I found this on one of the guy wire anchor points for a 400 foot tower:
#2 solid copper wire burned open by lightning strike
Had to be a pretty big hit to burn open a #2 wire. This is on one of six guy anchor points for the tower. The ground wire is U bolted to each guy wire before the turnbuckle and then goes to ground. This was noted between the last guy wire and the ground rod.
It is important to find and fix these things, as the next lightning strike on this tower would have a less than ideal path to ground at the guy anchor points, forcing the current to flow through other parts of the transmitter site, possibly through the transmitter itself, to ground.
I generally try to do a brief inspection of towers, guy anchors, lighting, painting and a general walk around the property twice a year. That helps prevent surprises like “Oh my goodness, the guy wires are rusting through,” or “Hey, did you know there is an illegal “hemp” farm on your property?” Well, no officer, I don’t know anything about that…
By now, you have heard of the great solar flare of 1859 that set telegraph wires afire across the US and Europe. This phenomena was due to a large electro-magnetic pulse from the earth’s magnetosphere interacting with the particles from a Coronal Mass Ejection (CME) caused by the solar flare. The long stretches of wire suspended above the earth acted like a large generator winding cutting through the magnetic field, which in turn caused voltage (Electro-magnetic force or EMF), which in turn, caused the fires.
In 1859 our understanding of electricity and magnetics is not what it is today, thus, the fires were likely caused by over loaded conductors with out means to bypass excess electrical energy to ground.
Schematic of Earth's magnetosphere, courtesy NASA
Fast forward to today. We are in the upswing of solar cycle 24, which is expected to peak in May of 2013. NASA predicts that this will be lowest sun spot peak since 1907. That does not necessarily mean the coast is clear. Over the last 11 years since solar cycle 23 peaked, computers and electronic automation have proliferated exponentially, becoming the norm. Things like GPS not only guide clueless travelers where to turn, but also sync up all those cellular telephone transmitters with timing signals. IP networks, SCADA, telephone networks and so on all run on some form of CPU. Newer Energy Star appliances like toasters and refrigerators also have CPUs. That technology has yet to experience a large electro-magnetic pulse (EMP) in the real world. Things could indeed get hairy if a moderate to large class X solar flare generated a CME that was polarized correctly to interact with Earth’s magnetic field and cause damage at ground level.
Solar flares and CME are slow moving events, with 1-2 days warning before the effects of a CME reach earth. One can stay apprised of solar flares and other solar activity by subscribing to NOAA Space Weather Prediction Center’s email service.
HEMP Mechanism for 400 kM high altitude burst
Of greater concern is other sources of EMP like high altitude nuclear explosions (HEMP). Those types of events, while rare, can happen. The good news is, the defense mechanisms for solar flare, high altitude nuclear burst and lightning induced EMP are the same. They are effective grounding, shielding, filtering and surge suppression.1 Of the three EMP scenarios, high altitude nuclear burst has the tightest design spec, so creating a building that incorporates the ideas in MIL-STD-188-125-1 specifications is a good start.
The question becomes, is all of this really necessary. It depends on how important it is to the radio station ownership to remain on the air during such an event. Based on historical information and global geopolitics, the probabilities of such an occurrence are:
Lightning strike – 1:1 Any radio station that has a tall structure, particularly a steel tower, will get struck by lightning, perhaps several times per year depending on the region.
Large Class X solar flare resulting in damaging CME – 1:21 Since 1859, there have been seven solar flare events that have disrupted communications or power systems on earth. This is a bit misleading since 6 of the 7 events have occurred in the last 22 years, making the real probability more like 1:3.6
High Altitude EMP – 1:30 Based on seventeen high altitude tests carried out by the US and USSR in 1962, the growing nuclear proliferation and a June 2005 Reuters article “Experts warn of substantial risk of WMD attack” in which the author stipulates a thirty percent risk of a nuclear attack of any type in the period of 2010 to 2015.
