How to protect sites from GPR
Content
Cellular/PCS Networks
White Paper
How to Protect Sites from Ground Potential Rise
_____________________________________________________________________
Eliminating equipment damage caused by lightning and increasing the reliability of cellular and PCS networks, involves more than following the National Electric Code requirements. An important step entails preventing large fault currents from propagating in the copper conductors, connecting the cellular equipment to the carrier’s central office facility.
Today’s telecommunications network and services are evolving rapidly toward more use of cell phone and PCS services over traditional copper wire line telephony. These services not only include voice-grade communications, but are now providing high-speed data, email and Internet services as well as meter reading services for utilities. Research studies have confirmed this migration from wired to wireless services, fueling the explosive growth in the cell phone sector. More than 30 percent of all wireline subscribers have cellular or PCS services and are opting to make cellular service their only service provider. Market data reveals that wired telephone service growth is less than 2 percent and cellular service growth is at 10 percent to 15 percent annually. Cellular subscribers expect the same level of quality and availability of service as they have come to expect from wired services. Recently the FCC mandated a 24-hour battery back-up capacity for central office facilities and an 8-hour backup capacity for cell sites. The intent of the ruling is to maintain public communications capability during prolonged power outages, such as seen after severe hurricanes and tornadoes. These weather conditions, also accompanied by lightning strikes, are a principal source of equipment damage to cell site towers. Personnel safety risk still exists for wireless sites. The following information explains the exposure of wireless sites to ground potential rise events and how GPR can be effectively mitigated.
Anatomy of a Cell Site
Cellular and PCS sites consist of a tower, an equipment room or equipment cabinets containing the radio — commonly called the BTS or base transmitter station — and logic equipment that routes the wireless signal to and from the site via leased copper facilities to a local telephone central office. This connection is usually one or several T1 circuits connected to a customer service unit, commonly known as a “smart jack.” To extend the reach of T1 signals, CSU and high-bit-rate digital subscriber line encoding equipment is required. This electronic equipment is vulnerable to lightning damage if not properly protected. Given that T1 lines carry voice and data-multiplexed signals, they are a critical link in maintaining wireless network continuity.
Page 1 of 3
Cellular/PCS Networks
White Paper
Cellular and PCS site installations are bonded and grounded according to the National Electric Code, which spells out methods, intended to minimize lightning damage to equipment at the site. However, code specifications do not prevent GPR from occurring during the lightning strike. GPR causes significantly large fault currents to propagate in the copper conductors connecting the cellular equipment to the carrier’s central office facility, which acts as a remote ground path for the current to follow. The magnitude of GPR is a function of several things: • Amount of grounding electrode in contact with the soil (ground rods only = poor; ground mat = better) • Soil resistance • Other factors, such as, number of parallel conductors leaving the site (including power lines)
During lightning strikes, the GPR can be as high as several thousand volts. The damage caused by GPR can range from destruction of the CSU to complete destruction of the telecommunication cables feeding the site. Current grounding practices tend to protect the tower, hard-line and RF equipment to a great extent during a lightning strike. The application of shunt-type surge protectors will not prevent GPR from sending large fault currents from the site and from causing damage to facilities. Even if these protectors operate and protect the CSU, outages will result, ranging from a few hours for electronic card replacement to perhaps days for cable replacement. In fact, when a shunt-type arrestor fires, it connects the tower site ground directly to the central office facilities and causes fault currents to flow. The only way to protect a cell site and associated cables from GPR damage is through isolating all metallic wires of the telco line feeding the site. This type of isolation is provided by Positron’s Teleline and TeleLite HVI, or high voltage interface, systems. For years, Telcos have mandated such isolation equipment where cellular and PCS sites have been collocated on high voltage transmission towers. The rationale is that if an insulator failure were to cause the high voltage transmission line to short to the tower structure, the central office cable would be protected. This sound reasoning provides service continuity and reduces failure during a fault.
