lightning

[bphgravity]

Re: lightning

All the research I have read seems to indicate that lightning tends to "dance" arond on the surface of the Earth never driving more than around 24" into the ground.

To me, this could mean it may be more effective to take the down conductors from an air terminal to a ground rod laying on the surface of the ground rather than driven into the earth.

 

[mc5w]

Re: lightning

Lightning is a form of high frequency alternating current and tends to take the lowest INDUCTANCE path. However, lightning tends to have enough current to it that it will take EVERY path to ground that it can find.

A single elctrode spot welder works on the same principle. Instead of having an electrode on top and another electrode underneath the work and passing all the current through the work, the transformers in a single electrode spot welder are connected across the 2 ends of a single elctrode. There is so much current through the electrode that metal in contact becomes a part of the circuit. Where you use a single electrode spot welder is when welding the wire guards for fans and air conditioning condensers.

If you were welding a seam with a single electrode spot welder the current would be running parallel to the seam instead of straight through the seam.

[ May 10, 2005, 03:57 AM: Message edited by: mc5w ]

 

[mc5w]

Re: lightning

I also forgot to add that if say lightning hits the mains out on the street, some fraction of the current will go up your service drop looking for a place to go just as a matter that lightning tends to take every path to ground that it can find. The distance also tends to act as a low pass filter that would cause only the lower freqency components to travel down a service drop.

You have to remember that in a parallel circuit not all of the current takes the shortest path. Otherwise, you would only be able to run 1 load at a time in a house.

The dancing on the surface phenomenon is more of both a skin effect due to the high freqency and that lightning tries to follow every path to ground that it can find.

 

[oceanaradio]

Re: lightning

Hi mc5w,

You cited some Army grounding parameters, and likened it to the 10' elevation problem we discussed earlier. I explained that bonding can't fix all problems that contractors or cable installers create. The performance of air terminals and ground rods does not cover that scenario as I think you suggested.

There's no contradiction that 10' of elevation can have a voltage potential unsurmountable by common bonding, while air terminals can be hundreds of feet above ground potential and work as designed.

We exit communications towers with feedlines above ground potential sometimes, when it is necessary to do so. But even a few feet above ground (elevation = potential) make catastrophic problems if not properly controlled further down the line. The down-conductors from air terminals to earth ground generally have much more surface area than the tiny #6 bonding jumper that NEC requires. The down conductors further connect to many earthing systems, where the lowly bonding jumper assumes by the rules that it can equalize massive potential differences between poorly constructed systems. Sadly, a #6 or even #00 can do no such miracles.

Bonding cannot protect a homeowner from poor construction practices either, such as separate utility entrance and/or ground rod locations. This is true in spite of the code allowing it, as long as that magical bonding jumper is installed. Reading the details of NFPA-780 go part of the way toward explaining the inadequacies of using only art.250 grounding, and then assuming that there is some form of lightning protection included therein.

Where 100,000's v potential is concerned, the surface of the earth will be so saturated that voltage division among many shallow conductors is more effective than one or two deep ones. DC rules just don't cover the behavior of a strike, although they are always a necessary first part of any protection equation.

Roofs of single family residences are not usually struck by lightning, and therefore most homes have no specific lightning protection. Proper code installation of service entrance and single point earth (or ufer) ground will protect them from "average" surges coming in from a nearby utility strike, tree strike, etc. Protect here means not start a fire, but they may lose all their electronics, etc.

But when a structure is located such that it CAN be struck by lightning, it can still be protected from both fire and electrical damage, it just costs more to provide this! I know some of you want to know why the soothsayers go around saying "nothing protects from a direct strike"! Well they are only right when mistakes are made in the design and construction, mistakes that lightning is unforgiving toward.

No sophisticated design or protection scheme can make up for major goofs such as the cable modem grounded to the cold water pipe, satellite dish on its own separate ground (and not bonded), etc.

Every case I have seen or read about has such an underlying cause - a mistake or underestimation was made somewhere, and lightning exploited it. Engineers have designed elaborate surge protection schemes to protect equipment, and then neglected air terminals on the structures that house our watchstanders. Go figure.

Jack
Virginia Beach, Va