Grounding notes by Polyphaser

 

Polyphaser Engineering Information

Polyphaser Company and their line of lightning protection products is well known in commercial and amateur radio circles.  They make an excellent line of products for amateur use.  The following information has been copied from the Polyphaser web site. It was copied here since their web site is very difficult to navigate effectively and the result is loss of useful information. Full credit is given to Polyphaser for this information. Link to Polyphaser

Notice: Much of the lightning protection discussion pertains to RF grounding also. Proper RF grounding can aid in enhanced lightning protection too!


Document Number PEN1016: An overview of Lightning Protection for Ham Radio Stations
Notes:Proper lightning protection for a ham radio station can involve more variables than any other type of radio site. The antenna location will establish the grounding requirements, while the station location will drive the protection requirements.The primary rule for surviving a lightning strike is still the same no matter which of the many possible variations you have: all equipment elements must be connected to a single, low impedance ground system. This includes the antenna, the antenna support (pole, tower, etc.), and all of your station’s input and output protectors. (I/O’s: antenna, power, telephone, rotor, etc.).Let’s examine the significant elements of a good grounding and protection scheme to help you construct an installation that will survive a direct lightning strike.

We begin with choosing the antenna location. This and the antenna type will dictate the size and location of the earth system needed to disperse the strike’s energy. The sooner the ground system is able to spread out the energy, the better the chances of preventing it from traveling to your equipment. Almost 90% of strikes will be electrons that, due to like charge, repel and spread out. The antenna ground system provides the interface to the earth body. As we will see later on, the ground system is formed by a set of ground rods interconnected below grade with bare radials.

Also fundamental to a good protection scheme is the creation of a single point ground within the ham shack. This single point ground is used to mount all of the protectors and to provide a ground for all of the equipment chassis. This interior single point ground is connected to an external ground system (composed of radials with ground rods) with a low impedance copper strap. The tower ground system and the single point ground system must be interconnected. This interconnection should be below grade and with a bare low inductance conductor. The coax cable shield must not be the only interconnection between ground systems.

Three Techniques:
Every conductor has measurable inductance. Similarly, ground conductors exhibit normal inductance before they go below grade. Once in the ground, the inductance of a bare conductor is shunted by the earth’s conductivity.

If the soil at the grounding location is not very conductive, three things can be done to help the situation.

• Increase the surface area of the conductor, decreasing its normal inductance.

• “Dope” the soil to increase its conductivity shunting the inductance of the in-ground bare conductors.

 Install additional bare radial lines with ground rods which will effectively parallel the inductance and reduce the overall system inductance.

In some locations it may be necessary to utilize all three of these techniques for the best results. Let’s examine each one.

1) Conductor Surface Area:
The most effective material for a ground system conductor is copper strap. Copper as a metal is a good electrical conductor, only moderately attacked by ground and air borne acids, and should have a life span measured in years.

Since lightning has a large portion of its energy in the LF range, it will behave like an RF signal. (See “Lightning Overview” and “Coaxial RF Protectors” for a more detailed discussion) That means the energy will only mostly conducted on the skin of the conductor (skin effect). Thus, the surge current will only ride on the outermost surface of the conductor. Such currents following a round-member conductor will not make extensive use of its large cross sectional area. With a 1-1/2 inch [38.1 mm] or larger flat strap of at least 26 gauge (0.0159 inches) [0.4 mm], both surfaces will conduct the surge.

2) Soil Doping:
Water in its purest form is an insulator. Ionic salts when mixed with water make ions. The earth is a conductor because of the number of ionic salts present in the soil. Therefore, conductivity can be improved by adding more ions to the soil.

Soil doping can be done by either adding water or a saline solution to the soil around the grounding system. If the soil already has a sufficient amount of naturally occurring salts, adding water will free the ions and improve conductivity. The more ions (salts) available, the less water that will be needed to reach a given level of conductivity.

If few natural ions are available, salts, such as Epsom salts, can be added to the soil to increase the conductivity. Depending on the amount of rainfall, doping the ground system radials with 4 pounds of salt per linear foot and 20 pounds per rod may last approximately two years.

3) Ground Radials:
Radials are the most cost effective grounding technique considering system impedance, material cost, and installation labor. If one radial gives “X” resistance, then two will deliver an equivalent “parallel rule” plus 10%. This rule only holds true when the soil has the same conductivity over the entire radial area. After the first two radials, you will need to double the number of radials each time to continue with the parallel-plus rule.

Radials do have a limit on their effective length. If the surge energy has not been launched into the soil within the first 75 feet [22.86 meters), the inductance of the radial will prevent any further effective prorogation. Therefore, as a general rule, all radials should be at least 50 feet 15.24 meters] long and no longer than 75 feet.

Ground rods should be placed along the entire length of each radial. The most cost effective spacing between rods for normal (grassy) soil is two times the length of a rod into the ground. If 8 foot [2.44 meters] rods are used, they should be placed on 16 foot [4.88 meters] centers.

[Notice: for RF purposes the ground rods are not necessary. See QST, Aug, 2003, P 39; “Optimum Radial Ground Systems”. -bd-]

If the soil is not normal (e.g., very dry or sandy), the separation may be reduced in order to minimize the interconnect inductance. It doesn’t hurt to have the rods too close, it only costs more in material and labor.

Ground Measurement:
Since most soils are stratified, the best way to determine the effectiveness of a ground system is to measure it. The simplest way to determine the sub-layer conductivity is to measure the first ground rod, one foot at a time, as it is hammered into place. This technique can provide a profile of the lower layers relative to the first foot. Most earth resistance meters measure only dc or low frequency ac resistance of the ground system. Although the lightning strike is dc, due to the fast rise time to peak current, there is significant RF energy. Since there is a high frequency component, the inductance (effecting transient response) of the ground system is important. Without using very expensive specialized test methods, the only way to ensure a low impedance ground system is to follow the suggestions given for conductors, doping and radials.

Tower Considerations:
No one should consider using a non-conductive structure for an antenna support. Only conductive towers or metal poles should be used for mounting antennas. If the tower or pole has sliding contacts (crank-up or push-up), the joints should be bonded using short sections of copper strap attached with PolyPhaser TK clamps. Normal self – supported and guyed towers will not need such jumpers.

