The changing weather conditions in the past few years have caused severe floods, draughts, fire, heat waves, cold waves, and ice and lightning storms in many areas with no similar recorded previous history. For typical or conventional macro wireless communications sites with metallic structures, such as towers, lightning can be a great concern, depending on where in the world the installations are (see Photo 1). Not only do weather phenomena have effects on existing sites, the design of future sites also will be affected.
For a start, the resistivity of the soil and its effects on the grounding electrode systems at communications sites should be closely evaluated. Resistivity of soil is measured as Ω-centimeter by sampling a cube of soil from the concerned site. The measurement describes the resistance of the sampled soil of the opposite sides of the cube that are 1 centimeter apart. Low soil resistivity means less resistance and high conductivity. Values of 10,000 Ω-centimeter or lower are considered to be acceptable, thus requiring less investment or augmentation of ground fields.
A secondary consideration typically omitted is the frost line of a site. The frost line, also known as frost depth or freezing depth, is where ground water in soil is expected to freeze during periods of freezing temperatures. Where lightning strikes induce current flow in the earth, the earth’s moisture affects how much and where the current flows. Lack of moisture caused by freezing or extended drought impedes the safe flow of lightning energy into the earth.
A third concern is the inductance of the metallic structures involved. Inductance is a characteristic of a conducting metallic surface or wire that induces a reversed electromagnetic force (EMF [or voltage]) during a period of rapidly rising electrical current in the opposite direction of the lightning current, thus creating an electrical choke that impedes the flow of undesired lightning current. Most people refer to the effects of inductance as resistance or impedance.
In Ohms Law, V = ZI, where V is voltage, Z is impedance and I is current. An electron traveling on a conductor or on a conductive surface encounters three electrical elements: resistance, inductance and inherent capacitance. Two of the components, inductance and capacity reactance, are frequency-dependent.
Inductance is expressed as XL = 2πfL, where L is the metallic composition or thickness of conductive materials and f is the frequency. Conversely, inherent capacitance is expressed as Xc = 1/2πfC, where C is the inherent capacitance value between metallic surfaces and earth or adjacent metallic structures. The dielectric for the inherent capacitance is typically air that becomes ionized in a distance as much as 6 feet upon intercepting and conducting lightning current. In electrical and electronic systems, reactance is the opposition of a circuit element to a change in current or voltage, caused by that element’s inductance or capacitance. As the frequency rises, so does the inductive reactance. Capacitive reactance diminishes.
For direct-current (DC) systems, the frequency is zero. Therefore, the only remaining element is the resistance value of conductors and metallic surfaces. For frequency-dependent alternating current (AC), the collective sum of all three components is known as impedance (Z), expressed as:
Z = √(R2 + ( | XL − XC | )2
The inductance (L) and capacitance (C) values are the byproduct of their equivalent vector summation. The corresponding impedance impedes the flow of electrons of AC-based currents. Commonly used frequencies in AC circuits measuring 400 Hz or less have published impedance values for conductors.
Lightning theories are not exact science. For the most part, it is agreed upon in the scientific community that lightning cannot be stopped or eliminated. The best practice is to predict its occurrence and the corresponding effect it will have on structures or areas.
Lightning bolts develop from an equivalent DC system. However, once captured and recorded, lightning-induced electrical current appears as the equivalence of a resonating, decaying waveform with an average frequency of 100,000 Hz. The corresponding current values range from as low as 3 kiloamperes (KA) to 200 KA or more, where the percentage of the latter value is fraction of 1 percent.
Where high frequency AC currents are conducted, the corresponding currents tend to flow on the surface of conductive materials. This phenomenon is known as the skin effect. Also at lightning frequencies, the only component that matters is the inductance of the conductive surfaces. Inductances have similar characteristics as resistance (Rdc ). They add up in series, and their values are reduced or lowered in parallel configurations.
Back to basic physics. Currents will flow in closed loop and will take all available passages exhibiting low resistance, impedance or inductance. They always return to the sources that created them, following Ohms Law: V = ZI. Lightning strikes are a product of imposed voltages between the top of a structure and the bottom of a charged cloud. Once the dielectric between the two components becomes ionized, a lightning strike is a byproduct the ionization in which the oppositely charged cloud initiates a step leader that connects with the return leader from the corresponding tall structure.
In summary, lightning currents at telecommunications sites with towers will tend to flow on the available metallic surfaces with lowest inductance and that have the best skin-effect characteristics — such one or more of the towers on the site. The lightning protection designer’s objective is to safely conduct the lightning currents back to earth by using adequately designed grounding fields.
