Rohit Narayan 2017-06-21 08:42:20
From capturing the lightning strike to protecting low-voltage data and telecommunications circuits and everything in between, a combination of devices and methods proves successful in protecting telecommunications facilities. A six-point protection plan will protect any facility from the damaging effects of lightning, transient voltages, noise and other disturbances. Capture the lightning strike — On a telecommunications tower as high as 140 feet, a simple rod of the Franklin air terminal type adequately captures lightning. When users install electronic equipment, such as remote radio heads, on top of the tower, they may find some merit in using an isolated Franklin rod air terminal to facilitate the use of isolated down conductors. To achieve the isolation, some install an air terminal on a 6-foot fiberglass mast. Other mission- critical telecommunications facilities, such as central offices, data centers and major repeater sites, also may need lightning protection. Lightning protection systems for buildings include conventional lightning protection installed to the NFPA 780 standard using a smooth-weave cable system, an early streamer emission system and proprietary systems, such as the Erico Dynasphere air terminal. Convey the energy to ground — Lightning protection specialists use two ways of mitigating side-flashing and flash-over risk: bonding and isolation. With isolation difficult to achieve, designers more commonly use bonding. Where communications systems use remote radio heads or where cable or feeder trays have been installed close to one tower leg, isolated systems have advantages. An isolated system can be used to bypass lightning energy and dissipate it into the tower leg at a distance below remote radio heads. Neither bonding nor isolation can mitigate all risks associated with lighting. Also, secondary effects, such as magnetic coupling, still will occur. An essential part of any telecommunications facility, including outdoor cabinets and outside plant, the outdoor grounding electrode is the part of the system that dissipates excess lightning energy into ground. Dissipate the energy into a low-impedance grounding system — The ITU K56 Recommendation, “Protection of Radio Base Stations Against Lightning Discharges,” provides an excellent depiction of the grounding system with ground rings around the building, around the masts and in the perimeter of the compound. The Telcordia Technologies Generic Requirements GR-3171-Core, “Generic Requirements for Network Elements Used in Wireless Networks,” makes extensive recommendations on the use of ring ground electrodes. This document states that the use of ground rings is a method to “minimize the differential potentials and induced current flow across the facility.” As does ITU K56, the Telcordia GR-3171 promotes the use of ground rings around buildings and towers. Where space constraints do not allow placing a ring that encircles the entire structure, alternative arrangements can be used. To ensure that the rings produce low earth potential rise (EPR) during lightning current flow, designers must pay attention and keep the ring installations as symmetrical as possible (see Figure 1.) Figure 1. A general view of earthing and bonding procedures external to a shelter, as shown in ITU K56 recommendations. To ensure that the rings produce low earth potential rise (EPR) during lightning current flow, designers must pay attention and keep the ring installations as symmetrical as possible. Resistance to Remote Earth It is widely accepted in the industry that 5 ohms of resistance to a remote ground is the highest allowable value for any telecommunications facility. Sometimes resistance this low isn’t achieved in areas with high soil resistivity or on sites with limited space for installing an earth grid (i.e., at roadside cabinets and in built-up areas). More complex sites, such as central offices, mobile switching centers, larger repeater sites and satellite stations, require lower ground resistances between 0.5 ohms and 2 ohms. Telecommunications operators themselves define the ground resistance values in internal standards and guidelines. Using predetermined design often achieves proper levels of ground resistance. Tower owners who want to have consistent designs at multiple sites often use predetermined designs. Sometimes the approach produces varying results because of differences in soil resistivity among sites. A more scientific approach to designing the ground electrode includes a soil resistivity test prior to starting the installation. Computer software or empirical formulas can use the results to predict the number and dimensions of ground electrodes required to meet the target ground resistance. Lightning protection equipment manufacturers recommend the scientific approach for larger sites where resistance values lower than 5 ohms are desired or where soil resistivity is high. Many use the Wenner method and measure soil resistivity at various depths at four points. Photos 1A and 1B. CU bond composite and CU bond solid conductors. Using modern conductors such as these helps to reduce copper theft at telecommunications sites. These conductors also cost less and last longer. Choice of Grounding Materials Conductor — Grounding systems most commonly use copper wire for horizontal connections in the ground. Copper has excellent electrical conductivity, and It offers good resistance from corrosion in a wide range of soil conditions. These characteristics make it an ideal conductor. However, copper is relatively expensive, and it often tempts thieves when used in exposed locations. U.S. standards recommend using copper conductors equivalent to AWG #2. Figure 2. A Cadweld exothermic weld has high reliability, long life, a low corrosion rate and a low relative cost. Commonly