Ieee standard for the electrical protection of communication facilities serving electric supply loca

IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

IEEE Power and Energy Society

Sponsored by the Power System Communications Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA

IEEE Std 487.1â„¢-2014

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IEEE Std 487.1â„¢-2014

IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment Sponsor

Power System Communications Committee of the

IEEE Power and Energy Society Approved 3 November 2014

IEEE-SA Standards Board

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Abstract: Workable methods for protecting wire-line telecommunication circuits entering electric supply locations are presented. The electric supply location environment; protection apparatus; service types, reliability, service performance objective classifications, and transmission considerations; protection theory and philosophy; protection configurations; installation and inspection; and safety are covered in this document.

Keywords: electric supply locations, high-voltage tower, IEEE 487.1™, power stations, protection, wire-line telecommunication •

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2014 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 7 November 2014. Printed in the United States of America.

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Participants At the time this IEEE standard was completed, the Wire-Line Working Group had the following membership: Percy Pool, Co-Chair and Technical Editor Larry Young, Co-Chair and Secretary Joe Boyles Steve Blume Tim Conser Bhimesh Dahal Jean DeSeve

Ernest Duckworth John Fuller Ernest Gallo Dave Hartmann

Dan Jendek Richard Knight Randall Mears Mark Tirio Thomas Vo

The following members of the individual balloting committee voted on this standard. Balloters may have voted for approval, disapproval, or abstention.

William Ackerman R. Baysden Joe Boyles Gustavo Brunello William Bush Bhimesh Dahal Douglas Dorr Ernest Duckworth John Fuller Frank Gerleve Jalal Gohari Randall Groves

Noriyuki Ikeuchi Yuri Khersonsky Richard Knight Jim Kulchisky Lawrenc Long Albert Martin William McCoy Joseph Mears Jerry Murphy Michael Newman Gary Nissen

James O'Brien Lorraine Padden Percy Pool Charles Rogers Bartien Sayogo Mark Simon Mark Tirio John Vergis Thomas Vo John Wang Kenneth White Larry Young

When the IEEE-SA Standards Board approved this standard on 27 October 2014, it had the following membership: John Kulick, Chair Jon Walter Rosdahl, Vice Chair Richard H. Hulett, Past Chair Konstantinos Karachalios, Secretary Peter Balma Farooq Bari Ted Burse Clint Chaplin Stephen Dukes Jean-Philippe Faure Gary Hoffman

Michael Janezic Jeffrey Katz Joseph L. Koepfinger* David J. Law Hung Ling Oleg Logvinov T. W. Olsen Glenn Parsons

Ron Petersen Adrian Stephens Peter Sutherland Yatin Trivedi Phil Winston Don Wright Yu Yuan

*Member Emeritus

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Also included are the following nonvoting IEEE-SA Standards Board liaisons: Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative Patrick Gibbons IEEE-SA Content Publishing Erin Spiewak IEEE-SA Technical Community Programs

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Introduction This introduction is not part of IEEE Std 487.1™-2014, IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment.

Wire-line telecommunication facilities serving electric supply locations often require special high-voltage protection against the effects of fault-produced ground potential rise (GPR) or induced voltages, or both.

Some of the telecommunication services are used for control and protective relaying purposes and may be called upon to perform critical operations at times of power-system faults. This requirement presents a major challenge in the design and protection of the telecommunication system because power-system faults can result in the introduction of interfering voltages and currents into the telecommunication circuit at the very time when the circuit is most urgently required to perform its function. Even when critical services are not involved, special high-voltage protection may be required for both personnel safety and plant protection at times of power-system faults. Effective protection of any wire-line telecommunication circuit requires coordinated protection on all circuits provided over the same telecommunication cable.

Some electrical environments, collectively called electric supply locations, require the application of unique electrical protection techniques because of their special nature. One such environment is the electric power station or substation. Another is at, or near, power line transmission and distribution structures such as towers or poles. Such structures often provide a convenient site for the location of wireless, personal communications service, and cellular antennas and their associated electronic equipment that is served by a link to the wired telecommunications network.

