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8/7/2019 How to protect sites from GPR
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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 ofcell phone and PCS services over traditional copper wire line telephony. These services notonly include voice-grade communications, but are now providing high-speed data, email andInternet services as well as meter reading services for utilities.
Recently the FCC mandated a 24-hour battery back-up capacity for central office facilities andan 8-hour backup capacity for cell sites. The intent of the ruling is to maintain publiccommunications capability during prolonged power outages, such as seen after severehurricanes 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 theexposure of wireless sites to ground potential rise events and how GPR can be effectivelymitigated.
Anatomy of a Cell SiteCellular and PCS sites consist of a tower, an equipment room or equipment cabinetscontaining the radio — commonly called the BTS or base transmitter station — and logicequipment that routes the wireless signal to and from the site via leased copper facilities to alocal telephone central office. This connection is usually one or several T1 circuits connectedto 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. Thiselectronic 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 wirelessnetwork continuity.
Cellular/PCS Networks
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White Paper
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Research studies have confirmed this
migration from wired to wireless services,fueling the explosive growth in the cell phonesector. More than 30 percent of all wirelinesubscribers have cellular or PCS services andare opting to make cellular service their onlyservice provider. Market data reveals that
wired telephone service growth is less than 2percent and cellular service growth is at 10percent to 15 percent annually. Cellularsubscribers expect the same level of qualityand availability of service as they have come to
expect from wired services.
8/7/2019 How to protect sites from GPR
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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, codespecifications do not prevent GPR from occurring during the lightning strike. GPR causes significantlylarge 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 GPRcan 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 sendinglarge fault currents from the site and from causing damage to facilities. Even if these protectors operateand protect the CSU, outages will result, ranging from a few hours for electronic card replacement toperhaps days for cable replacement. In fact, when a shunt-type arrestor fires, it connects the tower siteground 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 metallicwires of the telco line feeding the site. This type of isolation is provided by Positron’s Teleline and TeleLiteHVI, or high voltage interface , systems.
However, the number of insulator failures compared to lightning strikes is small. It is of utmostimportance 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 morethan 120,000 tower sites in the United States do not use HVI protection. This leaves the door wide openfor equipment and critical service outages when these are most required.
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For years, Telcos have mandated such isolationequipment where cellular and PCS sites have
been collocated on high voltage transmissiontowers. The rationale is that if an insulator failurewere to cause the high voltage transmission lineto short to the tower structure, the central officecable would be protected. This sound reasoningprovides service continuity and reduces failure
during a fault.
8/7/2019 How to protect sites from GPR
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Cellular/PCS Networks
The Issue is Reliability
Wireless providers have developed “best practices” in the design of equipment and installations toprovide 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 customeruses today. Cell sites must provide the same level of network reliability as that of traditional telcos and assuch must employ similar protection philosophies as developed by telcos. Customers expect the samelevel of service from cell sites as they do from traditional telephone services.
With more and more subscribers becoming completely dependent upon wireless service, reliability ofwireless service has become a forefront issue, as evidenced by the FCC’s recent backup powerrequirement 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, hasbegun 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 tothe telco central office through copper facilities. This philosophy will lead to more reliable service, whenit’s needed most, and raise the bar in service level offered by cellular operators. This will without a doubtenhance the marketing success of these services with customers, a win – win situation.
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White Paper
Positron Inc., 5101 Buchan, Suite 220, Montreal, Quebec, H4P 2R9 CanadaCopyright © 2008, Positron Inc. All rights reserved