Guyed towers are better from a lightning protection perspective if the guy anchors are grounded properly. Because the anchors are located away from the tower base, at least some of the strike energy will traverse the inductive guy wire to the ground. The more the strike energy is divided, the less there is to go to your equipment.

Dissimilar Metals:
Copper should never touch galvanized material directly without proper joint protection. Water shedding from the copper contains ions that will wash away the galvanized (zinc) tower covering. Stainless steel can be used as a buffer material. However, be aware that stainless steel is not a very good conductor. If it is used as a buffer between copper and galvanized metals, the surface area of the contact should be large and the stainless steel should be thin. Joint compound should also be used to cover the connection so water cannot bridge between the dissimilar metals.

Magnetic Energy:
Lightning has a large magnetic field associated with its typical high current pulse. The magnetic field will couple to all nearby conductive materials. There are two ways to minimize the amount of magnetic energy coupling, shield your equipment or place some distance between the equipment and the likely strike location.

A galvanized steel sheet may be used as a shield to attenuate the magnetic field pulse by 10dB.The steel should be at least 30 gauge (0.016 inch) [0.41 mm] and should be connected to the ground system.

Distance is the other means to limit magnetic field coupling. The strength of a magnetic field diminishes at the rate of one over the distance squared. Since a moderately high tower is much more likely to be struck than any other nearby structure, the placement of the tower with respect to your equipment deserves significant consideration. Factors that should be considered are not only the magnetic energy which will radiate from the tower, but also the benefit of the distance in terms of the inductive loss provided by the length of the orthogonally run coax. This added inductance of the coax line will buffer the energy entering your equipment area. In addition, the extra distance will provide a little more time for the tower ground system to dissipate the strike energy and thus have less to share with your equipment.

Both of these factors indicate there should be a reasonable >20 feet [>6 meters] separation between the tower and the operating equipment.. For towers already located closer than this, it may be necessary to utilize some shielding to minimize the magnetically induced energy.

Antenna Location:
A ground mounted vertical antenna is very similar to a ground mounted tower. Both have a low impedance connection to the ground system. However, if the antenna or tower is mounted on a roof, the inductance inherent in the conductors to the ground system will be very significant. So significant, that voltages in the order of several hundred thousands volts could be present during a strike. To reduce the inductance in the ground conductors, increase the surface area / circumference of the conductor (wider copper strap) as well as the number of conductors.

For roof mounted antennas and towers, multiple down conductors can be spread over the roof and brought down to ground in multiple locations. This will require a ground system run completely around the building (a perimeter ground). As an added benefit, this multiple down conductor approach will reduce the mutual coupling between down conductors and provide a low, unsaturated perimeter ground to absorb the conducted surge. The magnetic fields will also be divided and could, in theory, cancel in the middle of the building. This will help minimize magnetic energy coupling into the wiring inside the building.

Coax Grounding:
Since the tower is a conductor and is well grounded, all of the coax lines should be grounded (using a grounding kit) at the top of the tower close to the antenna and at the base of the tower before they come toward your equipment.

During the strike event, the tower and the coax lines will mutually share the strike energy. If the coax lines are not grounded as they leave the tower or they are completely isolated from the tower, more energy could traverse the coax toward your equipment than is conducted to the ground system by the tower. Such a large inductive voltage drop may cause arching between the coax lines and the tower that could cause deterioration (pin holes in the coax for moisture to enter) or destruction of the coax lines.

Notice the word “bottom” in this section. Since all towers have some inductance, leaving the tower at a point above ground will allow some of the strike current to continue on the coax line (both the center conductor and shield) toward your equipment. Once at the equipment, the current will follow the chassis to the safety ground. This could elevate the equipment cabinets to deadly voltages, deadly for both people and components.

Even though the inductive properties of the coax cable appear to be beneficial, and extra inductance can be created by adding a few turns to the coax; don’t do it. The added turns can also act like an air wound transformer coupling more energy into the line. Make sure coax lines leaving the tower remain at right angles to the magnetic field surrounding the tower.

Control and Coax Line Protection:
Rotor control lines should be protected using a protector at both the top of the tower where the lines go to the control motor and inside the shack at the single point ground panel.

If it is not practical to protect the lines at the single point ground panel, they may be protected at the bottom of the tower. The protected lines should then be placed within EMT (metal) conduit that is grounded only at the tower-base end. The EMT will act as a faraday shield from the tower’s magnetic fields and will minimize the amount of induced energy.

Single Point Ground:
The next step in a good protection scheme is to provide a single point ground, a plate where all of your equipment l/O protectors can be located. The panel is best located near the ground to keep the inductance of the ground conductor low. However, if this requires the plate to be far from your equipment and if the magnetic fields of a nearby tower can easily couple into the interconnecting wires and cables, then the panel should be located close to your equipment.

An alternative to the single point ground plate is to use a rack panel. This is recommended only if all of the l/O protectors are mounted on the panel and the ground connection is directly to the panel and not to any other piece of equipment.

The grounding of the plate or panel is very important. A low impedance path to ground is a necessity and only copper strap should be considered. Since the strap is flat, its susceptibility to magnetic fields is only towards its edges. To prevent coupling, the strap should be oriented with the flat side parallel to the tower (the most likely strike point and magnetic field source). The single point ground plate should also be oriented with its flat side parallel to the tower for the same reason.

In the equipment room, each piece of equipment must be bonded to the single point ground panel with a low inductance strap. This will maintain all chassis at the same potential during the strike event and minimize chassis-to-chassis current flow. The power, telephone and coax line protectors on each of the l/O’s must be mounted on the single point plate. This will minimize l/O to-l/O current flow.

Additional protectors may be used to protect the opposite side entrance locations for the power and telephone lines. They will provide added protection for jointly used equipment such as answering machines, appliances and etc. Ideally they should also be grounded and connected by a buried bare conductor to the ground system.

Remember that surge energy can enter your shack in either of two ways: from a strike down the road coming in on the power/telephone lines or from a strike to your tower. In either case, high quality protectors will dump the energy into the ground system. Because of varying propagation times, if the protectors are electrically spread out from each other, they cannot work in unison to keep the voltage levels between the equipment l/O’s within a tolerable range for equipment survival.

No Sharp Bends:
Route all ground straps and grounding conductors so they have a gentle bending radius. Bends sharper than 8-inch [203.2 mm] radius will add unwanted inductance to the desired ground path. Even for conductors buried in the ground, try to prevent sharp bends.

