LIGHTNING PROTECTION

for BROADCASTING STATIONS

by Phillip R Tompson BE(Hons) CPEng MIE(Aust) MIEE MIEEE NOVARIS PTY LTD

Abstract - Broadcasting transmitting stations and Thunderday maps are published by meteorological indeed all high power MF, HF and VHF transmitter organizations worldwide. As may be expected the sites are particularly prone to strikes and number of thunderdays is generally greatest subsequent damage. This paper examines the reasons for this phenomenon and discusses protection in tropical regions around the equator and falls off as techniques which may be applied to all high power one progresses north and south towards the poles. transmitting sites whether they are used for civilian broadcasting or military communications. Another commonly used statistic to record lightning activity is the “Lightning flash density”. This is defined as the number of lightning flashes to ground occurring on or over unit area in unit time. This is commonly expressed as per square kilometer per year -2 -1 INTRODUCTION (km year ).

It is not difficult to understand why broadcasting As may be expected there is a relationship between stations are so prone to lightning strikes. Medium thunderdays and ground flash density. Figure 1, frequency stations, MF, consist of tall, slender vertical reproduced from BS6651 shows this relationship. radiators generally located in a flat, often swampy area. The radiating mast is therefore highly prominent and being the tallest structure around, will be highly Thunderdays Mean flashes per sq km susceptible to receiving direct lightning strikes. per year per year Ng

High frequency stations, whether used for shortwave broadcasting or military communications 5 0.2 generally consist of vast farms with numerous 10 0.5 antennas supported by tall masts. It is again easy to see 20 1.1 why these structures are frequently struck by lightning. 30 1.9

Very high frequency, VHF, stations whether 40 2.8 broadcasting or FM are located on 50 3.7 mountain tops and other prominent sites. 60 4.7 80 6.9 In addition to direct strikes to transmitting 100 9.2 structures, strikes both direct and induced to power lines feeding transmitters must also be considered. Source: BS6651

Fig 1. Thunderdays vs Ground flash density

STRIKE INCIDENCE To assess the susceptibility of transmitting There are two common statistics used to measure structures to lightning, the number of likely strikes per the incidence of lightning strikes. The first is the term annum can be readily calculated. The attractive radius “thunderday”. This term is defined as a calender day of tall, slender structures of height greater than 60m, during which thunder is heard at a given location. The can be found by use of an equation for Ra given by international definition of lightning activity is given as Ericsson in reference 3. the number of thunderdays per year. This is also called the “isoceraunic level”.

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0.64 (0.66 + 2I x 0.0001) Ra = I x h potential is essentially caused by the self of the tower. Block in ref 6 presents a formula for where approximating the inductance of a typical slender tower. The self inductance of a 100m tower is 77 Ra = the attractive radius for the structure, in microhenries. meters I = the prospective lightning stroke current The potential at the top of the tower may be , in kiloamperes calculated from the following formula: h = the structure height, in meters V = L x dI/dt

Using the above equation, a transmitting mast with where a height of 100m and a prospective lightning stroke current of 50KA, has an attractive radius of 267m. L = Self inductance in microhenries dI/dt = Rate of rise of current in amps per The collection area is then given by: microsecond

2 -6 For a 50KA current rising in 1 microsecond, the Ac = π x Ra x 10 potential at the top of the tower will be approximately where 3.8MV.

Ac = the collection area for the structure, in square kilometers.

A 100m high transmitting mast will have a collection area of 0.224 km2.

Finally the prospective number of strikes per annum can be calculated from:

P = Ac x Ng where Fig 2. Self inductance of 100m slender tower P = the prospective number of strikes per annum -2 -1 Ng = the ground flash density km year

Earth Potential Rise In an area with 80 thunderdays, the mean ground flash density is 6.9. So a 100m high transmitting tower As the current pulse flows to ground a rise in will receive on average 3 direct strikes every 2 years. ground potential will also occur. By ignoring the effects of inductance and considering resistance of the alone this can be easily calculated. PROTECTION PRINCIPLES For example a 50KA impulse flowing to ground Direct Strike with a 10 ohm earth resistance will raise the earth potential by 500KV. When lightning strikes a tower that is either directly grounded or grounded via a spark gap arrester Since the local ground potential rises, any cables the current pulse, which typically may have a rise time leaving the vicinity of the tower will carry this of 1 microsecond and a decay to half amplitude of 50 potential to the transmitter building. Current will flow microseconds, will flow down the tower to ground. along coaxial cable sheaths and create a potential between the inner and outer conductors of these It is important to be aware that no matter what cables. form the lightning protection takes there will be a potential gradient developed up the tower. This

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At many stations the transmitting tower is often located some distance from the transmitter building so it cannot even be assumed that both the tower and building earth will rise to the same potential.

