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July 1959 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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 Lightning season is upon us once again.
The National Weather
Service says June, July, and August, are the most active lightning months in
the U.S., which is probably true in all of the northern hemisphere, and then December,
January, and February in the southern hemisphere. According to the
National Safety Council, the average American has a 1:114,195
chance of being killed by lightning in a lifetime (which ends abruptly upon being
killed). That's much less than your chance of dying due to cancer (1:7) or being
killed in a car accident (1:102), but is sucks if you're that one in 114,195. Not
all lightning strikes are fatal, but many cause personal and property damage. Mitigating
the chance of being harmed requires taking some simple actions to not expose yourself
to danger in the first place (don't be outside or touch grounded metal objects during
a lightning storm), and to otherwise employ schemes and devices that prevent lightning
discharge in your proximity and/or that safely directs its current to ground through
an appropriately sized conductor rather than through your body or property. Many
thousands of articles have been written on the subject. Here is another.
Facts About Lightning Protection
A property originally worth over $15.000. less than 20 minutes
after being struck by lightning. It was burned to the ground.
By Sidney C. Silver Service Editor, Electronics World
What is lightning? What makes it? Is the hazard real or rare? Is effective protection
possible?
What does this mean to the TV owner or technician?
Lightning is as inevitable a part of the human experience as the earth and the
sky that collaborate to create it. No corner of the world is immune to it. Yet knowledge
concerning these outbursts of Nature at her most dramatic and most violent is generally
inadequate or incorrect.
Many characteristics of this phenomenon will be appreciated better by the electronic
technician than by the electrician. With some cause, the public has come to make
some sort of association between lightning and television. As an area of interest
for the service technician, overlooked though it has been, the matter of protection
is a "natural" that goes beyond the simple association just mentioned.
Lightning and its effects are more widespread than is generally realized. It
does not move in the direction or manner that is generally believed. Objects most
vulnerable during a thunderstorm are often believed to be the safest. The storm
conditions and periods when hazards are highest are not widely recognized. How much
do you know about these celestial bursts of frenetic fury? How often does it really
cause trouble? Is effective protection truly possible? How big is a bolt? How many
volts of violence does one pack? If you know the answers, you are in scant company.
The development of a thunderstorm, meteorologists tell us, involves the intrusion
of a mass of relatively cool air over a layer of moist, warm air hugging the earth.
Every high-school physics student knows that warm air tends to rise above cool air.
This upward motion generates a formidable amount of static electricity, with the
upper masses of moisture becoming positively charged and the lower portions negative.
It is the latter cloud charge that concerns us.
Below this negatively charged entity is a great body of air, effectively a dielectric.
Still farther down is the earth itself. In essence, we have an enormous capacitor
with the earth forming one plate. With a negative charge on the opposite plate,
the shell of the earth becomes positive under a gathering storm cloud. (Under such
a cloud in an open field, you may feel your hair stand on end. If you do, dive for
the nearest ditch or hug the comforting earth!)
Fig. 1. Based on U.S. Weather Bureau data, this map shows
the number of lightning storms occurring over a 20-year period in various parts
of the U.S.
For the giant dielectric to break down, an almost irresistible attraction must
build up across it. Indeed the cloud-to-earth potential builds up to millions of
volts - perhaps a hundred million. At last, opposition collapses and a ferocious
stroke of lightning streaks upward.
Yes, upward. Positive to negative current? Well, not exactly. Actually little
leaders - fingers of lightning probing for a weak spot - dart down irregularly,
often running horizontally for thousands of feet, at only a few thousands of miles
per minute. This "slow motion," which we can see, gives us the impression of downward
movement. When a leader makes contact with the earth, the current path has been
completed. The blinding flash that follows is the main stroke, hurtling skyward
at over a million miles a minute, too fast for us to see motion, faster than a satellite
in orbit. The rapid heating, and ionization of air molecules in its path produce
thunder.