used for grounding system connections, exothermic welds even work with tape conductors and vertical surfaces. With an adequate coating, copperbonded steel conductors have a long service life similar to copper and the lower cost of steel, compared with copper. Steel makes the conductors rugged and more difficult to handle. They come in solid and stranded sizes equivalent to AWG #2. Photo 2. Copper-bonded ground rods with couplers provide the best long-term corrosion resistance relative to their cost. Reasons behind the need for modern conductors include increasing incidences of copper theft at telecommunication sites, a desire for long grounding system service life and a desire to keep the cost of conductors from increasing, in comparison with alternatives (see Photos 1A and 1B). Photo 3. A cross section of an exothermic weld shows how it combines metal between a stranded conductor and a solid conductor. These welds work with a wide range of materials and shapes of conductors. Ground electrodes — The three most common types of ground electrodes use copper-bonded steel. Copperbonded ground rods provide the best long-term corrosion resistance relative to their cost (see Photo 2). Grounding connectors — In telecommunications, the most common grounding system connections use exothermic welds, bolted connectors and crimped connectors. Among exothermic welds, Cadweld connections are the most common type used with copper-based grounding system in large parts of the world. Some reasons for their popularity include high reliability, long life, a low corrosion rate and a low relative cost. Exothermic welds can be used on a wide range of materials and shapes of conductors, including tape conductors and vertical surfaces (see Figure 2 and Photo 3). Figure 3. On the left, a Star-IBN, and on the right, a Mesh-IBN, with single-point connection windows (SPCWs). These isolated bonding networks are found in telecommunications facilities around the world. Ground bars — Tinned and bare copper has been the material of choice for telecommunications ground bars for many decades. Increasingly common copper theft at telecommunications facilities has led to carriers and tower companies to look for suitable alternatives for copper for use as ground bars. These include tinned aluminum and galvanized steel with a suitable amount of zinc coating (see Photo 4). Bond all grounding points together — In the opinion of the author, the indoor grounding arrangement is the most important aspect of the grounding system design. This aspect of grounding also is more likely to contribute to equipment faults, in comparison with the four other points. Among the six points of the plan for lightning protection, bonding all the grounding points together generally is the step with the lowest relative cost. Not surprisingly, it is sometimes given the least importance. Photo 4. A galvanized ground bar with a pigtail. ITU-K27 describes the two methods for bonding all the grounding points together commonly used in telecommunications facilities around the world: the Star-IBN and the Mesh-IBN (IBN stands for isolated bonding network). In the Star-IBN system, the indoor grounding system connects via a single-point connection window (SPCW) to the ground electrode system. The SPCW usually takes the form of a ground bar, but it also can be a ground ring inside the telecommunications facility (see Figure 3). Figure 4. Controlling energy that confronts a telecommunications facility from outside the facility requires the proper installation of surge protection devices. In the Mesh-IBN system, components inside the telecommunications facility (e.g., equipment frames) are interconnected to form a mesh-like structure. This may, for example, be achieved with multiple interconnections between cabinet rows or by connecting all equipment frames to a metallic-grid bonding mat or signal reference grid extending beneath the equipment. The bonding mat is insulated from the common bonding network (CBN) of the adjacent room or building. Although there are multiple connection paths within the equipment room, there is only one single point via which the Mesh-IBN connects to the external ground electrode system (see Figure 3). The comprehensive Telcordia GR-295 document “Mesh and Isolated Bonding Networks: Definition and Application to Telephone Central Offices” details how to construct Star-IBN, Mesh-IBN and Mesh-BN systems. There is consistency in methodologies recommended in ITU-K27 and Telcordia GR-295. Photos 5A and 5B. Erico models TDX100M and SES40P surge protection devices. The devices must switch on quickly and handle large amounts of energy in a short time. Protect incoming AC and DC power feeders — The installation of the indoor and the outdoor grounding system addresses the safety, electromagnetic compatibility, lightning energy dissipation and noise control system at a telecommunications facility. Grounding alone does not fully control what happens outside of the facility that could transfer a transient or a surge to the facility. Obtaining this level of control requires proper installation of surge protection devices (SPDs) (see Figure 4). AC surge protection — Lightning strikes near power lines or other power system disturbances, such as switching, can couple voltage transients or surges onto power lines. Lightning can couple onto power lines via a direct strike to the power line or via capacitive and magnetic coupling when lightning strikes nearby. The IEC 61643 suite of standards and the Institute of Electrical and Electronics Engineers (IEEE) trilogy of standards documents (C62.4.1, C62.4.2 and C62.45) cover SPDs for AC application in detail. The IEEE has published additional standards for testing and using SPDs. UL 1449 4th Edition defines the requirements for SPDs designed for repeated limiting of transient voltage