LIGHTNING!
  Did I get your attention?   Can we survive lightning?  Did you ever wonder how F.M. Radio & T.V. stations stay on the air through thunder storms?  They do, as a rule, of course when the lightning causes short power failures, they do GO OFF briefly. (long power failures start the emergency generator)  It is rare for anything at a broadcast transmitter building to be damaged by a lightning storm.  There are several combined features that make stations "Lightning proof" so to speak.

  The transmitter buildings are usually placed 50 to 100 feet from the tower, because the horizontal transmission line flexes up & down, because the vertical copper and aluminum transmission lines (up the tower) expand much more than the steel tower when the sun strikes them (and from "R.F. loss" heating) in the morning, after cooling down overnight.

  Although steel is a poor conductor of electricity, compared to GOOD conductors like aluminum or copper (or silver,) the "tall towers," 1,000. feet or more tall, are made of three to six inch or more thick steel or iron legs.  This results in a high current carrying capacity.

  When a company pays from a quarter to three quarters of a million dollars or more for a tower (installed) they don't mind paying to drive two 30 to 60 foot long copper (or copper clad steel) ground rods for EACH of the three tower legs and each guy point and 1/2" or thicker copper wire to connect the six ground rods to the tower legs.  Normally using "cadwelds" to make the connections.

  The stations know (from experience) that all of the grounds and grounded items in and around their building must be bonded together like one big ground rod, using conductors with enough current capacity for the "stroke current *."  (frequently a 4" wide copper strap.)

  The antenna is never the top item on the tower, in addition to the top tower light, there are some blunt lightning rods, above everything else, designed to be hit by lightning.

  Lately towers are being topped with "STATIC DISSIPATERS" (stainless steel brushes, wreaths or barbed wire umbrellas with lots of sharply pointed wires) which PREVENT the lightning strokes!  They change the value and distribution of the electrostatic charge on the tower and the nature of "multiple small sharp points" ionizes the air above the tower in a low current mode, discharging the cloud slowly.  Discharging the cloud eliminates almost 100% of the tower strikes.

  The results:
     The heavy tower grounding allows the mass of the tower to be pre-charged with electrons or holes (depending on the cloud's charge polarity.) When a strike occurs most of the electrons (or holes) needed are in the tower's metal and the ground current is minimized.

     The high tower to cloud current and low ground to tower current keeps the resulting VERY HIGH magnetic field (which kills I.C.'s) in the area of the tower, away from the equipment in the building.  The reduced ground current results in the different ground connections around the building (building power, telephone system, water supply/well, and the transmission line/s connected o the tower) to have less potential difference, which cause less currents between them (through the equipment connections) and less equipment damage.

*    *    LIGHTNING FACTS    *    *
  Our towers come into greater peril when they become taller than the nearby power poles' "static wire" or aren't close too such protection.

  A lightning stroke is often preceded by an air ionizing "leader," this is what knocks down golfers (often leaving permanent injury) without killing them.  The electron flow in the leader goes in the same direction as the stroke that MAY follow and the electron flow goes from the ground to the cloud about 60% of the time.  The ionization starts at both ends & work's it's way to the center, the thinner air at the cloud becomes luminous first (at a lower current) making it LOOK like lightning always goes down.

 The leader electrically breaks down a "path," ionizing a column of air molecules from sharp point/s on the ground, up to the charge in the cloud.  Once the leader current flows the column of ionized air can and will be blown by the wind (if any) to be over lower conducting objects like people and houses.  This is why lightning struck the side of a brick smoke stack, knocking a six foot diameter hole through one side, right between two 750 MCM bare grounding cables going up to lightning spikes on top.

  The conclusion:
  We need to have a blunt tip or a static dissipating "brush" as the top most item on the tower with a REALLY HEAVY connection to the tower grounds.  We should file down, cut off, or cover with weather proof insulation ALL sharp points and edges.  (tips of thin elements, hose band edges, "U" bolts, clamp plates etc. with plastic caps, BLACK silicone sealer etc.)

  We need to ground our towers, (NOT antennas on the roof!) REALLY GROUND THEM!  Drive or jet down two or more 3/4" or larger galvanized water pipes or coupled together ground rods 20' or longer.  (right at the base of the tower, perhaps one on each side).

  We need to bond all of our grounds together with short direct heavy wire AWG #4 or heavier, if possible.  Whether WE put them there or not!  The R.F. nature of lightning (2 micro-second rise time) requires us to use thick short bonding cables with as direct paths as possible.  (Slow sweeping curves, NO corners!) Perhaps you can find used welding cable cheap.  (with rotten rubber?)

  Test our old grounds, clamp an INSULATED AWG #12 (or heavier) wire from a ground rod with everything disconnected from it, directly to a 20 Amp. breaker (without any other load on it,) if it trips in less than a second (when you turn on the breaker) it should be a good ground.  (if not, not)  8 & 10' ground rod are usually useless.

  Many people say "Tampa is the LIGHTNING CAPITAL OF THE WORLD."  We REALLY are on the southern edge of corridor from the northern Tampa area to around the Brooksville area that extends across the state in a band to just south of the Jacksonville area.  This area has the second highest yearly "stroke count" on earth, the peninsula of India is #1, Western Washington state is #3.  The Tampa area is definitely a "hot" place to live, in more ways than one!

* The "stroke current" can range from less than 1/2 amp. from a "leader" or a nearby hit, (induced through magnetic lines of force) through 10,000. to 200,000. amps for about 8 milliseconds per direct hit, which can be a series of 1 to 12 or more in less than a second.  Watch lightning from a porch and you will see a surprising amount of multiple strokes.

  I have only covered the tower aspect of lightning damage.  We suffer major equipment damage from strokes that strike the power and telephone lines that (eventually) enter our homes.  Even a direct strike to a tree near a telephone line (in your neighborhood) can kill your computer, (even if it's turned off) if your telephone line is connected to your computers MODEM.  Keep your modem's PHONE LINE unplugged when you are not actually using it.  Did I mention the cable T.V.'s ground . . .

Ron KA4INM  ka4inm@qsl.net
(C) 1993 Ronald M. Youvan