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How to "Arrest" Lightning
May 1955 Popular Electronics

May 1955 Popular Electronics

May 1955 Popular Electronics Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

Before the advent of electrical distribution systems and electronics, the hazard of lightning was primarily from fire ignition and, to a lesser extent, bodily injury (to humans and animals). In fact, it was Benjamin Franklin's discovery that lightning was a form of electricity that led to his subsequent invention of the lightning rod system that, after being installed on Philadelphia's tallest wooden structures, significantly reduced the incident of lightning-related devastating fires which had been ravaging the city for years. Once cities began installing electric power lines, they were to lightning what trailer homes* are to tornadoes - strong attractors. Line protection systems were soon developed to help stop strikes which sometimes caused electrocution to people inside homes who happened to be touching a light switch or plumbing fixture (back when metal pipes were used everywhere). Then came electronics and electrical appliances which could not withstand a high voltage surge and burned out as the result of a lightning strike. Ungrounded television antennas were notorious for attracting bolts of lightning and frying the innards. The TV antenna on my parents' house never had a grounding wire so we must have lucked out since the Annapolis, Maryland area experienced a lot of lightning storms in the summer.

* I have owned and lived in trailers twice in the last four decades, once in Vermont when attending UVM, and again in south central Minnesota, a major tornado region.

How to "Arrest" Lightning

By Elbert Robberson

Map  days each year severe thunderstorms and lightning - RF Cafe

This map will give the reader an approximate idea of how many days each year severe thunderstorms and lightning may be expected. San Francisco readers generally have only 4 or 5 severe storms each year, but readers in New Orleans will experience 70 or more lightning storms.

Experimental layout for demonstrating the safety factor of lightning arresters - RF Cafe

Experimental layout for demonstrating the safety factor of lightning arresters.

Examples of TV lead-in wires shown above have been attached to antennas struck by lightnin - RF Cafe

Examples of TV lead-in wires shown above have been attached to antennas struck by lightning. On the left is a short piece of tubular TV lead-in. All that is left is the tube since the copper wire has vaporized. On the right a flat-type TV lead has suffered the same fate, although the copper vapor punctured the wall.

Construction of a typical TV lightning arrester - RF Cafe

Construction of a typical TV lightning arrester. The metal strap connects to a ground pipe or rod. On top of the strap is a piece of insulation. The TV lead wires are speared by the points of the gap electrodes.

steps required for safe installation of a lightning arrester - RF Cafe

These are the steps required for safe installation of a lightning arrester. In this photo a pipe is being driven into the ground close to the building where the lead-in will enter the window frame. The photo is for demonstration purposes only, as obviously it is safer to do the hammering before clamping on the arrester.

TV lead-in at the right is placed in the arrester - RF Cafe

The TV lead-in at the right is placed in the arrester so that the electrode points pierce the strands of wire. Secure the lead-in with the cap of the arrester.

Lightning choke, which adds to the safety of the installation - RF Cafe

A lightning choke, which adds to the safety of the installation, is easy to make. Loop the lead-in twice on the "protected" side of the arrester and secure with cellophane "Scotch" tape. The lead-in should now enter the building, preferably at a right angle.

Ninety times a year, lightning hits somewhere around Tampa, Florida. But wherever a person lives, he begins to wonder about his antenna when one of those storm clouds begin spit fire.

I have watched my own antenna during a lot of thunderstorms and it looks like it keeps getting higher and higher, the closer the thunder rolls. When the bolts start hitting close, the antenna looks as if it's the highest place around for miles, and it seems to be begging for an electronic visitor.

"Look, now," I tell myself, "with a lightning arrester or a grounding switch, an antenna is perfectly safe during a thunderstorm. Stop worrying!"

Then I answer myself right back. "That may be true," I say, "but does the lightning know it?"

Lightning does know about lightning arresters. If one is installed properly, the bolts from the blue will obey the natural law and high-tail it to ground without touching the equipment. But if there has been a slip-up somewhere, a whole basket full of arresters is not going to provide protection.

First, let's prove that lightning is educated to know how it should act around an arrester. A simple experiment which can be hooked up on a workbench will show the principles involved. If it is tried before the installation of any lightning arresters, the constructor will be more likely to do the job right because he will know the principles as well as the lightning does.

A miniature lightning bolt could be made with the ignition coil from a car. It bangs out a shot of high voltage that will leap almost a half inch through the air. To do this, use the hookup shown in the diagram.

The electrode connected to the high-tension coil terminal takes the place of the charged cloud in a thunderstorm. The terminal directly beneath stands for the antenna. The additional gap directly underneath goes to ground, and is the lightning arrester. The coil off to the side represents the lead-in wire going inside the house to the equipment, while the extra spark gap with the close spacing is what I would call an "air-gap voltmeter" because high voltage at this point will cause a spark to appear.

