Mythbusters

| Did anyone see Wednesday's Mythbusters on Discover Channel? | | They were out to test the myth of being electrocuted while talking on a | phone during an electrical thunderstorm. They made a little test | house/shack, and wired it up, and put a dummy talking on a phone, phone was | strapped to his ear. Anyway, it's a long story. They powered up this big | test machinge to 300,000 Volts and it arced to the house. They couldn't get | the test dummy to read any current until they removed the house ground wire. | | Anyone see this episode and want to comment on the validity of the test? | I'm curious to see any part of the experiment was unreal.

I didn't see that episode. But I have witnessed a real life case. I was calling a friend of mine who lived about 20 miles out of town in a rural area. I reached his mother who sent someone to go get him, which took a while since he had gone to bed early. So I was chatting with his mother for a couple minutes when I hear a loud pop and a lot of crackling on the phone and then no one was talking on the other end. About 30 seconds later my friend got on the phone and told me he'd call back later because his mother had been struck by lightning on the phone and knocked out. He called about a half hour later and she was OK. He found some scorch marks on the phone around the base and plug. Amazingly the call was not dropped by the lightning. But I can imagine it could have been worse.

I didn't see this, but when I was young, we had a summer cottage on a lake. When thunderstorms were around, the phone would often ring during a lightning strike, particularly when the storm was east of the place. We'd joke "It's Mr. Lightning calling; don't answer it!". Sometimes just a ding, sometimes with more energy. It was a party line; the ringer was attached to a ground stake. (when they eliminated party lines I don't think lightning caused it to ring anymore) You may think overhead wiring would be a cause of all this, but surprisingly the phone wiring in the area was all underground.

any attempt to make generalizations out of this would be invalid. there are way too many variables in the equation.

if you are all that worried about it just always use a wireless phone or cell phone.

Even the museums with the largest electric lighting simulators cannot approach the voltages present in an actual electric strike. It's a magnitude of millions or billions of volts vs. hundreds of thousands for the manmade lighting generators.

It's no secret that lighting is destructive and kills hundreds each year. In a severe electrical thunderstorm, the telephone is just one such household item to avoid that just happens to be part of an electrical system in the house. I would avoid taking baths and showers and usings sinks, basins, washing machines, and toasters, and hair dryers. The path of a monster lightning strike is mostly unpredicatable once it actually gets in the house.

The thing is you may have a perfectly working ground that could be overwhelmed by the tremendous currents from a direct strike. There are techniques that radio & TV broadcasters use to mitigate lighting strikes from entering their transmitter shacks. Few homes, though, have the same degree of industrial protection. For one thing, its very expensive.

Beachcomber

I'll be looking @ the dummy through a camera/monitor holding the phone in a cabin in the middle of nowhere USA };-) with a charge while lightning is thundering strikes all around. the lightning documentary i saw on pbs was hot....but who cant predict or provoke electricity};-)

I have to warn you the big guys don't tell you you're about to Szzt before you pop something .......... be alert

howoboutit

------------------------- yeap , when you see your hairs about to stand long on ends you know it's near :-) imagine gettin shunted to earth by one silent translucent bolt, powder :-)

how about a motorized cage/mesh like compartment or/ with a switchable ground terminal and on rubber wheels. how close was it monitored ????

we'll need the monster capacitor I have right? an, the monster blade switches from last month for starters }:-)>

ohmm, i need a job or a sum/thing

(-:{ =AEoykey

That's freaking hilarious! ~:-)

Dwayne

"Dwayne" wrote in news:jC%2e.864085$6l.193353@pd7tw2no:

It actually goes UP from the ground :)

I didn't see the episode but here is how lightning works. It takes the least conductive path to ground. Every house has an earth wire buried that is connected to a big metal spike in the ground. If you have an overhead telephone system (ie: your telephone come from a nearby pole), then you are at risk. However, if there is a big metal flag pole nearby (at a restaurant for example) then your fine (as long as you not in the restaurant). If your worried about your house being struck by lightning just give a neighbour's kid a kite to play with and make sure the string is actually a wire.

Dwayne

I've heard the arguments about lightning going up from the ground. But I know less about lightning than I do about electricity as a whole. So, until I have time to do my own homework on this one, I'll just take your word on it. :)

It actually goes UP from the ground :)

I've heard the arguments about lightning going up from the ground. But I know less about lightning than I do about electricity as a whole. So, until I have time to do my own homework on this one, I'll just take your word on it. :)

Actually i heard there are 2 kinds the kind that goes up and another that comes down :-o

It is rather complex. When the potential difference between a cloud and the ground gets to be excessively high (i.e. hundreds or thousands of millions of volts) two things happen.