EMP Theory
High altitude nuclear burst EMP has three components; the fast component (20/550 ns pulse) is an electromagnetic shock-wave, the medium-speed component (1.5/5000 μs pulse), and the slow component (0.2/25 s pulse) resulting from the expansion of the explosion’s fireball in the Earth’s magnetic field.1 Compare that to a lightning strike, which typically has a 1.8 µs rise time. That means the first pulse frequency is from about 72 to 200 MHz, second pulse frequency is from about 800 Hz to 2.5 MHz and third pulse is basically DC and effects mostly long wires. Thus, any shielding, grounding and suppression needs to consider the highest frequency down to about 10 KHz.2
The rise time to frequency comparison is an important consideration in the design and construction of grounding systems. Grounds need to present the least amount of inductance possible. This means using solid, not stranded wire, keeping bends to a minimum and where required, use long radiuses. Bond all junctions by welding, exothermic welding or soldering. Use a single point ground system. Zone grounding should also be employed. The definition of zone grounding is concentric grounding areas connected to each other by a single low inductance ground conductor.2 The idea of isolating grounds by use of a ferrite suppressor may seem odd, however, if there is a separate RF ground, tied together with the building ground using wide copper strap laid on the floor, this will minimize ground system “reception” of incoming EMP.2,3
Zone grounding diagram
Shielding means surrounding the protected equipment with a conductive material such as copper plate, aluminum plate, copper mesh, aluminum mesh, brass mesh, steel plate or steal mesh.1,2 There are advantages and disadvantages to each. Seams should be welded to prevent “leaks.” Doors need to have finger stock or metal compression gaskets to ensure proper sealing of opening. Other openings for ventilation, cable ingress, etc. need to be “significantly” less than one wave length. MIL-STD-188-1 gives the allowable opening size of 10 x 10 cm or 3.9 x 3.9 inches. If openings in the shield become greater than approximately wavelength/6 meters (at 250 Mhz, about 8 inches), significant fields can penetrate to the interior.2
Suppression deals with those connections to the areas outside of the facility. These include incoming electrical service, data service and RF transmission lines to and from antennas. Since the fast component of the HEMP falls within the VHF spectrum, FM broadcast installations are particularly vulnerable. Suppression devices for incoming AC power are readily available from commercial sources and are well proven. LEA makes a series surge suppressor that uses a combination of fast acting silicone diodes, MOVs and an LC filter made up of series inductance and parallel capacitance to ground. The LEA DYNA family series surge protectors have a system response time of less than one nano second and are tested to greater than 1,000 operations.4 The response time depends on a good, non-inductive ground connection.
LEA DYNA systems series surge protector
Suppression for incoming RF and data cables is more difficult because the normal operating frequencies fall within the HEMP rise time frequency. Incoming data at a transmitter site usually consists of a DS-1 circuit however, larger capacity circuits are sometimes used. Fiber optic cables are immune from HEMP as they have no metal conductors. Copper data lines must have a data line surge suppressor between the TELCO demark and the CSU.
RF cables must have their shields ground to the zone 0 ground, then go through a ferrite toroid to add inductance to the outer shield and isolate it from the zone 1 ground. After the ferrite toroid, a gas discharge type inline surge suppressor should be used. These come in a variety of configurations, frequency band and power levels. It is best to keep the suppressor rating as close to the peak carrier power as possible, affording the most protection to the transmission equipment.
Design and implementation of EMP hardened facilities
Of the four strategies for mitigating HEMP; Grounding, Suppression, filtering and shielding, shielding is the hardest and most expensive to implement. Good grounding should be included in any good radio station design same as suppression and filtering.
Grounding. Grounding for a transmitter site must include an outside ring ground around the periphery of the building.2 This is bonded to several ground rods installed at regular intervals. The ground is brought into the building and all coax shields, electrical service entrance, TELCO equipment, suppression equipment and safety grounds are connected to it. This forms the zone 0 ground. One conductor then goes to the zone 1 ground which is the transmitters and racks. Any other conductors at any potential that go from zone 0 to zone 1 are bypassed at EMP frequencies by use of ferrite toroids or other high mu ferrous metal filters. Inductors may need to be bonded to ground to prevent saturation. At studio locations, the building electrical safety ground should be evaluated for adequacy. Additional grounding may need to be installed depending on effectiveness of existing ground. Any outside antennas, supporting structures, satellite dishes and generators need to be bonded together and grounded to the building electrical grounding system. Studios and engineering rack rooms need to be bonded to the ground using star topology. Facilities that are not adequately grounded should be retrofitted.
