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TV Antennas Invite Lightning to Strike
October 1960 Radio-Electronics

October 1960 Radio-Electronics

October 1960 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

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Photovoltaic (PV) system installation. This was the early stage before the battery backup and AC wiring was completed. Everything worked perfectly the first time it was turned on. I measure around 315 Vdc from the PV arrays on a sunny day.

The Lightning Protection Institute (LPI)has been around for a long time; in fact, it might have been informally founded by Benjamin Franklin ;-) Last year while working on an installation plan for a friend's solar panel array and equipment room, I found lots of useful info on the LPI website for best practices of lightning protection. I was surprised to learn that there was still disagreement among experts as to whether a raised, grounded surface like the metal roof onto which the photovoltaic (PV) arrays are mounted tended to reduce or increase the likelihood of a lightning strike. There are good theoretical arguments on both sides. I tend to think the raised, broad surface decreases the cloud-to-ground potential and disperses the charge across a larger area, thus reducing chance of a strike. There is also disagreement whether the ground for the PV array should be physically connected to the ground point for the inverters and AC distribution panel. Thus far there have been no problems, but luck plays a large part of it unless a full-scale mitigation system is implemented, and that requires a lot of expensive rods, bleeder lines, and ground stakes.

TV Antennas Invite Lightning to Strike

TV Antennas Invite Lightning to Strike, October 1960 Radio-Electronics - RF CafeBy Lightning Protection Institute

An increasing number of TV-related lightning fires occur each year. Most could be prevented if proper grounding techniques were used

If you're among the thousands who think that an ordinary television antenna atop the roof is safe from lightning, you're wrong, says the Lightning Protection Institute.

Studies of some 1,500 lightning-fire and damage losses, reported in newspapers during a 3-month period, reveal that upward of 4% of them involve television sets and their antennas. That's no small matter - $4,800,000 annually.

Those facts pointedly illustrate that an unprotected antenna actually invites lightning to strike. There are more than 40,000,000 television masts in this country, and it is inevitable that this forest of high-rising metal towers will help to increase the losses suffered from lightning strikes - unless the masts are protected.

Total annual lightning losses in the US are placed at 600 persons killed, more than 1,500 injured, along with $120,000,000 in property losses. Lightning is the biggest single cause of fires in rural and outlying suburban areas, being responsible for 37% of all such fires.

Tying the TV antenna into the home's lightning-protection system is one means of helping to reduce losses. Where a building does not have a complete lightning protection system, grounding the TV mast properly will only partially protect it. To ground a TV antenna adequately, in addition to a 10-foot ground rod, tie-in should be made to the water system, electrical and telephone grounds and any metallic bodies within 6 feet of the down conductor. Ordinary TV installations in themselves are inadequately grounded, and the antenna is not a lightning-protection device.

Lightning traveled "down" a TV lead-in and made a new entrance for this kitchen.

Basic Information

Lightning will most likely strike the highest object in an area. Because of its height above surrounding objects, the antenna becomes this most likely lightning target.

A TV antenna, even though grounded according to most set manufacturers' recommendations (and with a lightning arrester), does not offer sure lightning protection for the set. It is not connected with a heavy enough cable nor does it provide the proper paths to earth to function as an efficient lightning rod.

Most antennas, contrary to popular belief, do not have lightning arresters. And an arrester is not full protection in itself - the mast must also be tied in with the correct conductor cable to a ground rod sunk 10 feet deep in moist earth.

When lightning does hit an ungrounded antenna, its path is most likely to be down through the house. It may leap over to the wiring system or water pipes, or follow the antenna lead. At the least, you can expect damage to whatever mechanical equipment the bolt, passes through (Many TV sets have ended in a ruined tangle of wires) and at the worst, injury or death to occupants.

Scores of lightning losses involving TV antennas lead the Lightning Protection Institute to endorse strongly this requirement in the Underwriters' Laboratories code:

"Radio and television masts of metal, regardless of location on building, shall be bonded with standard conductor and fittings to the main conductor of the lightning protective system. It is also recommended that a lightning arrester be installed on the lead-in wire, tape, or cable."