During what part of the storm is lightning most ferocious? Nature speaks with
her most terrifying voice at the beginning of her outburst. The charge has built
up to its highest peak and the lower air is driest before the storm breaks. As the
ensuing downpour develops, the damper air is a better conductor. Thus the lady vents
much of her rage in the first wild shriek; then grumbles, mutters, and finally quiets.
A main bolt may be miles long and about as thick as a man's waist. It is likely
to do less damage if it comes later in the storm. Such earthbound objects as trees
and houses are less likely to be damaged when struck as they become increasingly
wet, since they are better able to pass the charge to ground.
Isn't the likelihood of lightning striking someone or something rather small,
actually? You can draw your own conclusions from available statistics. The map of
Fig 1, based on data compiled by the U. S. Weather Bureau, shows the number of lightning
storms that have occurred in various parts of the country over the last twenty years.
Some areas average over a hundred a year, other as low as five or ten. No area is
immune. The number of strokes in each storm can only be guessed.
Also consider this: The National Board of Fire Underwriters, in assessing the
causes of all fires in rural and suburban areas, indicates that lightning is the
villain in close to 40 percent of these! It is responsible for almost as many as
the next five leading causes combined. What's more, a lightning-caused conflagration
can differ in degree. As in the case of the photo at the beginning of this article,
a stricken property can be utterly devastated before the owner has had a chance
to recover from the initial shock.
We have mentioned that probing lightning leaders seek out earthbound weak spots
to complete the path. It is generally known that anything projecting up from the
ground is such a weak spot. It may be a church steeple, a tree, or a flagpole. It
may be a TV antenna.
Aware that unprotected antennas may invite lightning, many people take the precaution
of insisting on antenna lightning arresters and, if they don't, their conscientious
service dealers are likely to urge these devices on them. When properly installed,
these arresters will prevent the antenna from inviting disaster. However, except
in the case of the relatively small home, these legitimate safeguards give some
people a false sense of security. The area under the antenna that will be protected
is relatively small.
A carefully planned, over-all system is needed. Yet, even with such an installation,
many people tend to ask: "How effective are protection systems? We hear of so many
cases where lightning has struck in spite of safety devices." For the answer to
this one, we go to another agency. Underwriters' Laboratories, Inc., on the basis
of its own extensive records, reports that a lightning protection system is 99 percent
effective - if it is properly planned, constructed, and installed! A defective system
may actually have an effect opposite to the one sought, however, in that it can
increase the hazard.
How does one know that a particular installation is properly completed? The Underwriters'
Laboratories issues individually registered and numbered master labels on satisfactory
systems. The inspections on which issuance of these labels are based include the
materials used as well as the completed installation itself. For those interested
in safeguarding their properties, no over-all system without such a label should
be considered. Also, the presence of this label is often a determining factor in
insurance savings.
A protection system is not always as simple as is often thought. A single lightning
rod, for example, does not necessarily do the job for a single structure, even where
the former is located in the best spot and properly grounded. There are too many
factors for an inexperienced or untrained amateur to take into account. Structures
adjacent to the one to be protected must also be evaluated. For example, an unprotected
tree next to a house that has a system may be a weak spot. Later changes in the
structure must also be taken into account. Periodic inspections should be made to
check on the condition of the system. Clearly a professional hand is needed here.
For the TV service dealer, an exceptional opportunity is open in this field.
One insurance company estimates that three quarters of the nation's rural property
needs such services. Add to this the rapid rise in private suburban dwellings. Systems
manufacturers, simply unable to exploit the potential through existing facilities,
are looking for new dealers. At least one such firm, the Independent Protection
Company of Goshen, Indiana, has been alert enough to see the manner in which this
type of work complements that of the TV service dealer. Robert Cripe, IPC vice-president,
points out that equipment used is very much the same as that required in antenna
work. His firm welcomes TV service dealers, whom it will train and assist. With
TV service work slowing down in the summer months, when interest in lightning protection
is high, there is a built-in balancing factor. In addition, the sale of an antenna
is an open door to suggesting lightning protection, or vice versa.
For anyone who lives where lightning. may strike, the ounce of prevention is
available.
Lightning Protection Articles on RF Cafe
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