Suppression and filtering. Good surge suppression and filtering should be a part of all transmitter and studio site designs. Hanging a few MOV’s off of the service panel is not enough to prevent damage to a facility. All incoming lines from the street; electric, telephone, and cable need to have surge suppression connected and be bonded to a low inductance path to ground. The only exception to this is fiber optic, which is immune to the effects of EMP. Additional layers of filtering for sensitive, mission critical computer systems such as FERUPS, shielded category wiring that is properly installed, etc. Facilities that do not have adequate suppression can be retrofitted.
Shielding. Shielding is the most expensive, time consuming and difficult to install correctly. The High Altitude nuclear test Starfish Prime in July of 1962 produced a field of 5600 V/m in Honolulu, some 1300 KM away from the blast. Building a shielded structure against those intense magnetic and electrical fields is very difficult. Attenuating the field through layers of shielding is the most effective means provided the distance between the shields is wavelength/6 or more (about 27 inches at 72 MHz) to prevent coupling.2 For example, using a concrete structure with steel mesh creates 35-40 dB of attenuation in zone 0, a well designed transmitter with good RF shielding in it’s cabinet design creates a shield for zone 1. Equipment racks can also be used to create shielding zones by using copper or brass mesh with good metal to metal contact around front and back doors. At studio locations, engineering rack rooms should have copper or brass mesh embedded in the wall structure to create a shield. This will create a safe area to locate computer file servers, routers, switches, STL gear, satellite receivers and the like equipment. Layered shielding with use of metal, gasketed door will improve shield performance. Retrofitting shielding is more difficult to accomplish than grounding and suppression. It is best done in new construction. There are many different ways to accomplish even moderate shielding, which may serve well for lightning and solar flare induced EMP.
From personal experience, investing an extra $10-20K in grounding and suppression at a lightning prone transmitter site in Florida solved all of the issues at that site. Prior to installing the ring ground and bonding, the transmitter was knocked off the air several times per year. Since the work was done in 2005, there has not been one lightning related outage at that site.
References:
1. Protection Technology Group, System Approach to EMP Mitigation, February 2011
2. US Army Corps of Engineers, Engineering and Design – Electromagnetic Pulse (EMP) and Tempest Protection for Facilities EP 1110-3-2, December 31, 1990
3. US Department of Defense, HIGH-ALTITUDE ELECTROMAGNETIC PULSE (HEMP) PROTECTION FOR GROUND-BASED C4I FACILITIES PERFORMING CRITICAL,TIME-URGENT MISSIONS, MIL-STD-188-125-1A, February 15, 1994
4. Protection Technology Group, Modular Hybrid Series Connected Surge Protection Device LEA DS21 data sheet, 2010
The rumble of thunder this morning let me know that lightning season is upon us here in the Northeast and likely the rest of the country as well. I used to enjoy the odd summer thunderstorm, especially the late afternoon pop ups that cool off a hot summer’s day. Now whenever I see lightning or hear thunder I wonder if the phone is going to ring. Chances are good that it will not, as I invested many hours of my time and my previous employer’s money into lightning protection at the transmitter sites.
upper atmosphere lightning depiction
I go on the assumption that all tall steel towers will get struck, often times repeatedly, during any particular electrical storm. Back in the day, I took a course by Polyphaser called “The grounds for lightning and EMP protection.” It was a great primer on how to ground and bond equipment at a transmitter site to eliminate current flow, which is the cause of all EMP and lightning inducted failures. When I was in military communications, no expense was sparred as they took uptime very seriously. Any downtime was a personal affront to the commanding officer of the unit in question.
Lightning is DC however, it behaves more like 10 KHz – 2 MHz AC. Therefore, lightning and EMP grounding systems need to be designed and installed to accommodate DC through 10 KHz AC voltages. This is easily done by choosing the correct conductors, ground buss bars and bonding systems.