Protectors:
Coax protectors should be units that have dc blocking on the center pin. This serves as a high pass filtering that prevents the lightning’s low frequency energy from continuing to your equipment. The strike energy is picked off and diverted into the ground system in a controlled way. The dc blocking ensures the operation of the protector regardless of the input circuitry of the equipment.

Did you know that spark gap protectors with dc continuity will not work on receivers and shunt fed duplexers? The shunt to ground inside a receiver (coil to ground for static draining) prevents the low frequency lightning energy from turning on the dc continuity protector. The coil shunts the energy to ground all right, but it is at the wrong place. If the coil can’t handle the energy (half the coax surge energy is on the center pin), the coil will open up and the current will translate to a large open voltage source capable of arcing anywhere within the radio.

Lightning protection can be summed up simply: You have control of the lightning strike energy and not Mother Nature. Once control is lost, all can be lost.

The basement is the best location for the ham shack. It is closest to ground and will have the lowest inductance connection to the grounding system. Because it is below grade, some magnetic shielding may occur. Most basements have concrete floors. Since concrete is a conductor, your equipment must not sit directly on the concrete. Doing so will allow surge energy to enter the shack and find a ground path through your equipment to the floor. Insulate your equipment with material that does not absorb water. Wood is not a good choice. Polypropylene is better than nylon to use as a full footprint sheet insulator. Obviously, you should not be on the concrete floor touching the equipment when a storm is near!

The first floor is the next best location. The magnetic shielding is less than the basement and the inductance to ground is higher than the basement. If your tower is close to the building, the recommended grounding strap, running down the outside wall, may inductively couple some energy from the tower. This is also true for other lines such as coax, tower lights and rotor lines. The longer this parallel run, the more energy will be coupled. Our recommendation is to protect these lines at the tower base then run them in EMT (electrical metal tubing) steel conduit. The conduit should be grounded to the tower base ground point. This will act as a faraday shield for the cables inside. Do not run unprotected lines in the EMT. The protectors must be grounded to each other as well as to the tower ground. The best way to do this is to place the protectors inside a weatherized NEMA type box. Make sure the box is grounded, as well as the inside mounting plate. To do this correctly, remove the paint from the box’s outside and inside surfaces at the ground point and use proper joint compounds to weatherize the connections. Stainless hardware may be used. Crimp lugs must be crimped, soldered and weather covered. Solder (60/40) will not hold up to sunlight and ozone without protection. Use a short section of strap to bond between the inside surface of the box and the inside protector mounting plate.

High Rise Buildings:
Our definition of a high rise building is different than the upper stories of a house. The antennas on a high rise are not on a ground mounted tower, but are usually attached to the building structure. Therefore, a single point grounding plan is a must for a high rise equipment room. Grounding both the antenna and the single point ground connection in the equipment room is easy for buildings with structural steel frames – just bond to the building steel. Buildings other than steel construction are not as simple. Some high rise buildings have a fire riser with a straight run to the basement where a super charger pump is usually connected. The riser may be used as a ground path if the pump’s power is protected and a strap jumper installed to take the strike energy past the pump’s gasket on both its input and output ports. If the riser is over 50 feet away, it may not be the best ground path to use. Check for other paths such as existing building lightning rods with down conductors or large electrical conduits. Do not use drain pipes or vent stacks. If none are available, regardless of the path distance, and it is impossible to run a strap down the side of the building, then the antenna just can’t be grounded! When an ungrounded antenna is hit by lightning, the energy will traverse the coax line to your single point equipment ground location. This may be many meters from earth and the inductance/ resistance voltage drop will be very large (hundreds of thousands of volts).

The ideal plan is a single point ground with no sneak paths. Sneak paths are loops that allow lightning current to flow into the equipment room. The easiest sneak paths to miss are the safety ground and the concrete floor (discussed above). The safety ground can be fixed by adding a distribution panel and protector at the single point ground location or, for small sites, a plug-in protector grounded on the single point ground panel. All l/O’s (input/output) must be protected at this single point. The next thing to measure is distance. During a strike, distance equates to voltage drop to earth, the entire room of equipment will be elevated. The sharp corners of equipment cabinets can breakdown the air, causing current to flow. This will be a very low current unless another path is found by these streamers. Heater vents and electrical conduits that are not grounded to the single point can become such paths. It is a good idea to bond (ground) all conductive objects within 1 meter of any single point earthed equipment in the room.

Tower mounted equipment is similar to the above high rise situation. The l/O’s must be protected and the protectors must be located and bonded together. Single point grounding should be easy to do if the equipment is mounted inside a metal enclosure.

Antenna Support:
Ground mounted vertical antennas require the same type earthing for lightning protection. A vertical antenna’s impedance is half of a dipole’s. Don’t stop short of a good ground plane. The better the ground plane for RF, the better the earthing for lightning. This is assuming that the RF ground plane is in the ground.

If you have a antenna tuner fed long wire and the pole is just supporting the antenna wire, it would be a good idea to have the grounded straps extend higher to intercept a strike or to divert energy to ground if the wire is struck. This can be done by either placing a high voltage gas tube between the long wire and the straps or by making an arc gap between the wire and the ground straps. A gas tube will not be adversely affected by temperature, humidity, pollution, or wind, while the air gap will be affected. It may be difficult to calculate the voltages present at the gas tube and it will change when switching bands. A rule of thumb is for about 7kV. An air gap would be about 0.175″ at sea level with 50% humidity and grows larger with elevation/humidity. (Humid air is less dense)

Another gas tube or gap may be added closer to the antenna tuner. For dipole antennas with baluns, use the same gas tube technique. Place gas tubes around the balun. Place one across the balun at the dipole wires and one from each side of the balun to the ground straps. This will protect the balun from a strike to the dipole wires. The more strike energy you can divert to the ground before it reaches your equipment, the better off you and your equipment will be.

Just a word to those who tell us that they are safe from lightning because they always disconnect the coax from their equipment. When asked what they do with the disconnected line(s), they usually respond that it is placed on the floor. Now if you stop and think about the last few thousand feet that the lightning has jumped, you can see the fallacy of their thinking. In fact, they made it worse since arcing involves ignition temperature plasmas inside your house. True, the radio may still work, if it survives the house fire. Throwing the coax out the window is not a solution, especially if the coax has already entered the house from the antenna or the antenna is roof mounted without a ground path. Grounding switches will not last long with direct hits unless other good ground paths are provided. Grounding the antenna line and not disconnecting the coax shield can still allow strike energy to be shared with the equipment The shield connects to the chassis and if a single point ground is not present with power/telephone protectors, the equipment will be damaged.