However, the number of insulator failures compared to lightning strikes is small. It is of utmost importance to follow through with the same protection philosophy to lightning damage, which is more predominant, than to isolator failures. The same applies for standalone cell sites. The majority of the more than 120,000 tower sites in the United States do not use HVI protection. This leaves the door wide open for equipment and critical service outages when these are most required.
Page 2 of 3
Cellular/PCS Networks
White Paper The Issue is Reliability
Wireless providers have developed “best practices” in the design of equipment and installations to provide for the best service possible. Wireless services are no longer viewed as an “optional convenience” but instead are considered to be an “essential” service. This perspective has become paramount in view of the accelerated growth of cell service, and in many cases, cell service is the only service a customer uses today. Cell sites must provide the same level of network reliability as that of traditional telcos and as such must employ similar protection philosophies as developed by telcos. Customers expect the same level of service from cell sites as they do from traditional telephone services. With more and more subscribers becoming completely dependent upon wireless service, reliability of wireless service has become a forefront issue, as evidenced by the FCC’s recent backup power requirement rulings. The key area of improvement in the chain nationwide is the use of proper GPR protection at all vulnerable cell sites in North America. The Wireline subcommittee of the IEEE Power Engineering Society, which is open to all stakeholders, has begun work on IEEE Standard 1692. The standard will provide reliable and “best practice” engineering methods and practices to help the industry correctly mitigate this vulnerability. Because reliability of wireless services has become a key issue, “best practice” would indicate that an appropriate Wireline or Fiber Optic High Voltage Interface be installed at all cell /PCS sites that connect to the telco central office through copper facilities. This philosophy will lead to more reliable service, when it’s needed most, and raise the bar in service level offered by cellular operators. This will without a doubt enhance the marketing success of these services with customers, a win – win situation.
Positron Inc., 5101 Buchan, Suite 220, Montreal, Quebec, H4P 2R9 Canada
Copyright © 2008, Positron Inc. All rights reserved
Page 3 of 3
White Paper
How to Protect Sites from Ground Potential Rise
_____________________________________________________________________
Eliminating equipment damage caused by lightning and increasing the reliability of cellular and PCS networks, involves more than following the National Electric Code requirements. An important step entails preventing large fault currents from propagating in the copper conductors, connecting the cellular equipment to the carrier’s central office facility.
Today’s telecommunications network and services are evolving rapidly toward more use of cell phone and PCS services over traditional copper wire line telephony. These services not only include voice-grade communications, but are now providing high-speed data, email and Internet services as well as meter reading services for utilities. Research studies have confirmed this migration from wired to wireless services, fueling the explosive growth in the cell phone sector. More than 30 percent of all wireline subscribers have cellular or PCS services and are opting to make cellular service their only service provider. Market data reveals that wired telephone service growth is less than 2 percent and cellular service growth is at 10 percent to 15 percent annually. Cellular subscribers expect the same level of quality and availability of service as they have come to expect from wired services. Recently the FCC mandated a 24-hour battery back-up capacity for central office facilities and an 8-hour backup capacity for cell sites. The intent of the ruling is to maintain public communications capability during prolonged power outages, such as seen after severe hurricanes and tornadoes. These weather conditions, also accompanied by lightning strikes, are a principal source of equipment damage to cell site towers. Personnel safety risk still exists for wireless sites. The following information explains the exposure of wireless sites to ground potential rise events and how GPR can be effectively mitigated.
Anatomy of a Cell Site
Cellular and PCS sites consist of a tower, an equipment room or equipment cabinets containing the radio — commonly called the BTS or base transmitter station — and logic equipment that routes the wireless signal to and from the site via leased copper facilities to a local telephone central office. This connection is usually one or several T1 circuits connected to a customer service unit, commonly known as a “smart jack.” To extend the reach of T1 signals, CSU and high-bit-rate digital subscriber line encoding equipment is required. This electronic equipment is vulnerable to lightning damage if not properly protected. Given that T1 lines carry voice and data-multiplexed signals, they are a critical link in maintaining wireless network continuity.