The other path lighting takes into a transmitter is through the AC mains. Utility company high voltage primary feeds act like large antennas for lightning induced EMP. Fortunately, much of that is filtered out by the step down transformers just before the building service entrance. It is still possible, however, for some impulse voltage to make it though the transformers and into the service entrance panel. On older tube type transmitter, this could often damage the plate voltage power supply because of the voltage multiplication factor of the plate transformer. Often times, the transformer secondary would have “holes” punched through the insulation to ground. This is an expensive and time consuming repair.
I would conservatively estimate that for every $10.00 spent on lightning protection, $1,000.00 dollars worth of damage and downtime is saved. Overall, a pretty good return on investment.
The basics for lightning ground bonding are thus:
Use the lowest inductance wire possible. The industry standard is #2 solid copper, however, if bonded properly, there will be very little current flow inside the transmitter building, so if #2 is not available, then any solid wire up to #8 will work. Tower ground bonding should be as heavy as possible.
Ground all guy anchor points, bond all guy wires together and to the same ground rod or ground rod system.
Keep the bonding conductors as straight as possible, bends should be long sweeping turns to minimize series inductance.
All metal equipment should be bonded, no rack, telco demark, electrical panel, dummy load, bulkhead entrance grounding buss, combiner, door frame, etc should be left unbonded.
All coax outer shields should be grounded where it comes into the building.
All coax cables should go through a toroid before being connected to the transmitter.
All outdoor bonding connections should be exothermically (CAD welded) bonded to ground rods.
All grounding must go to a common ground point, AKA star grounding point. No individual ground points should be allowed in the building.
Multiple ground rods installed around the outside perimeter of the building as deeply as possible. Some mountain top transmitter sites may require special grounding material (Bentonite) and or to have a ground well drilled. Ground conductors should have as much surface area contact with earth as possible.
The whole idea is to present a low resistance ground path and keep all of the equipment at the same potential to minimize current flow between equipment.
For the electrical building service entrance, a series surge protector installed before the service panel is the best method. Several are made and they need to be sized for the building service. A fall back is a parallel surge protector will provide some protection. On the AC mains connections, any series inductance that can be added to increase resistance to the lightning pulse is good. All AC mains connections to the transmitter should go through a toroid before they are connected to the transmitter. This is a good idea for remote control and mod monitor wiring as well.
It is that time of the year again, at least in the northern hemisphere, for thunderstorms. I am a big proponent of grounding everything, there is simply no such thing as too much grounding. I took a course when I was in the military given by Polyphaser in which grounding for lightning protection and EMP was emphasized. It was very interesting in several respects.
One commonly held belief is that when lightning strikes an object, the ground immediately absorbs all of the charge. That is not true in most cases due to ground resistances. Eventually, the ground will absorb the charge but it can take several seconds to do this, especially with a big strike. Equipment is damaged by current flow, therefore, every effort must be made to keep all of the equipment at the same potential, even if that potential is 10KV. That is where a single point ground buss comes in. Bonding every piece of equipment to a common ground buss ensures that no one device is at a lower potential while the charge dissipation is occurring.
The second misunderstanding about lightning is that it is DC voltage. That is true, however, a lightning strike has an extremely fast rise time, on the order of 30 microseconds. That makes it behave more like AC voltage around 10 KHz. Therefore, ground buss wires need to have a minimum inductance. Solid #2 wire is best, keeping it as straight as possible and using long sweeping turns where needed. All bonds should be exothermically welded (CAD weld).
Ground system installed at WKZY, WHHZ and WDVH-FM transmitter site in Trenton, Florida. Central Florida is the lightning capital of the US. Prior to doing this work, the Harris FM25K transmitter was knocked off the air at least once a month. Since this was installed in 2005, they have had zero lightning related damage. The ground rods are 20 feet long, driven down into the water table, spaced 20-30 feet apart.