surges and is arguably the most onerous standard for safety testing of SPDs. The most common topology for an SPD is the shunt connection. Various types of SPDs include metal oxide varistors (MOVs), silicone avalanche diodes (SADs), gas arrestors, spark gaps and triggered spark gaps. No one device type is superior to other device types. Instead, each has advantages and disadvantages, and they need to be chosen correctly for the application. In a shunt application, a user installs a surge diverter between the phase and the neutral lines and between the neutral line and earth ground. SPDs are normally open-circuit, but turn on when a higher voltage appears across its terminals during a transient voltage or a surge current. They momentarily create a short circuit to ground to allow the surge energy to divert to the ground instead of going to the load. Lightning surges and other power system transients are quite fast (durations of a few tens of microseconds) and can have extremely high amplitudes (many thousands of volts). Therefore, to be effective, SPDs must switch on quickly and handle large amounts of energy in a short time. Normally, upstream circuit breakers or fuses do not have time to trip when the surge diverter activates because the reaction time of circuit breakers and fuses is much slower (see Photos 5A and 5B). DC surge protection — Modern cellular and microwave equipment uses a remote radio unit (RRU) or a remote radio head (RRH) fed from the base station via optical fiber. This method eliminates feeder loss and allows transmission to occur at much higher frequencies with wider bandwidth. Power cannot be transferred from the base station to the RRU or the RRH via optical fiber. Thus, the power supply feeds power to the remote units separately as DC on copper cables. The copper cables are either separate from the fiber or are part of a composite fiber-copper cable. The DC feed acts as a source of lightning surges back into the equipment room. In the traditional radio settings of the past, damage normally would have been limited to the radio equipment. In modern installations, damage can occur to the rectifiers or the entire DC power system, which then would jeopardize other equipment installed at the site. This development has heightened the need for DC surge protection. Protect low-voltage data and telecommunications circuits — Surges and transients caused by lightning can couple to telephone lines and RF feeders via magnetic or capacitive coupling. Where telephone lines run parallel to power lines for longer distances, the surges and transients can also couple to the lines through electrical induction. Leaving these communication lines unprotected may still leave the facility open to potential damage, even if other elements of this plan are implemented. Telephone and data lines — Twisted pair (ordinary copper wire) transports the telecoms services subject to this discussion. The services may be telephone lines or services such as Category 5 (Cat 5) and Category 6 (Cat 6) cable. Each service has two wires, or lines, sometimes called the a and b wires. With telephone and data lines, surges can occur from each line to ground, known as L-G or common mode, or can occur across the lines, known as L-L or differential mode. Protection against these surges requires the use of appropriately designed surge protective devices. Coaxial feeders — The magnitude of surges that can be coupled onto the signal in a coaxial feeder is relatively small in comparison with total lightning energy. This is because the telecommunications tower and the cable ladders provide significant shielding to the feeder. Furthermore, the construction of the coaxial cable provides excellent shielding to the inner conductor. Differential mode transients are not possible because there is one signal line and the other is the feeder screen, which is directly grounded at several locations. Furthermore, there is some level of sharing of the induced and coupled currents among multiple parallel runs of feeders. Despite this level of shielding and sharing, it remains possible to have several thousands of amps at a fast rise-time coupled or induced onto coaxial feeders. As a minimum, the coaxial feeders should be grounded at the top of the telecom tower, at the point where they bend close to the ground and at the point of entry to the equipment room or cabinet. The feeder tray should be kept continuous in its trajectory along the tower. The feeder tray should be continuous when it leaves the tower and extends toward the equipment room or cabinet, preferably using a curved section at bends. Where additional precaution is needed, coaxial surge protectors with appropriate connector type, bandwidth and surge ratings can be installed at the point of entry to the equipment building or the cabinet. The Six-point Plan of Protection from Erico Capture the lightning strike. Convey lightning energy to ground. Dissipate energy into a low-impedance grounding system. Bond all ground points together. Bond all ground points to eliminate ground loops and create an equipotential plane. Protect incoming AC and DC power feeders. Protect equipment from surges and transients on incoming power lines to prevent equipment damage and costly operational downtime. Protect low-voltage data and telecommunications circuits. Protect equipment from surges and transients on incoming telecommunications and signal lines to prevent equipment damage. Rohit Narayan has 27 years of experience as an electrical and telecommunications engineer and has a passion for telecommunications power systems and grounding. He has helped develop grounding standards, drawings and specifications for various telecommunications carriers. He is global sales director for telecom at Erico. Visit www.erico.com.
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