Power/Telco entrance:
Full protection for a ham shack must cover not only strikes to your tower, but also hits from down the road to utility lines. By using single point grounding, your ham equipment will survive the hit to your tower. If the outside (tower/perimeter) ground has a low impedance at lightning frequencies, most of the strike energy will be dispersed into the ground and little energy will enter the shack. This is fine, but what if your ground has deteriorated over time or was never very good because of yard size?

The ground system can absorb only so much energy before it becomes saturated. In 90% of the strikes, a traffic jam of electrons will be coming down your tower. If they cannot spread out in a reasonable time frame, the back up pressure (voltage) will find or create another path. The ground system, if too small in area, will cause more energy to traverse the cables and other lines to the shack. The I/0 protectors can keep the voltage levels between the single point ground and the signal line(s) at survivable limits, but the energy is only diverted elsewhere. This could be the house phone lines and power lines.

Other house appliances may be at risk. When the ground system is saturated, the energy is actually coming from the (utility) ground system and can go through your TV, for example, in an effort to leave the area by way of the cable TV drop. Satellite dishes will also have the same problem. The best way to protect the rest of the house is to provide protection at a single point. The easiest single point will be at the power and telephone entrance. The utility ground rod (which should have been already interconnected to your ground system) is used by both the power neutral and the telephone protector installed by the phone company. By placing a power mains protector and a secondary phone line protector at this location, the entire house will be protected. The cable TV or outside antenna coax should be rerouted and a good coaxial protector installed at this point. The cable company installed protector is usually just a grounding block earthing only the outside shield and does nothing to the center conductor energy that can have as much energy as the outside shield! As the ground system rises in potential from a strike, the protectors will take the ground system energy and place it on the power, telephone, and cable TV lines while keeping the voltages between earth and the active lines within the limits of equipment survival.

The utility ground rod for the house should have already been interconnected to your ground system. What if this can’t be done? If this is not done, the energy from the tower strike will traverse the house safety ground wires to this rod, causing problems. The reason to interconnect them in the ground using bare conductors was to reduce the inductance of the interconnecting path. It is true that the house wires are a parallel path and there is nothing we can do about it. If the interconnect path is better (lower inductance and resistance) the majority of the current will bypass the house wiring. The only alternative is to provide a copper strap path through the house. This may not be a sufficiently low inductance path and it will radiate to other wires/equipment inside the house.

The power and telephone feeds to your house can be either aerial or underground. Most people think underground is better from a lightning standpoint. Buried underground, it will not be hit directly, but if a nearby tree is hit, the amount of energy coupled through the conductive ground medium can be almost equal to a direct hit. By being underground, the shielding effect to the wires is not great. The buried depth does little when compared to the depth low frequency strike energy penetrates. When you consider the cost of underground utilities, these and the aesthetics must be weighed.

Ground System Materials:
Solid copper wire/strap and copper clad steel rods, makes copper the most commonly used earthing material. Your below grade ground system should be made with the same material throughout. Mixing of materials, like galvanized rods with bare copper radials, will create a battery action and the zinc of the galvanized rods will become sacrificial, dissolving into the soil. This leaves bare steel to rust and not provide an optimum connection to earth. (Note: when wet, rust can conduct, but not very well.) Using stainless rods in order to prevent corrosion will not provide the best conductivity. Since stainless wire will be required to interconnect the rods, the resistance of the system will increase. An all aluminum ground system should only be considered in very acid soil conditions and even then it should be chemically tested for other attacking soil compounds.

Joints between copper radials and copper clad rods should be made by exothermic welds or by using joint compounds in high compression clamps. Solder connections, even torched silver solder connections will not last as long as the above. An exothermic weld is created when a graphite mold around the connection is filled with copper oxide and aluminum powders. An additional starter powder ignites the exothermic process. The resultant molten copper is deposited into the lower mold cavity where it burns away any oxides and creates a larger fused connection. The larger cross sectional bond decreases the resistance and increases the surface area, reducing the inductance of the joint. Since the materials are all the same, the connection will last as long as the rest of the grounding material. High pressure clamps provide a meshing of copper to copper since the material is soft (malleable). The use of joint compounds further enhances the weather tightness of the bond. The high pressure will need to come from another material stronger than copper.

If you find a rock layer is making the ground rod insertion difficult and you can’t remove the rod to start over a few feet away, the best idea is to cut off the rod and connect it to the system. A rock layer will hold water and salts so the conductivity above should be good. Making more connections to areas of higher conductivity will reduce the overall impedance of the ground system (resistance and inductive reactance).

The ground system has a resistance and an inductance value. (It has capacitance too!) The amount and location of the inductance can choke off the effectiveness of radials. When a radial is in poorly conductive soil such as buried in a dry, sandy layer, the radial inductance can be calculated as being in air (a very poor conductor). When the radial runs in highly conductive moist soil (or doped soil), the inductance of the wire is shunted by the soil’s conductivity, making it unimportant.

Since copper strap has lower inductance than wire, it is recommended for the radial run. The strap’s extra surface area reduces the inductance and the sharp edges allow for a high E field concentration forcing more charge into the soil. Short multi-point (like barbed wire) type grounding systems have been tried and have not been as effective as the sharp edge of copper strap for ground rod interconnecting material or for radial runs without rods. Copper strap radials have been proven successful on bare mountain top solar powered sites where ground rods could not be used. The strap edges helped disperse the strike’s deposited charge to the tower by arcing onto the mountain surface, saving the solar powered radio equipment at the site.

Adding ground doping material to your radial trenches and rods can be helpful. Stay away from gels and other chemicals that can shorten conductor life. All add-on conductive earthing materials do little except make your copper conductors larger (more conductive surface area). This gives some percentage of improvement but it still must interconnect to conductive soil where it has both salts and moisture. If the soil is dry around the earthing material, the connection to earth will be poor, regardless of the advertised claims. If the area is not large enough, the earth connection will suffer. By increasing the area of your ground system with the addition of more radials, the same improvements can be obtained for less money.

