reported a massive lightning strike at about the same time I experienced it at my friend's house. Unfortunately, there was no automatic data logging system at that time and I couldn't prove that they happened at the exacr same time.

I was also running a TV repair service at the time and we were overloaded with cases of lightnng damage for a long time. That must have been one HUGE strike.

As Tim Williams points out, dB/dt counts for a lot, but then so does dumb luck in such situations.

ca. 1968 I was watching (the family's Curtis-Mathes) TV when there was a lightning strike thirty feet from the house. The TV indeed just blinked.

Mark L. Fergerson

Nashville, TN (well Joelton) circa 2000, I'm sitting on our front porch watching a T-Strom. My wife and baby daughter are just inside the front door in a rocking chair. I turn to come inside and... Wham! (Bright light out of the corner of my eye.) I hit the deck... and turn to see flame shooting up and down one of several huge oak trees in our front yard, about 50 feet away. (Two had already been killed by previous strikes.) Giant pieces of bark are blown all over the place. It's like the inside of the tree exploded.

I read later that lightening is a major cause of death in the old oak trees. Other trees species are known to survive lightening strikes. The theory is that other trees have a smoother bark and (in a rain storm) there is a continuous sheet of water running down the outside. The lightening strike tends to stay on the outside of the tree, causing less damage. Oak's have this rough corrugated bark and the lightening gets inside the tree, frying it.

George H.

Hmm, excuse my spelling of lightning.

Odd things happen with lightning... a number of years ago there was a lightning strike at an SRP substation ~3 miles from my house.

Between me and the substation is all underground feed.

I was awakened by a boom and a flash so bright I thought the house was on fire... the surge blew every light dimmer in the house and the flash was the nominally 400W dining room chandelier 50' away going off like a flashbulb.

Even devices "protected" by UPS were trashed, but several monitors survived... some TV's bit the dust, some survived.

I was so adrenalin-rushed that I managed to access both attic hatches without aid of a ladder... to check for fire ;-)

Fortunately insurance replaced it all. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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In my case, I was doing the "one-mississippi" trick (count seconds between flash and thunderclap to gauge distance) while watching TV. The TV is located at the end of the living room adjoining the garage. The garage is typical 2-car (perhaps 20 ft wide) with the walnut tree *literally* on the other side (many other trees were felled when the house was built and my folks sought to preserve these others).

When this strike came, the house shook from the thunderclap which almost seemed to precede the lightning flash (of course, it didn't; but when you are expecting it to be "seconds later" and it is exactly coincident...)

"Wow, that one was *close*!"

It wasn't until the next morning that I saw all the bark on the ground and wondered where it had come from -- looking up to see the side of the tree "shaved clean"! I suspect the water in the phloem boils nearly instantly and "explodes" the bark off the tree.

"Cool!"

Positive lightning (aka a "bolt from the blue") can be many times more powerful than a normal (negative) stroke. See e.g.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

I was a witness to this:

It was a good and hearty thunderstorm, so I was watching from the safety of our front porch. This was one city block (lengthwise) from the church.

Think it was some time after 1:00 am, KABOOM, one thin, spindly stroke right onto the steeple.

I didn't think much of it at the time (besides the rush of the strike) -- one would hope their grounding was good -- but by morning, it was smoldering, and the day after, little more than masonry walls and a pile of charred timbers. FD couldn't do anything. RIP.

They rebuilt it, but it's boring sheer brick, nowhere near as ornate as the original.

Tim

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