Surge Protection

Whether the tower is struck by lightning or lightning strikes the incoming power line, surge protection on all incoming services is essential. The aim is to reference all incoming services to the local ground either directly or via surge diverting components such as metal oxide varistors, spark gaps etc.

Fig. 3 Spark gap incorporating Jacob’s ladder DIRECT STRIKE PROTECTION Typical spark gap dimensions may be set depending upon transmitter power, base impedance A direct strike to a transmitting tower is unlikely to and altitude. Figure 4 below from ref. 7 shows the damage antennas unless the antenna itself is struck or relationship between breakdown and spark gap correct earthing and bonding principles have not been for various gap geometries. adhered to. The peak RF antenna voltage is given by: Antennas which form the highest point of the structure and are not at tower potential are particularly Vpeak = 2.83 x Za x Ia vulnerable and it is difficult to protect these effectively. High gain whip antennas mounted at tower where top are typical examples. The best form of protection is to carry some spares. Vpeak = peak antenna voltage Za = antenna base impedance, ohms Where antennas are mounted on the lower faces of Ia = antenna current, amps RMS the tower, it is usual to erect a vertical spike, or Franklin rod, at the top of the tower to act as the air Whilst it is preferable to minimise the spark gap, terminal. To be effective the top of the rod needs to be dimensions below 5mm are impractical where a build at least 3 metres above the highest point of the up dirt etc may cause the gap to arc over with the antenna. presence of RF power alone. Furthermore the gap must be sufficient to allow the arc to extinguish once No special precautions with regard to triggered by the lightning strike. downconductors on all steel towers are necessary. The four legs of a self supporting tower provide an At MF sites where a single feed wire connects the excellent path for the lightning impulse current. antenna to a tuning hut, a common method of reducing Special air terminals and proprietary downconductors lightning current in the wire is to form it consisting of custom made coaxial cables etc are into one or more loops to produce a low but finite totally unnecessary. They do nothing to reduce the series inductance. potential rise at the top of the tower and cannot possibly be insulated to the level required to prevent HF systems incorporating balanced feeders to high flashover to the tower itself. power baluns employ spark gap protection across the balun terminals. MF and HF antennas of which the mast itself is the radiating structure pose a special problem. When the mast is mounted on a base it is usual to provide a spark gap across the insulator to conduct the lightning to impulse to ground. The spark gap may consist of either spheres, points or a Jacob’s ladder to assist in extinguishing the arc. A typical spark gap with Jacob’s ladder is shown in figure 3.

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Figure 5. Earthing and bonding points

Fig. 4 Spark gap dimensions BUILDING LAYOUT

The geometry of the interconnections in and around the transmitter building are of vital significance EARTHING AND BONDING to the effectiveness of the lightning protection system. The objective is to provide a path for the potentially Since a direct strike to a transmitting tower will destructive lightning current flowing from the antenna raise earth potentials and cause current to flow in to the AC supply line via a path that does not include feeders and coaxial cable sheaths, it is essential to pay the interior of the building. particular attention to correct earthing and bonding practices. The ideal building layout would be one where the coaxial feeders, AC supply and other services enter the 1. Ensure that the antenna system is securely building at one point. At this point all services are bonded to the tower structure. connected to the building ground either directly in the case of coaxial cable sheaths, water pipes etc or via 2. Bond the sheath of the feeder cable to the tower surge protection devices in the case of AC power, structure at the antenna. telephone, programme line etc.

3. Bond the sheath of the coaxial feeder to the It is impossible to reduce earth resistances to zero tower structure at the point where it leaves the so there will always be earth potential rises developed tower. Do this just prior to the bend in the feeder. when lightning strikes. By carrying out the above procedures a single earth will be produced such that an 4. Ensure that the tower is securely earthed. For a equipotential earth rise will occur and current flow in 50KA lightning impulse every one ohm reduction the station through vital equipment will be eliminated. in earth resistance will reduce the earth potential rise by 50KV.

5. Bond the sheath of the cable to the station SURGE PROTECTION ground at the point of entry to the transmitter building. AC Power

The AC supply line to the transmitter building usually represents the lowest impedance to remote grounds and will therefore carry much of the lightning current flowing away from the site. The surge

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protection installed must thus have sufficient capacity This is not always reliable if the gas device is to carry these currents. installed at the end of a long feeder and the arcing path has some finite impedance. This situation is altogether different from the case of induced in power lines for which many surge protection devices are designed.