When Lightning Strikes
It is important to understand the increased exposure to lightning strikes and their potential effect on sensitive electronic equipment usually installed at communications sites. Doubling the voltage across the power and data input/output (I/O) terminals of an integrated circuit (IC) board will cause the IC board to fail. Conversely, impressed voltages below the double voltage values on IC board I/O terminals will cause deterioration of the IC boards, leading to errors and malfunctions.
Air terminals should be strategically located on top of tall structures to properly intercept the lightning strikes. It is equally important that all available structures and equipment are bonded together and grounded to the site’s grounding electrode system. This bonding will assist in safely conducting unwanted lightning energy to the source that created it, cloud to earth.
Successfully managing the flow of lightning strike relies on properly designed and maintained ground fields adequately installed below the frost line. Many standards address such installations with associated earth ground resistance required values. In general, the communications industry has adopted the long-standing military standard of an average value of 5 ohms of ground resistance or less. However, in order to properly implement such standard, the resistivity of a site and its corresponding frost-line value should be equally considered in association with the desired value of 5 ohms or less. Ground fields must be permanent, continuous and able to conduct electrical current. They must have the lowest possible impedance or inductance path all year round where sensitive electronics equipment is installed.
The traditional ground rings supplemented with ground rods at each corner of a structure and each leg of the tower must be evaluated and possibly augmented, modified or both in order to achieve the desired year-round earth ground resistance value.
The continuous validation and ongoing measurements of the grounding fields should also be part of the overall maintenance and operating scopes of such sites. Initially, the desired earth ground resistance value must be validated prior to site acceptance. This step happens during the commissioning process. Typically, a three-point-fall potential instrument or ground clamp-on instruments are used to validate the desired earth ground resistance readings (see Photos 2 and 3). Both tests are important; however, they are commonly misapplied and misinterpreted.
Communications equipment also needs to be protected by surge protective devices. The ground field is the drainage where surge protective devices almost instantaneously deflect unwanted lightning energy with the sole purpose of redirecting the energy safely back to earth while keeping the site on the air.
Effect of Climate Change
Many national and global standards contain discussions of the foregoing information and expressed concerns, together with valuable data recorded and collected over several decades. However, the recent fast changing of earth’s climate has introduced new variables without the support of sufficient data to assist with implementing solutions to offset the severity of climate changes. This situation is not only problematic for existing sites, it also is a great concern for the implementation of future sites, including, but not limited to, site selection, geographically speaking. You cannot manage what you cannot measure.
Aside from the obvious economic cost, chaos and unpredictability, vulnerability has accompanied recent severe and intense events. More frequent and severe hurricanes, droughts, fires, floods and record-breaking rain, snow and lightning seem to have become the new norm. Global temperatures higher than usual seem to be affecting many areas with no recorded historical data of such occurrences.
We cannot say with absolute certainty that climate changes are causing weather phenomena to change. Conversely, we know for a fact that for every cause there will be an effect. Consequently, innovation and new initiatives must be put in place to ensure the availability and reliability of existing and new communications sites. Conventional and traditional methods need to be challenged and rethought. Equally important is to keep an open mind by accepting and adopting new initiatives. Resolutions should be put in place for modifying conventional methods while adopting new ideas. A one-size-fits-all solution may not always be the proper approach.
It is never too late to act or to react to weather changes and their effects. Existing communications sites must be upgraded, and newly designed sites must be properly installed to make them resilient to the present and potential weather conditions. Proposed solutions, actions and measures have already been put in place to consistently assist in tackling the population growth and carbon foot print of industrialized and emerging economies. Most countries are in agreement that nature is the primary candidate in lowering, and possibly reversing, the climate change that many believe leads to be severe and destructive conditions.
Restoring or creating additional green spaces is crucial and vital in managing climate conditions by assisting with reducing the world’s carbon footprint and its environmental effect. Equally important are continuous innovation and investment in existing and new renewable sources of energy such as solar, wind, smart grids and smart cities (see Photos 4 and 5). The time to act is now, for many technical and economic reasons.
Aside from moral issue of the quality of life of future generations, the time to act is now for many technical and economic reasons because they are directly related to the communications industry. More and more reliance on communications equipment makes it necessary for the equipment to be available and reliable during catastrophic and severe weather. Towers and their associated ground fields should be periodically checked and validated to ensure their ongoing resilience despite their augmented exposure to more frequent severe weather and lightning.
To read more of the April AGL Magazine, click HERE
Sam Chreiteh is an independent critical systems consultant with Transtector Systems.