All coax shields and metal conduits that come into the building should be bonded to the ground system where they leave the tower and where they enter the building. At most tower sites, I install a ground ring around the outside of the building with rods every 20 feet or so. From that ring, 5 to 6 radials outward 40 feet with ground rods every twenty feet works well. I also install 5 to 6 radial out from the tower base with the same configuration. The tower and building grounds are bonded together. This is important because when the tower gets hit, the ground will quickly become electrically saturated. If the building and the equipment is inside is at a different potential, current will begin to flow toward the lower potential, thus damaging gear.
All Coax, control and AC cables in and out of sensitive equipment should have ferrite toriods on them. Transmitter manufactures normally supply these with new solid state transmitters, as MOSFETS are particularly sensitive to lightning damage.
This is a Potomac Instruments AM-19 directional antenna monitor. It was damaged by a lightning strike two weeks ago on the WBNR tower in Beacon, NY. The case arced to the rack it was mounted in. This was a large strike, as several components in the phasor control circuit were also damaged. The fact that this arced means that somehow the sample lines are not attached to the single point ground for this site, which needs to be corrected.
Insulated AM towers present special design problems when it comes to lightning protection. Generally speaking, tower arc gaps should be set so there are side by side and there is no arcing on positive modulation peaks. Depending on power levels, this can be anywhere from 1/2 inch to 2 inches. Tower impedance also plays a roll in setting arc gaps. The final link between the ATU and tower should have several turns in it. The idea is to make that path a higher impedance path for the lightning, causing it to dissipate through the arc gaps. Incoming transmission lines from the towers should be bonded to a copper buss bar at the entrance to the building. All of this grounding needs to be tied to the RF ground at the base of the tower.
Arial phone cables can act like large lightning antennas for strokes several miles away. It is very important that the cable shield and the cable termination device is bonded to the building ground buss. I have seen installations where the TELCO tech pounds in a separate ground rod outside and connects the TELCO equipment to that. That defeats the concept of single point grounds and should be fixed ASAP.
Electrical services entrances also can act like big lightning antennas. Normally, pole mounted transformers will filter some of this energy out. Internal electrical distribution systems can also add impedance, thus act as inadvertent filters for lightning. In most mountain top transmitter sites, however, some type of power line surge protection is needed.
Inside view of LEA surge suppressor
There are two types, series and parallel. Parallel types are the least expensive and least intensive to install. They are usually found mounted next to or on the service panel and fed with their own breakers. They usually have some type of MOV or similar device that acts as a crowbar across the AC mains, conducting spikes to ground. Series types go in between the service entrance and the main panel. They include a large inductor designed to force spikes off into shunts. A series type protector offers more complete protection than a parallel.
Another example from my blown up shit collection, artifacts division:
Delta TCT-1HV current sample toroid destroyed by lightning
This is a Delta TCT-1HV current sample toroid that was pretty well destroyed during a thunderstorm. I mounted it on a piece of plexi-glass because I think it looks cool. This unit was installed at the base of the WGY transmitting tower. One June evening, I received a call from the station operator (back when they had live operators) that the air signal sounded kind of “funny.” So I turned on the radio and sure enough, if one thinks a radio station that sounds like a motor boat is funny, then, why yes indeed, it did sound funny.
Since I only lived a few miles away from the site, I jumped in the trusty truck and headed over. Upon arrival, I found the MW50B on the air at full power, with the carrier power swinging wildly from 20-90 KW with modulation. Hmmmm, bad power supply? Turned the transmitter off and tried to place the backup transmitter on the air. Now the old Gates BC5P had never been super reliable in the first place, but it was odd that it would not even run at all.
Then I had a hunch, lets walk out to the tower I said to my assistant who had showed up to help. When we got to the ATU building it was filled with blue smoke. Ah ha! Somebody let the magic smoke out of one of the components! I was expecting a capacitor blown in half but was surprised to fine the copper tubing that connected the ATU to the tower melted in half. Lightning must have caused an arc between the tubing and the toroid and for some reason the transmitter kept on running while it was arcing. The copper tubing in the picture with toroid is only missing about six inches, the way the system was mounted at the tower base, fourteen inches of copper tubing was missing, or rather melted into a puddle on the bottom of the ATU.