It is usual to choose mains power filters in preference to connected surge diverters. The filters provide multistage protection and redundancy in the event of a component failure. Figure 6 shows the configuration of a typical mains power filter. Fig 7. Gas arrester coaxial protector

L1 A recent development in the protection of high E1 power broadcasting transmitters has been the development of a coaxial cable spark gap GND GND incorporating an optical sensor to detect an arc. Once the arc is detected, an optical fibre the L2 E2 information to the transmitter and can initiate a brief interruption to transmission to allow the arc to MOV s LOW PASS FILTER extinguish.

Fig 6. Mains power filter The spark gap can be made very small as it no longer has to be wide enough to extinguish the arc so Generally three phase versions with surge ratings firing voltages are low and equipment is protected. no less that 70KA would be employed. Current ratings Such a device with an adjustable spark gap is shown in depend upon the station’s requirement but for high figure 8. This device is designed for 3 1/8” cable using power transmitters, 630 amps per phase at 220V is not EIA flange type connectors. uncommon.

These filters would be installed at the building point of entry of the AC .

Coaxial Cable Protection

At high frequencies the components used in power filters are unusable. Metal oxide varistors have a high self capacitance which shunt RF energy. The only useable device is the gas filled arrester which may be connected between the inner and outer sheaths of coaxial cables to clamp differential voltages.

Such devices are readily available for low power transmitters up to a few hundred watts. Fig 8. Adjustable spark gap protector for 3 1/8” coaxial

cable. At higher powers gas filled arresters also become unusable unless special precaution are taken to ensure that, once fired, they will not be held in a conductive These coaxial cable protectors are installed at the state by the RF energy. It is possible to utilise some building point of entry. types of arresters for high power use assuming that the transmitter will automatically shutdown once the arrester fires when its reverse power rises. Line Protection

Incoming programme lines and telephone lines also require protection at the point of entry. Typical

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protection devices that may be utilised include, gas arresters and multistage protection in configurations similar to figure 9.

L1 E1

GND GND

Fig 10. Mast guy static drain L2 E2

gas MOVs suppressor arrester diodes CONCLUSION

Fig 9. Multistage signal line protector The protection from lightning of high powered broadcasting stations poses some special problems. There is no doubt that broadcasting structures are STATIC PROTECTION highly prone to direct lightning strikes.

It has long been recognised that static voltages can With proper attention being paid to earthing, build up on the insulated guy wires of MF and HF bonding and correct station layout in addition to the transmitting masts, ref 8. This phenomenon can be correct choice of surge protection, effective lightning caused by the build up prior to a lightning protection can be provided. strike or can be caused by friction due to wind in very dry conditions. Even at sites where building layout is poor, ie where services enter from different parts of the Up until the advent of fully solid state transmitters building, where earthing is incomplete, it is still this problem caused little inconvenience. Transmitters possible to retrofit protection and earthing schemes employing valve power amplifier stages were robust which can provide effective lightning protection enough to withstand the momentary impedance mismatch as the guy insulators arced over discharging the static.

Modern solid state transmitters with sensitive reverse power detection actually shutdown under such conditions.

The solution is to provide a form of static drain across the insulators so that charge build up is slowly dissipated. A common approach is to connect inductors across the guys and across the base insulator in the tuning hut. One problem with this solution is the presence of unwanted .

An alternative approach is to connect high resistance elements across the guys to dissipate the charge. Such a device is shown in figure 10. It consists of a conductive material with a resistance of a few megohms concentric with a spark gap to carry the current from a lightning strike. These devices can be connected across guy insulators without the problems associated with .

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REFERENCES

1. NZS/AS1768-1991, Australian, New Zealand Standard “Lightning Protection”

2. BS6651, British Standard on Lightning Protection.

3. A.J Eriksson “An improved electrogeomagnetic model for transmission line shielding analysis” Trans. IEEE, July 1987, Vol. PWRD-2, No 3.

4. M. M. Frydenlund “Lightning Protection for People and Property”. Van Nostrand Reinhold.

5. J.L Norman Violette, Donald R. J. White, Michael F. Violette “Electromagnetic Compatibility Handbook”. Van Nostrand Reinhold.

6. Roger R Block “The Grounds for Lightning and EMP Protection” Polyphasor Corporation

7. “Reference Data for Radio Engineers” Howard W Sams & Co.

8. George H Brown “A Consideration of the Radio- Frequency Voltages Encountered by the Insulating Material of Broadcast Tower Antennas” Proc of I.R.E September 1939

9. Henry Jasik “Antenna Engineering Handbook” Mc Graw- Hill

Phillip R Tompson graduated from the University of Queensland with an honours degree in in 1972. His early experience was gained as a communications engineer with Telecom Australia, then with power utilities specialising in communications and control systems design and management. His work now involves consultancy in the field of lightning protection, power quality as well as product design work for surge and overvoltage protection products. He is a chartered member of IE(Aust), IEE, and IEEE as well as a member of the Australian Standards Committee on lightning protection.

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