Longevity:
After doing all this work, Mother Nature still has a way of making anything we do temporary. Once a ground system is in the ground it will start to age. Copper and other metals are attacked by acids, while aluminum is attacked by bases. Other chemicals may be present in the soil causing decreased effectiveness of the grounding materials. This is why maintenance testing is important. While some ground systems last 30 years, others don’t even last two years! There are two ways of finding out if your ground system is in need of work. One is after a lightning strike and is too late! The other is to measure the system. An old timer once told me that he tested a ground by disconnecting it from everything and connecting it to power “hot” through a 30 amp fuse. If the ground was good, the fuse would blow. This is not the way to test a ground and it could change the soil conductivity by attempting such a test. The proper way is to use an earth resistance meter providing a fall of potential type test. Be careful when connecting a ground system to your electrical utility ground rod. Depending on ground conductivity, harmonic and other currents, there could be current flow causing a spark when connected.

Most of the above topics are covered in more detail in our other technical documents. Read on! (73, KF4MT) Last Reviewed: 1/9/03


 

Summary:
Document Number PEN1030: An interconnected concrete tower base can help the tower ground system.

Notes:
Concrete is a fair conductor and can be used safely and effectively to augment the tower grounding system. The concrete’s ability to quickly absorb moisture and release it slowly over a long period of time makes this possible. The pH of the released moisture in turn enhances the conductivity of the surrounding soil.

It is a common misconception to think that a lightning strike will blow up a concrete pad. However, consider first, a myth-perpetuating case of an improperly designed earth ground system where the tower leg “J”-bolts are imbedded directly into the concrete pad. In this case, due to the poor nature of the tower ground system, each of these J-bolts will actually share a significant amount of strike current which in turn will flow through the concrete. Since the surface area interface between the J-bolts and the concrete is small, and the surge current density from the strike very large, the corresponding heat generated by the energy transfer can turn the concrete moisture into steam and possibly crack the pad. We have only seen this happen once on a mountain top in the Nevada desert. However, a few poorly implemented occurrences can give a valuable technique a bad reputation.

If during construction, all rebar in the concrete pad becomes an integral part of the ground system, the overall surge current density will be several orders of magnitude lower than the myth-perpetuating case above. With the surge current distributed over all of the rebar there will be little opportunity to develop the temperatures necessary to vaporize the imbedded moisture. The pad will not crack.

To successfully implement a Ufer ground system it is necessary to bond each of the independent pieces of rebar together. This is best accomplished using an exothermic process. Failure to weld all elements of the rebar could allow for a spark gap between the unconnected elements and thus an opportunity for localized heating of the imbedded moisture. The electrically unified rebar is connected to the tower leg. The buried ground system radials, used with ground rods to further disperse the strike energy, are also bonded to the rebar. The Ufer ground, enhanced by the local earth resistance, will be lower due to the leaching of the concrete pH into the earth that in turn lowers its impedance at lower frequencies. The better the ground system, the more current flows through the tower leg into the Ufer ground. Also, since the strike charge is all of the same polarity, it naturally wants to spread out. With the large surface area of the rebar closer to the earth than the tower J-bolts, the current passes easily through the concrete to get to earth where it continues to spread out even further.

A Ufer ground should not be used alone. We always recommend that radials with ground rods be used as the main ground system and that the Ufer ground be used to further reduce the ground resistance of the system. Many tests have been done, dating back to 1968, that prove the Ufer is a safe and effective way of augmenting a ground system.


Document Number PEN1031: How to “ground” the coaxial cable shields as they enter the equipment building.

Notes:

Any input or output (I/O) line can carry surge current that can harm equipment. Of the primary I/O lines (power, phone, and coax), the coax line(s) from the tower can bring in more surge current to equipment than any other source. The large surface area of the shield, the correspondingly low inductance of the coax line, and the proximity of the coax to the tower present the fast rise time lightning current pulse with an ideal path towards the equipment.

If the bottom coaxial cable ground kit (where the coax leaves the tower) is at any elevation above the earth, the inductance of the tower section between that ground kit and the earth is sufficient to cause a substantial voltage drop. The resultant voltage on the coax will drive current on the coax line to the equipment where the electrical safety ground provides a path to ground. Once the surge is inside the building and on the equipment chassis, it is almost too late to try to protect the equipment.

The best way to prevent such coaxial currents from reaching your equipment is to keep them from entering the equipment building. This may be accomplished by installing, on the outside of the building, a panel connected to the ground system with large surface copper strap(s). The large surface area strap is necessary to provide a low inductance path to ground for the surge energy as well as provide for the high frequency component of the strike energy. Each coaxial line as it enters the building is attached to the panel with an additional ground kit before connecting to a protector.

A PolyPhaser bulkhead entrance panel provides all of the necessary attributes required above. The bulkhead panel has a weather protected built-in grounding kit that will handle a wide range of coax lines. The low inductance ground connection is provided by multiple 6 inch [152.4 mm] wide copper straps attached with a sandwich bar to the 0.125 inch [3.2 mm] thick solid half-hard copper bulkhead. The panel is designed to mount/ground a PolyPhaser coaxial protector facilitating cable installation. The overall physical installation of a bulkhead entrance panel is shown in the PolyPhaser online catalog. Please note the below grade interconnection of the bulkhead panel with the other elements which comprise the grounding system. Last Reviewed: 1/9/03


Document Number PEN1024: Multiple I/O port protection, Single Point Ground considerationsNotes:One of the basic ideas in developing a protection strategy is not allowing stray energy to flow through the equipment. There are several ways to accomplish this. One is to totally disconnect the equipment! Another is to provide some form of impulse protector for each of the equipment’s Input or Output (I/O) ports. These ports are usually the ac power connection, a telephone or control line, and an antenna transmission line. A protector on each of the l/O’s will protect that path from damage. However, it is also necessary to be careful about voltages that may exist between the I/O’s during a strike event.

For example, if each of the protected l/O’s of a remote transmitter are connected to a different ground, which could happen very easily in the best of installations, the following situation will exist during a strike event that could damage the transmitter.

The transmission line is grounded at its protector as the line enters the building. The power line to the transmitter is protected and grounded at the distribution panel where the power line enters the building. The telephone line is also protected and grounded where it enters the building. The protection on each of the l/O’s at the building entrance is good practice and has the advantage of keeping the strike energy toward the outside of the building and away from the transmitter.