Page 1 of 3
Cellular/PCS Networks
White Paper
Cellular and PCS site installations are bonded and grounded according to the National Electric Code, which spells out methods, intended to minimize lightning damage to equipment at the site. However, code specifications do not prevent GPR from occurring during the lightning strike. GPR causes significantly large fault currents to propagate in the copper conductors connecting the cellular equipment to the carrier’s central office facility, which acts as a remote ground path for the current to follow. The magnitude of GPR is a function of several things: • Amount of grounding electrode in contact with the soil (ground rods only = poor; ground mat = better) • Soil resistance • Other factors, such as, number of parallel conductors leaving the site (including power lines)
During lightning strikes, the GPR can be as high as several thousand volts. The damage caused by GPR can range from destruction of the CSU to complete destruction of the telecommunication cables feeding the site. Current grounding practices tend to protect the tower, hard-line and RF equipment to a great extent during a lightning strike. The application of shunt-type surge protectors will not prevent GPR from sending large fault currents from the site and from causing damage to facilities. Even if these protectors operate and protect the CSU, outages will result, ranging from a few hours for electronic card replacement to perhaps days for cable replacement. In fact, when a shunt-type arrestor fires, it connects the tower site ground directly to the central office facilities and causes fault currents to flow. The only way to protect a cell site and associated cables from GPR damage is through isolating all metallic wires of the telco line feeding the site. This type of isolation is provided by Positron’s Teleline and TeleLite HVI, or high voltage interface, systems. For years, Telcos have mandated such isolation equipment where cellular and PCS sites have been collocated on high voltage transmission towers. The rationale is that if an insulator failure were to cause the high voltage transmission line to short to the tower structure, the central office cable would be protected. This sound reasoning provides service continuity and reduces failure during a fault.
However, the number of insulator failures compared to lightning strikes is small. It is of utmost importance to follow through with the same protection philosophy to lightning damage, which is more predominant, than to isolator failures. The same applies for standalone cell sites. The majority of the more than 120,000 tower sites in the United States do not use HVI protection. This leaves the door wide open for equipment and critical service outages when these are most required.
Page 2 of 3
Cellular/PCS Networks
White Paper The Issue is Reliability
Wireless providers have developed “best practices” in the design of equipment and installations to provide for the best service possible. Wireless services are no longer viewed as an “optional convenience” but instead are considered to be an “essential” service. This perspective has become paramount in view of the accelerated growth of cell service, and in many cases, cell service is the only service a customer uses today. Cell sites must provide the same level of network reliability as that of traditional telcos and as such must employ similar protection philosophies as developed by telcos. Customers expect the same level of service from cell sites as they do from traditional telephone services. With more and more subscribers becoming completely dependent upon wireless service, reliability of wireless service has become a forefront issue, as evidenced by the FCC’s recent backup power requirement rulings. The key area of improvement in the chain nationwide is the use of proper GPR protection at all vulnerable cell sites in North America. The Wireline subcommittee of the IEEE Power Engineering Society, which is open to all stakeholders, has begun work on IEEE Standard 1692. The standard will provide reliable and “best practice” engineering methods and practices to help the industry correctly mitigate this vulnerability. Because reliability of wireless services has become a key issue, “best practice” would indicate that an appropriate Wireline or Fiber Optic High Voltage Interface be installed at all cell /PCS sites that connect to the telco central office through copper facilities. This philosophy will lead to more reliable service, when it’s needed most, and raise the bar in service level offered by cellular operators. This will without a doubt enhance the marketing success of these services with customers, a win – win situation.
Positron Inc., 5101 Buchan, Suite 220, Montreal, Quebec, H4P 2R9 Canada
Copyright © 2008, Positron Inc. All rights reserved
Page 3 of 3