I quickly found another piece of 1/2 inch copper, cut it to length and flattened out the ends with a hammer and drilled mounting holes. Luckily I was able to get everything back in order quickly and the station returned to the air about an hour or so after it went off.
Everything has a cause. Investigation showed that the VSWR circuit on the MW50 had been disconnected from the directional coupler. The lead was un-soldered and taped off, so it was quite intentional. I spoke briefly with two of the three prior engineers that had serviced the MW50 over the years, they both blamed the other one. I surmise this; The WGY tower was prone to lightning strikes because of it’s height. Even if the tower was not directly struck by lightning, often times the guy wires would arc across the insulators, causing the MW50 to momentarily interrupt the PDM signal and drop the carrier for about a second. Some programming people at the station did not like this, it sounded bad on the air, so one of those guys undid the VSWR circuit and voila! No more momentary outages during a thunderstorm! Brilliant! Except for the 60-90 minute outage one night…
Sometimes it is better to tell the program directors that their idea is not good, then move on.
For about 4 years, I lived in a house next to a four hundred foot radio tower. Although I never actually saw lightning strike the tower, I heard it several times. Like everything else, after a while, you get used to it.
This is a video of lightning hitting the WSIX STL tower in Nashville, TN. The camera work is a little unsteady, the strike occurs around the 1:36 mark:
Most (if not all) radio engineers cringe when they here a clap of thunder. Then the waiting begins. What are we waiting for? The cellphone to start ringing, of course. Over the twenty or so years I have been doing this, I have learned a few things. One of them is you cannot over ground something.
That being said, you can, of course, ground something improperly.
The worst areas we have for lightening damage is the Gainesville/Pensacola markets. Those places are in the lightning capital of the US. Time was our class C FM station was getting knocked off a couple of times a month.
US thunderstorm Days map
There is hope. When we upgraded the stations and installed new transmitters in 2004 I insisted that the tower and building be properly grounded. I even got into an argument with the CFO about the “mission creep” as he put it. Never mind that I put $20K in the initial work specification for grounding.
There are a couple of strategies to use when dealing with lightning at transmitter sites:
Grounding: First, foremost and always. Grounding should consist of multiple ground rods driven as deeply into the earth as possible. At the Trenton Florida transmitter site we used 20 foot long ground rods driven in 20 feet apart all the way around the building and in five 60 foot spokes around the tower. All of these ground rods and tower base were bonded with #2 solid copper wire CAD (exothermically) welded to the ground rods. All turns were kept to a large diameter radius to keep inductance down. When lightning strikes the tower, this creates a large electron sink to dissipate the strike energy into.
Bonding: All equipment cabinets, racks, and everything metal is bonded together and to the same ground point presented by the grounding system. When lightning strikes, often the ground cannot dissipate the energy fast enough. When this happens, the entire ground area around the tower gets charged up. Current will only flow down a less resistive path. If everything is bonded together, the potential between any piece of equipment or component is the same, even if that potential is +10,000 volts. No flow of current means no damage.
The transmitter building is located away from the tower. Almost every FM and TV transmitter site I have visited, the building is right smack at the tower base. By moving the building away about 100 feet or so, the EMP from the tower strike has dissipated (log function) significantly before it passes through the transmitter building. It is a little more expensive to install due to the added transmission line lengths and losses, however it works.
I have been at the Trenton Florida transmitter site when lightning struck the tower. The result, not even a transmitter overload. Nothing noticed on the air, no damage sustained by any equipment. For the last five years, there has been no off air time due to lightning damage at this site.
Studio building with lightning rod, Gainesville, Florida
The studio site has a similar story. We built a new studio building in 2005, there is a 100 foot monopole that holds the STL antennas. You know that it gets hit during a storm. I remember the manager and IT guy from Pensacola commenting about how nice the new SAS Rubicon consoles were. Both of them also said that they wouldn’t last through the first summer because of lightning damage. Four years later, not a single incident of damage to the consoles, computers or anything else in the building because we grounded everything as I described above.
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~Benjamin Franklin
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