Each of the I/O’s are an entrance point for strike event energy. During a strike event, the energy will propagate along a conductive path (power line, transmission line, or telephone line) until it meets the protector. The protector will shunt the majority of the strike energy to the earth ground. The earth immediately surrounding the ground point will begin to take up the energy charge and dissipate the energy by propagation within its “sphere of influence” (see ground systems topic). The local earth ground will rise in potential ( see GPR) for a few microseconds. For a brief instant, one port of the transmitter is elevated above ground while the other ports are at ground potential due to other protector connections not yet elevated.

As the surge energy attempts to go to earth ground using the transmitter as a connecting path to the other grounds, it is likely to also use some of the internal circuitry as a current carrying conductor and cause equipment damage. A complicating factor is that the other l/O protectors are at a distance with respect to the equipment. The greater the distance between the protectors, the more serious the problem.

Another complication in this scenario is the inductance of the conductor between the I/O protector and the ground system. The inductance will determine how much of the strike energy is conducted into the ground system and how much is left to elevate the transmitter chassis. Since strike energy is a high frequency pulse, a low inductance path to ground becomes a critical factor. Copper strapping is preferred over large diameter wire as an inter-connecting media. Copper strap has a large circumference and low inductance per unit length. The strike energy, like water, will follow the easiest (least inductive) path to ground. In the above example, each of the I/O protectors is connected to separate grounding points. This can be corrected, but will require some physical rearrangement of the transmitter installation.

First and foremost, there should be only one ground system .

Second, the individual l/O protectors need to be co-located on the same electrical ground plane. This means establishing a single point ground system within the equipment building. An ideal way is the PolyPhaser Bulkhead Panel, PEEP, or Single Point Ground Panel. The single point ground system will keep all the I/O protectors at the same level with respect to each other.

Third, the transmitter equipment chassis must be insulated from conductive flooring and connected to the ground plane using a low inductive connector. Last Reviewed: 1/9/03


Document Number PEN1014: Where the lightning energy goes and what can happen. Ground Potential Rise (GPR)

Notes:
A lightning ground system should be capable of dispersing large amounts of electrons from a strike over a wide area with minimum ground potential rise. It should be capable of doing this very quickly (fast transient response).

• Ground potential rise (GPR) means any difference in voltage between the strike’s local earth sphere of influence and the “outside world.”

• Outside world means any other ground outside of the lightning strike’s local sphere of influence.

• By spreading electrons out over a wide area with a fast transient response ground system, the ground potential rise (step potential) for any smaller given area would be reduced.

• The speed, or transient response, of the ground system would be dependent on the combined inductance of the below grade conductive components and the resistivity/conductivity of the soil “shunting” those components. The lower the inductance of the system components and soil resistivity, the lower the impedance at higher frequencies, the faster the ground system could disperse electrons.

• Copper strap has lower inductance per unit length and more surface area in contact with conductive soil than the equivalent amount of copper in a circular conductor.

Local ground potential rise (GPR) can be the source of damage to any equipment interconnected to the outside world. A local ground potential rise looks for any path to the outside world. Some of these paths could be through your equipment.

• Any signal line with a ground return through the input/output circuitry is subject to damage. The local elevated potential would seek to equalize the signal conductor through your circuitry, and use it as a path to a lower potential. Local and remote interconnected equipment would be at risk.

• Any insulated signal lines are subject to damage as is the source power supply.

• A coaxial cable shield can carry a large amount of current at speeds exceeding 90% the speed of light with possible damage at both ends.

• The velocity factor of the coaxial cable center conductor with a fast rise time pulse can vary from 66 to 90% the speed of light. The directly applied or induced voltages on the center conductor would “roll off” and arrive after the shield pulse, producing large differential voltages between shield and center conductor.

• Secondary ac power conductors are a two way “street” for electrons. They are usually large low inductance conductors. A strike to the power lines some distance away can conduct damaging energy to equipment. Also, a strike to the tower or building can produce a local ground potential rise with damage to your equipment from energy attempting to “escape” to the outside world through the ac secondary conductors.

A good source for more information on GPR is: www.gpr-experts.com Last Reviewed: 1/7/03


Document Number PEN1011: Does your equipment room have a single point ground?

Notes:
Equipment racks and cabinets can provide an unwanted path for lightning surge energy. The common practice of bringing the antenna cable into the top of the cabinet and securing (bolting) the cabinet to the floor could damage the equipment.

Concrete floors, particularly those at grade level, can provide a conductive path to earth ground during the strike event. Any surge energy arriving on the coaxial cable, power and control wiring, or inadequate equipment ground system could find a lower inductance path to ground through your equipment and conductive concrete floor.

Your first line of defense is on the outside of the building. The value of a good tower ground/radial system (5 ohms or less) cannot be over emphasized. Most of the surge energy from a tower strike event is dispersed away from the building and equipment by the tower ground system. The remaining energy on the coaxial cable shield and center conductor can be directed safely to earth by using a PolyPhaser entrance bulkhead and proper coaxial protector. The entrance bulkhead provides a single point low inductance connection to the building’s perimeter ground and the tower radial system.

Your second line of defense is inside the building. Connect all overhead cable trays and equipment ground conductors back to the entrance bulkhead creating a single point ground system. AC power line protectors and control/data line protectors are connected to the single point ground to complete your defensive strategy.

A lightning strike is a series of fast rise time “pulses” that become a constant current source once the path to ground is established. A typical pulse has a rise time of 2us to 90% of peak and a 10 to 40us decay time (to the 50% level). Three pulses per event is the median. The average current is about 18,000 amps for the first pulse, then dropping to half that value for the second and third pulses. Ten percent of all strikes will exceed 60,000 amps on the first pulse and one percent will exceed 120,000 amps. For the following discussion, a 2us by 18kA negative strike pulse will be considered.

When lightning strikes, a tremendous rush of electrons move down the tower and out into the ground/radial system. Since we are dealing with a fast rise time event, the inductance of the tower and paralleled coaxial cables become the primary factor in determining the amount of instantaneous peak voltage developed between the strike point and the bottom of the tower.

The peak voltage drop can be calculated from the formula: E = L di/dt; where L is the total inductance of path (tower and coaxial cables in uH); di is 18,000 amps (average strike current, abbreviated 18kA); and dt is 2us (rise time).

When the coaxial cable leaves the tower at any elevation above grade level, a voltage divider is created at the take-off point to ground. The divided current, conducted by the coaxial cables towards the building, places the equipment in series with the energy path if the bottom of the rack is grounded.

