Just have to know...

[zardoz]

Overmod,
My understanding was that even though the charge is directed around the cage, if there is another path for the charge (through a person touching both the cage and another point of current flow), some, but not necessarily all, of the current will flow throught the other path. If that path is more electrically conductive, more of the current will flow through it.

It has been a few years since we covered this issue in the NWS training classes, so I may be somewhat in error. I shall reasearch it further and will advise.

The closest I ever came to lightning was one night when I was out photographing a storm over the lake. The front of the storm was a good two miles away; suddenly a bolt shot out of the cloud and hit a pole about 25 yards from me. I had about a two second warning of the strike when my arm hairs stood up and I got all tingly. And WOW, was the thunder loud!! I decided to cease my photography for the evening. Sure made a believer out of me!

[chad thomas]

QUOTE: Originally posted by Overmod

Zardoz -- my understanding was that if you were inside an electrostatically-charged Faraday cage and touched the wall, there would be little if any spark.

Remember the formulas for charge distribution in a conducting sphere, and what the charge density INSIDE the sphere is? There might be some asymmetries in a metal cab that would allow small currents to flow -- but I'd expect these to be about on the order of the spark you'd get shuffling across a carpeted floor and touching a doorknob: high voltage, very low current. And, of course, not passing through the heart in the critical range of either current or voltage...

Much more significant, I think, would be a strike introduced into the cab through some of the insulated wiring, which might allow very large current for a brief interval which would then happily discharge to the "Faraday cage." Randy! What's the breakdown voltage for the path between the engineer's hand and the most likely parts of the throttle control? ;-}

The breakdown depends on the dialectric strength of the handle (K factor)
For 1 (air) it's about 100,000V per quarter inch. Plastics typicaly have a K factor of around 8. So assuming 1/4" of handle it would be in the neighborhood of 800,000V.




Lightening "searches" for its path before the brunt of the flow occours. This "searching" is what does the most damage to electronic devices.

There are two kinds of lightning strikes.
Positive- Strikes from the clouds down.
Negative- Strikes from the ground up.
There are about 100 positive strikes for every one negative.
The forrest services monitors lightning strikes on there property.
When a negative strike occours they automaticly dispatch a fire crew as they almost always start a fire.
I

[zardoz]

Chad, a small correction: it is the positively charged lightning that is the nasty type (see last paragraph).

Lightning education, part 2 (this is from the NWS Storm Spotter taining site):

Each spark of lightning can reach over five miles in length, soar to temperatures of approximately 50,000 degrees Fahrenheit, and contain 100 million electrical volts.

During fair weather, a potential difference of 200,000 to 500,000 Volts exists between the Earth's surface and the ionosphere, with a fair weather current of about 2x10 to the -12 amperes/meter squared. It is widely believed that this potential difference is due to the world-wide distribution of thunderstorms.

Present measurements indicate that an average of almost 1 ampere of current flows into the stratosphere during the active phase of a typical thunderstorm. Therefore, to maintain the fair weather global electric current flowing to the surface, one to two thousand thunderstorms must be active at any given time. While present theory suggests that thunderstorms are responsible for the ionospheric potential and atmospheric current for fair weather, the details are not fully understood.

Ice in a cloud seems to be a key element in the development of lightning. Storms that fail to produce quantities of ice may also fail to produce lightning. In a storm, the ice particles vary in size from small ice crystals to larger hailstones, but in the rising and sinking motions within the storm there are a lot of collisions between the particles. This causes a separation of electrical charges. Positively charged ice crystals rise to the top of the thunderstorm, and negatively charged ice particles and hailstones drop to the middle and lower parts of the storm. Enormous charge differences (electrical differential) develops.

A moving thunderstorm gathers another pool of positively charged particles along the ground that travel with the storm. As the differences in charges continue to increase, positively charged particles rise up taller objects such as trees, houses, and telephone poles. Have you ever been under a storm and had your hair stand up? Yes, the particles also can move up you! This is one of nature's warning signs that says you are in the wrong place, and you may be a lightning target!

The negatively charged area in the storm will send out a charge toward the ground called a stepped leader. It is invisible to the human eye, and moves in steps in less than a second toward the ground. When it gets close to the ground, it is attracted by all these positively charged objects, and a channel develops. You see the electrical transfer in this channel as lightning. There may be several return strokes of electricity within the established channel that you will see as flickering lightning.

Not all lightning forms in the negatively charged area low in the thunderstorm cloud. Some lightning originates in the cirrus anvil at the top of the thunderstorm. This area carries a large positive charge. Lightning from this area is called positive lightning. This type is particularly dangerous for several reasons. It frequently strikes away from the rain core, either ahead or behind the thunderstorm. It can strike as far as 5 or 10 miles from the storm, in areas that most people do not consider to be a lightning risk area. The other problem with positive lightning is it typically has a longer duration, so fires are more easily ignited. Positive lightning usually carries a high peak electrical current, which increases the lightning risk to an individual.

Jim

[chad thomas]

Jim (zardoz),
Thanks for the explaination. And sorry for my mistake.

[Anonymous]

A few years ago, a few maintainer friends of mine were working on a siding switch and had cleared for a train. Lightning had struck the main line rails and a blue fire ball flew down the rails and just followed the rails. It had damaged the Hot Box Detector (HBD) about 1/2 mile north, the electronics inside the cabinet for the siding switch, and an event recorder at an interlocking 2 miles south of where the guys were standing. They were just about to start working on the switch when the lightning struck. Lightning doesn't always come from the sky and heads downward - it can start from the ground and work it's way up to a cloud. Difference in electrical potential. Case in point, a neighbor of mine who lives right behind my house had a lightning strike that required the fire dept's help. The huge tree in their back yard was untouched. They used to tell us a tall object will attract lightning to it for an area that is up to 3 times the height of the object. So, if a tree were 100 feet tall, then, lightning that is headed downwards would strike the tree 1st and would "protect" everything up to 300 feet away from the tree. Now, I should point out that the lightning strike that flew down the rails by my maintainer friends were working on amtrak's northeast corridor and they were surrounded by the catpoles that support the electrical catenary wiring system for the trains. These poles are on average 280 feet apart, up to 100 feet tall, and are on both sides of the tracks. Those guys were basically surrounded by "lightning rods", but, that didn't help them!

[Anonymous]

QUOTE: Originally posted by Hugh Jampton

Lightning finds the path of lowest resistance to earth,, whatever it may be.

[8D] i have a feelin that i would be the tallest object to be hit if i step foot in a church