Where a television antenna is installed on a building provided with lightning protection (a lightning-rod system), the metal mast or supporting tower should be connected to the system. If the tower rises appreciably above the building, it should also have a down conductor connecting it directly to ground to conform to National Bureau of Standards requirements.

Mast should be grounded to copper rod sunk into moist earth.

Alternate Method

Where a building is not provided with a complete lightning protection system, and safety with economy is the watchword, the institute advises full protection of the TV antenna and set. This will partially protect the building proper. This is recommended on the premise that some protection is better than no protection at all.

If the TV antenna is centrally located, grounding the mast can be tied in with complete lightning protection for the home by continuing the cable in both directions (and adding terminals at 20-foot intervals if the building is 40 feet or longer) from the TV mast along the roof ridge and down to ground on two sides of the building. This is not a job for the usual TV installation man. Considering the danger to life and property, the importance of trained engineering skill becomes even more vital. Don't hesitate to seek professional help or advice in all instances, whether for protecting a TV set, and antenna or for a complete lightning-protection system. However, if the antenna is at either end of the building, the TV mast alone is comparatively simple.

The drawing illustrates how the lightning arrester is connected to the TV lead-in and then fastened to the mast with an approved pipe clamp (which generally comes with the arrester). Many good arresters are on the market; the important thing to watch for is the Underwriters' Laboratories label.

The drawing also illustrates how the ground cable is connected to the TV mast, using a UL-approved cable clamp.

The ground cable is run from the connection at the mast, along the roof ridge (hidden by placement along ridge rolls), down the gable side (hidden under eave), then down the building side.

There are several rules governing cable installation. The cable shall have no bends sharper than 90°. The radii of all curves should be at least 8 inches. Lightning, with its sudden rise time, will not travel through a cable that has sharp bends if it can find a shorter path to ground (a jump across several feet to a water pipe is not uncommon). The cable is to be supported every 3 feet on open-air runs, and maintain a horizontal or downward course, free from down and up (V or U) pockets.

The cable is connected to an approved ground rod with a standard ground-rod clamp. The ground rod is driven 10 feet into permanently moist soil, at least 2 feet away from the house foundation.

Standard Materials

Copper and aluminum are acceptable materials for lightning-protection systems, but should never be used together without special connectors. Aluminum parts must be larger than equivalent copper parts for equal conductivity.

The copper cable should weigh at least 187.5 pounds per 1,000 feet (individual wires making up the cable should be at least of No. 17 gauge). This cable is three times the size of a No.6 cable which most manufacturers and installers recommend. Aluminum cable should weigh at least 95 pounds for the same length.

Ground rods, solid copper or copper-clad, should be at least 1/2 inch in diameter and 10 feet long.

Connectors and fasteners must be of the same material as the cable. Galvanized or plated-steel nails, screws or bolts are not acceptable on copper or aluminum installations.

Service Available

The Lightning Protection Institute, a non-profit Chicago organization, offers its help whenever it is needed by anyone - home owners, plant engineers, organizations or individuals. Sponsored by reputable, long-established lightning-equipment manufacturers, the institute was set up to promote the science and improve the methods of lightning protection (and to advise and give information) .

Another of its purposes is to formulate and press for the practice of ethical standards in all phases of the lightning-protection industry. In this role, it works toward the goal of full inspection and official approval of all lightning-protection systems, new or old.

While the institute does not take any part in actual equipment testing or field inspections of installers and their work, the group works closely with Underwriters' Laboratories.

If you plan to protect your TV antenna and set, entire home or plant against lightning and there is any question regarding materials or installation, the best way to get the answer is to write to the Lightning Protection Institute, 53 W. Jackson Blvd., Chicago 4, Illinois, for Lightning Facts and Figures.

 

 

Posted July 5, 2024

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