Lets look at a practical example of a typical cell site installation, consisting of a 150 foot [45.72 M] by 35 inch [889 mm] triangular tower and three 1-5/8 inch [41.3 mm]coaxial cables. The tower and coax cables would have a combined inductance of approximately 15uH assuming that the cables ran from top of the tower to the grade level ground. During a typical 18kA strike event, a 135kV voltage drop would exist between the top of the tower and ground.

Even though the bulkhead entrance panel and appropriate protectors at a single point ground window do their job by conducting shield and center pin energy to ground, equipment damage or interruption may still result since shunt path currents could traverse your equipment cabinet.

In the real world, most coax lines do not run all the way to the bottom of the tower; but usually leave the tower at a convenient height to make an entrance near the ceiling of the equipment room. For example, if the coax lines were to leave the tower at the 10 foot [3.48 M] level, traverse 10 feet horizontally to the equipment building, attach to a ground bar via a ground kit prior to entering the building, and connect to the perimeter/tower ground system with two #6 AWG copper wires, the additional series inductance in parallel with the lower 10 foot section of the tower is calculated as follow:

Three parallel coax lines bent (+90°) leaving the tower = 0.05uH

Three parallel 10 foot lengths of 1 5/8 inch coax = 1.33uH

Ground kit and ground bar bends = 0.3uH

Two #6 AWG copper wires to earth ground = 2.2uH

The inductance of the 10 foot horizontal coax run and ground bar to earth ground is 3.88uH (3.9uH rounded for simplicity). The inductance of the lower 10 foot section of the tower without any coax lines is 2.7uH. When these two parallel inductors to ground are combined (3.9uH and 2.7uH), the lower section of tower has an inductance of 1.6uH.

The following questions may then be answered regarding conditions during the strike event.

Peak voltage at the 10 foot level on the tower: E=L di/dt = 14.4kV

Current through the ground bar to ground k(E/L)dt = 7.385kA

Peak voltage after the ground bar (at the protector): E = L di/dt. 9.231kV

Since the coaxial cable shield is connected to the ground bar via the ground kit, 9.2kV would be present on the shield and will be conducted directly to the antenna connector on your equipment rack. After charging the rack’s capacitance, the entire peak voltage would appear at the rack/concrete interface. Depending conductivity of the concrete and rebar placement with respect to the charged rack, 9.2kV could be sufficient voltage to arc through the concrete creating a conductive path and allowing current to flow through the equipment racks. The preferable alternative is to allow all racks to rise and fall with the overhead ground system potential without any other paths to ground. For this to happen you must insulate the racks from the conductive flooring.

Rack cabinets can act as a “faraday shield” with respect to the components inside by either converting the magnetic field energy into a voltage feeding your single point ground system or as a conductor between the overhead ground system and a conductive floor breakdown path. The breakdown path will produce a large voltage drop and magnetic fields inside the cabinet which are likely to destroy components.

To insulate rack cabinets from a potentially conductive concrete floor some installations place cabinets on wooden platforms. Others use isolation kits (0.062 [1.6 mm] phenolic insulating material and isolation grommets – PolyPhaser RACK ISO-KIT) for racks that must be physically secured to the floor.

The PolyPhaser Entrance Bulkhead serves as a low inductance entry ground, entry plate, and mounting point for PolyPhaser center pin protectors. Cable trays and system ground wires inside the building should be connected to the bulkhead to provide true “single point” grounding.

If a PolyPhaser Entrance Bulkhead with its two 6 inch [152.4 mm] copper straps were used in the previous example, the inductance path would be lowered to 2.7uH, and the voltage at the entrance bulkhead would be reduced to 5670 volts. With three copper straps the voltage would be even lower. The more surface area to ground the better.

Nothing can stop a lightning strike! How you direct the energy is the difference. Last Reviewed: 1/7/03


Document Number PEN1010: Oxidation and Joint Compounds.

Notes:
There are many different types of metals and each has desirable properties. However, when two dissimilar metals are joined to make an electrical connection there can be problems. Corrosion will begin when the connection is exposed to moisture or any other liquid acting as an electrolyte.

Corrosion is an electro-chemical process resulting in the degradation of a metal or alloy. Oxidation, pitting or crevicing, de-alloying, and hydrogen damage are a few descriptions of corrosion. Most metals today are not perfectly pure and consequently, when exposed to the environment, will begin to exhibit some of effects of corrosion.

Aluminum, as used in PolyPhaser’s coaxial protectors, has an excellent corrosion resistance due to a 1 nano-meter thick barrier of oxide film that instantaneously forms on the metal. Even if abraded, it will reform and protect the metal from any further corrosion. Any dulling, graying, or blacking that may subsequently appear is a result of pollutant accumulation.

Normally, corrosion is limited to mild surface roughening by shallow pitting with no general loss of metal. An aluminum roof after 30-years only had 0.076mm (0.003 inch) average pitting depth. An electrical cable lost only 0.109mm (0.0043 inch) after 51-years of service near Hartford, Connecticut. Copper such as C110 used in our equipment shelter coax cable entrance panels has been used for roofing, flashing, gutters, and downspouts. It is one of the most widely used metals for atmospheric exposure. Despite the formation of the green patina, copper has been used for centuries and has negligible rates of corrosion in unpolluted water and air. At high temperatures some copper alloys are better than stainless steel.

If copper were joined to aluminum or copper to galvanized (hot dipped zinc) steel with no means of preventing moisture from bridging the joint, corrosion loss will occur over time. This is the accelerated corrosion (loss) of the least noble metal (anode) while protecting the more noble (cathode) metal. Copper, in this example, is the more noble metal in both connections. (See the Noble Metal Table for a ranking of commonly used metals.)

Where the connection is with galvanized steel, the zinc coating will be reduced allowing the base steel to oxidize (rust), which in turn will increase the resistance of the connection and eventually compromise the integrity of the mechanical structure.

The aluminum will pit to the copper leaving less surface area for contact. The connection could be become loose, noisy, and even allow arcing.

This type of corrosion problem can be prevented by using a joint compound, covering and preventing the bridging of moisture between the metals. The most popular compounds use either zinc oxide or copper particles embedded in silicone grease. As the joint pressure is increased, the embedded particles dig into the metals and form a virgin low resistance junction void of air and its moisture.