A step leader ionized path of lightning comes down from the clouds in discrete steps descending lower and lower and seeking out a path to ground. Usually this step leader is invisible to the naked eye, but it sometimes can be heard as the loud swishing sound just before a close-by lightning strike.

Similary, the charge on the ground becomes so great that similar ionized streams of the opposite polarity will come up from the ground, or more commonly, flagpoles, trees, grounded tv antennas, lightning rods, church steeples or basically anything that is conductive and grounded including humans and animals.

When the two streamers meet, the conductive path is complete and the lighting flashes with the distinctive light and high current discharge.

A fraction of a second later, what is called the return stroke discharges in the direction from the earth to the cloud. This is on the order of from 10 to 1000 times more powerful than the initial stroke in terms of current magnitude and is the main big bright flash that is seen and heard as thunder.

To the average person, it looks like the lightning has struck the ground because it happens in just a fraction of a second. More properly, the initial step leader and ground streamer have established the lowest resistance ionized path and, in a fraction of a second, the return stroke takes place over this ionized path. A special camera is needed to see the details of how the connection is established because it is to fast for the human eye to follow.

Beachcomber

Numbers in hyperbole were posted. Lightning is not the massive destructive energy as implied. Lightning is high power but not the high energy event so often promoted in myths. Lightning is a current source - not a voltage source (as was implied). From basic EE, voltage will rise as necessary to maintain that current flow. And no, one need not worry about using a telephone if the human has performed his job. How a horse is killed when not even struck demonstrates principles of single point earthing.

Why a single point ground? Same reason why horses can be electrocuted when a nearby tree is struck. Lightning seeks earth some 4 kilometers west. So it strikes a tree directly below the cloud that is east of the horse. Shortest path westward is up that horse's hind legs and down horse's fore legs. Horse is killed because it became a good path for lightning to flow west. Horse conducts electricity better than earth beneath. Horse became part of an electric circuit.

Same applies to house (building) protection. If using multiple grounds, then a transient can rise up on a left side ground rod, find destructive paths through appliances, then reenter earth via a right side earth ground. Concept is demonstrated in a NIST figure that demonstrates why bad earthing can cause fax machine damage:

How to make a single point ground beneath the horse? Surround the barn with a buried halo ground. Then earth beneath the horse becomes one big single point ground - equipotential. Horse can be killed by a concept that also kills computers. Therefore better buildings also install halo grounds or Ufer grounds. Earth ground - not a UPS or power strip protector - being so important for computer hardware (and horse) protection.

An introduction to concepts of building (and electronic) protection is provided at:

Protection from lightning is first and foremost about earthing. Yes, even the household earth ground provides a massive improvement in both human and transistor protection. But for protection from rare, higher energy lightning strikes (that you may never observe in your lifetime), utilities install massively larger earth grounds. Major expense only to marginally improve their earthing system - because reliability is that critical.

Removing a building's earth ground (or connection from each incoming utility to that single point earth ground) puts the human (and transistors) at greater risk from lightning. Reason to not use a phone during a thunderstorm? A human has failed to install or has compromised the building's earthing system. This should never happen in an EE's building.

The myth busters dem> Even the museums with the largest electric lighting simulators cannot

--------------- The horse is not killed because the lightning wants to "go west" ground due to high ground current radiating out from the tree (natural ground rod) through resistive ground. There will be a step potential between the horses legs because of this so that there will be a small part of the current through the horse (if large then the horse is cooked, not electrocuted). If the horse could stand with all feet together it would be OK if not too close to the trunk of the tree.

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This source does indicate the transient problem but doesn't fit your description. It is a case of a long path between the entance and a single ground. This is also a factor in the case of lightning striking a power transmission tower. If multiple grounds, there may or may not be an appreciable voltage build up, depending on many factors but the net effect will be lower voltages than with a single ground. However, you are right with respect to "proper grounding" .

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---------- Possibly I am missing your point but Multiple well separated (too close together and only a slight benefit exists with respect to a single rod) ground rods will be more effective than any single rod - IF the rods are interconnected properly. If not, then there will be no benefit. Note that utilities will use many rods in a switchyard but will interconnect these at or under the surface by a grid- to achieve both low ground resistance as well as an approximately equipotential surface.

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------ Agreed but the key factor here is low ground resistance more than short distances. It is a matter of incoming line surge impedance vs ground resistance. It is also a factor that protective equipment should be as close as possible to the equipment being protected. It is less about "shunts" than about the behaviour of surges and reflections where changes in surge impedance occur.