Press the key, then release it quickly. On the release, a spark should jump from the "cloud" to the "antenna" then across the "lightning-arrester" spark gap to ground. No spark will show at the "air-gap voltmeter," even if this gap is somewhat shorter than the one in the "lightning arrester." Something has made shorter this gap, which would be thought to be the "path of least resistance ", actually a path of highest resistance. So the bolt is discharged across the lightning-arrester gap to ground, without ever getting inside the house.

The "something" which has made the path inside the house a high-resistance route is the inductance of the wiring and the equipment. Inductance has the electrical property of reactance and this looks just like an "open circuit" to the fast-travelling front of the lightning bolt. The more curly and roundabout the wiring is, the greater the reactance.

Likewise, something has made the path through the lightning arrester to ground, even across a longer gap, look like the path of least resistance to the lightning. This "something" is the lack of inductance. The straighter and more direct this path to ground can be made the easier it will be for lightning to get through.

There are over twenty different lightning arresters on the market, but all of them depend on these principles for operation. Most arresters are simply a spark gap in a convenient package, although there have been models that use a neon tube which "fires" at about eighty volts, instead of an air gap. The idea behind this is that the arrester path to ground will break down at a lower voltage than an air gap, and so will have still less resistance to the lightning bolts.

One arrester should work as well as another if it is installed right. Suppose we want to protect a TV antenna. First of all, the pipe supporting the antenna elements should be grounded with a wire of 10-gauge or heavier. And no matter how many protective devices there are down below, do not omit this heavy ground wire. If a lightning bolt should decide to come down the antenna, and its only path is a comparatively flimsy "twinlead," the bolt might very easily melt the small wire, then jump to the nearest other object around. In this process, it could blow a hole in the house. Be safe: put a heavy ground on the support. Aluminum, copper, or copper-clad steel wire can be used.

Make the ground "run" straight and direct, with no sharp turns or bends. At the bottom, the wire should connect to a water pipe if one is handy-but if the nearest one is around the corner somewhere, forget it.

In this case, make a "ground" by driving a pipe or ground rod four or five feet into the earth. Connect the ground wire to the pipe with a clamp. If a water pipe is used make sure it goes into the ground right now and does not wander around the cellar before connecting up with mother earth. A pipe that goes through the house before reaching ground can induce voltages in the thousands in any nearby metal - very undesirable in the coal bin! This is not impossible - remember how the transformer works; a current in one wire induces a current in another one alongside. Well, this principle works quite well outside transformers, too. A high current in the ground conductor can induce voltages in other nearby pipes or wires. Very messy! For the same reason, keep any other wires at some distance (and this means feet) from the ground wire down the side of the building.

Many TV antennas have the center of the dipole connected to the support pipe. This automatically protects the antenna cable at this point. If the antenna is not grounded to its support, the transmission line will last longer if an outdoor type of arrester is installed at the very top of the line, grounding it to the support pipe. This will help keep heavy current in its place - the ground lead.

Whether or not an arrester is put at the top of the line, an arrester must be installed at the very bottom, where the lead enters the house. Just be sure that the connection to ground is direct and solid. Most arresters are made with terminal teeth that automatically make the proper transmission-line connection when the wire ribbon is clamped in place. It is not necessary to skin off the insulation.

The protection can be made even more certain by winding about a foot of the lead-in wire on the equipment side of the arrester into a 2- or 3 -turn "lightning choke." This coil will greatly increase the reactance of the lead-in to lightning strokes; the signal will not be affected except on the higher-frequency channels. Many commercial and military stations use lightning chokes in their lead-ins.

Some arresters are advertised as having a "static" drain. All this amounts to is a high-resistance ground which is permanently connected to the antenna circuit. The idea is that any build-up of electrons from wind friction or induction will leak off instead of growing to the danger point.

Now, what happens when lightning strikes the arrester? It is certain that the sparking electrodes will melt. If there is insulation in the gap, this material will char or vaporize. Any moisture inside will turn to steam. This will happen fast.

So if the arrester is a type which is sealed, without large exit holes for the large cloud of gas that is going to be generated in a hurry, it is quite likely to blow up. Install such arresters so flying parts won't damage anything nearby.

With practically any kind a new arrester will be needed if lightning strikes. Be thankful that this one was put in right, and shell out for a new one right away, because lightning can strike twice in the same place.

A person can keep it from raining by wearing a raincoat - maybe installing an arrester will do the same for lightning.

Further information on lightning and lightning arresters may be obtained from the U. S. Department of Commerce Handbook 46, Code for Protection Against Lightning obtainable from the Superintendent of Documents, Washington 25, D. C. The latest price is listed at 40 cents. Somewhat similar booklets are also available from various safety agencies and insurance companies.

Editor's Note: The following concerns manufactured lightning arresters. Our readers are referred to their nearest TV serviceman or radio parts jobber and distributor for information on prices, installation, etc.

Amphenol, Birnbach, ICA, JFD, Radelco, RCA, RMS, Radion, Telco and Ward.

 

 

Posted August 14, 2019

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