The use of a joint compound is the recommended means for joining our coaxial protectors to our bulkhead panels for non-climate controlled installations. We have tested this compound with a “loose” 1 square-inch (6.5 sq-cm) copper to copper joint and have found it to handle a 25,500 ampere 8/20 waveform surge with no flash over and no change in resistance (0.001 ohms). We have even moved the loose joint before and after the surge and experienced no change in resistance.

The connection of a copper wire to galvanized tower leg should be avoided even if joint compound is used. The primary problem here is the low surface-area contact of the round wire with the (round) tower leg. Consider using a PolyPhaser TK series stainless steel clamp. The TK clamp will help increase the surface area of the connection as well as provide the necessary isolation between the dissimilar metals. Use joint compound on exposed applications of the TK clamps. For an even more effective connection, use copper strap in place of wire with the TK series clamp.

Silver oxide is the only oxide (that we know of) that is conductive. This is one reason why PolyPhaser’s N-type coax connectors are all silver with gold center pins. Copper oxide is not conductive and the proper application of joint compound will prevent oxidation.

Elements: MAG ALUM ZINC IRON CAD NICK TIN LEAD COPPER SILVER PALL GOLD
MAGNESIUM 0.0 -0.71 -1.61 -1.93 -1.97 -2.12 -2.23 -2.24 -2.71 -3.17 -3.36 -3.87
ALUMINUM 0.71 0.00 -0.90 -1.22 -1.26 -1.41 -1.52 -1.53 -2.00 -2.46 -2.65 -3.16
ZINC 1.61 0.90 0.00 -0.32 -0.36 -0.51 -0.63 -0.64 -1.10 -1.56 -1.75 -2.26
IRON 1.93 1.22 0.32 0.00 -0.04 -0.19 -0.30 -0.31 -0.78 -1.24 -1.43 -1.94
CADMIUM 1.97 1.26 0.36 0.04 0.00 -0.15 -0.27 -0.28 -0.74 -1.20 -1.39 -1.90
NICKEL 2.12 1.41 0.51 0.19 0.15 0.00 -0.11 -0.12 -0.59 -1.05 -1.24 -1.75
  TIN 2.23 1.52 0.63 0.30 0.27 0.11 0.00 -0.01 -0.47 -0.94 -1.12 -1.64
LEAD 2.24 1.53 0.64 0.31 0.28 0.12 0.01 0.00 -0.46 -0.93 -1.11 -1.63
COPPER 2.71 2.00 1.10 0.78 0.74 0.59 0.40 0.46 0.00 -0.46 -0.65 -1.16
SILVER 3.17 2.46 1.56 1.24 1.20 1.05 0.94 0.93 0.46 0.00 -0.19 -0.70
PALLADIUM 3.36 2.65 1.75 1.43 1.39 1.24 1.12 1.11 0.65 0.19 0.00 -0.51
GOLD 3.87 3.16 2.26 1.94 1.90 1.75 1.64 1.63 1.16 0.70 0.51 0.00
LESS NOBLE:
Noble Metal Table: Accelerated corrosion can occur between unprotected joints if the algebraic difference in atomic potential is greater than + / – 0.3 volts. Last Reviewed: 1/7/03


Document Number PEN1009: Galvanic action. Corrosive effects of soil conductivity. 
Notes:Most people have a tendency to use copper as for grounding because it is readily available, relatively economical, a good conductor, and one of the more noble metals. However, it does have a significant drawback. Since it is near the upper end of the table of noble metals, copper when put in direct contact with most common metals, which are lower in nobility, will cause accelerated corrosion of the lesser metal. The significance of being more noble means any other metal buried and connected to your copper ground system will be more anodetic and thus become sacrificial. (Also see Topic: Dissimilar Metals.)

A galvanized (hot dipped zinc) tower on a rebar reinforced concrete base is not at risk with a copper ground system. Since concrete is conductive and forms a Ufer ground, and rebar is a very hard (heat treated) steel which will rust, the rust on the surface of the rebar, which is a non-conductive oxide when dry, will prevent further oxidation. Since concrete readily retains moisture for long periods of time, the rust is actually a good conductor. Further corrosion reduction of the rebar, due to galvanic action, will be limited by this oxide layer.

A buried galvanized tower section (not the “J” bolts) or guy anchor embedded in concrete will have some galvanic currents that could cause the depletion of the zinc coating into the concrete. This will leave exposed steel that will continue to pit and may even de-alloy. This type of corrosion may take 20 to 30 years or more before the structural failure of the tower may occur.

In 1990, several towers were lost in Minnesota alone due to guy anchor corrosion and failure. Some contractors and manufacturers have now gone to the extent of tar/pitch dipping the anchors so this galvanic corrosion does not occur. This means that the anchor is now insulated from the surrounding Ufer ground! Proper guy wire and anchor grounding is essential under these (or any) circumstances. An improperly grounded insulated guy anchor can arc through the anchor’s pitch coating, cracking the concrete with resulting structural failure.

The better the soil conductivity, the more galvanic corrosion could occur. Doping soil with salts can increase the speed of the corrosion. Consequently, it is better to have an extensive radial and ground rod system rather than a smaller ground system with doped soil

For existing towers, look at the ratio of surface areas and ground resistance. The current density for a given current will be greater for material with a smaller surface area. The more extensive the copper grounding system close in around the anchor or tower base, the more the current density and galvanic corrosion on the anchors or tower base for a given earth resistance.

If you follow the recommended PolyPhaser method of using radials and ground rods that lead away from the tower base, guy anchors, and equipment building, the resultant distributed surface area and current must return through ever increasing ground distance/resistance. This makes the currents smaller than a ground system using concentric rings or a ground grid.

It may appear obvious that the use of similar materials would eliminate the galvanic problem While this is true, galvanized ground wire and ground rods are not normally recommended since the electric utility company will probably use a copper clad steel ground rod. Replacing the utility rod may be dangerous or impractical.

Another alternative is to install an active cathodic protection system. The system consists of a power supply (lightning protected of course) and a buried sacrificial anode element, such as zinc. The power supply will electrically elevate the tower section or anchor (negative) forcing galvanic currents through the sacrificial zinc anode (positive) element. The anode will deplete over time in the soil instead of your tower and guy anchors. If magnesium were substituted for zinc, the power supply can be eliminated since magnesium is more anodetic that galvanized steel. Last Reviewed: 01/07/03


 

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