This standard presents workable methods for protecting wire-line telecommunication circuits entering electric supply locations. It is important to note that special high-voltage protection for the purpose of personnel safety and plant protection may be required even when critical services are not involved. In the case of leased circuits, mutually agreeable methods for the installation of protective equipment owned by either party are presented.

This project is part of a reorganization of IEEE Std 487™ a in which the main document is broken down into a family of related documents (i.e. dot-series) segregated on the basis of technology: ⎯

IEEE 487™: general considerations

⎯

IEEE 487.1™: for applications using on-grid isolation equipment involving metallic wire-line

⎯

IEEE 487.2™: for applications consisting entirely of optical fiber cables

⎯

IEEE 487.3™: for applications of hybrid facilities where part of the circuit is on metallic wire-line and the remainder of the circuit is on optical fiber cable

⎯

IEEE 487.4™: for applications using neutralizing transformers

⎯

IEEE 487.5™: for applications using isolation transformers

This standard covers the use of modular-type on-grid isolators, either transformer or optical, for the electrical protection of wire-line (metallic) telecommunications facilities serving electric supply locations.

The use of discrete hard-wired isolation transformers is covered in IEEE Std 487.5 [B21]b.

a b

Information on references can be found in Clause 2.

The numbers in brackets correspond to those of the bibliography in Annex A.

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This standard has been prepared by the Wire-Line Subcommittee of the Power System Communications Committee of the IEEE Power and Energy Society. This standard represents the consensus of both power and telecommunications engineers.

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Contents 1. Overview .................................................................................................................................................... 1 1.1 Background .......................................................................................................................................... 1 1.2 Scope ................................................................................................................................................... 2 1.3 Purpose ................................................................................................................................................ 2 2. Normative references.................................................................................................................................. 2 3. Definitions, acronyms, and abbreviations .................................................................................................. 2 3.1 Definitions ........................................................................................................................................... 2 3.2 Acronyms and abbreviations ............................................................................................................... 4 4. High-voltage environment .......................................................................................................................... 5 4.1 Locations ............................................................................................................................................. 5 4.2 Practical implications........................................................................................................................... 5 4.3 Earth potential profile .......................................................................................................................... 6 4.4 Cables in high-voltage environments .................................................................................................. 7 5. Protection theory and philosophy ............................................................................................................... 7 5.1 Background .......................................................................................................................................... 7 5.2 Concepts and concerns ........................................................................................................................ 9 5.3 Service performance objective (SPO) classifications .........................................................................10 5.4 Responsibilities for installation and maintenance ...............................................................................10 5.5 General-use telecommunication cable in the electric supply location GPR ZOI ................................10 5.6 Dedicated cable...................................................................................................................................11 5.7 Special wire-line protection design requirements ...............................................................................13 6. Protection configurations ...........................................................................................................................14 6.1 Ground potential rise (GPR) plus induced voltage levels ...................................................................14 6.2 Basic protection system ......................................................................................................................14 6.3 Protection configurations employing isolation devices ......................................................................16 6.4 General isolation protection configuration .........................................................................................19 6.5 Protection configuration employing modular high dielectric and optic isolators ...............................21 6.6 Protection practices for electric supply locations services ..................................................................21 7. Periodic inspection considerations ............................................................................................................26 8. Safety .........................................................................................................................................................27 8.1 General safety considerations .............................................................................................................27 8.2 Safety considerations in equipment design .........................................................................................27 8.3 Safety considerations related to installation and maintenance ............................................................28 Annex A (informative) Bibliography ............................................................................................................30 Annex B (informative) Telecommunications cable in the electric supply location GPR ZOI.......................32

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IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, security, health, or environmental protection, or ensure against interference with or from other devices or networks.

Implementers of IEEE Standards documents are responsible for determining and complying with all appropriate safety, security, environmental, health, and interference protection practices and all applicable laws and regulations.

This IEEE document is made available for use subject to important notices and legal disclaimers.

These notices and disclaimers appear in all publications containing this document and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/disclaimers.html.

1. Overview

1.1 Background Wire-line telecommunication1 facilities serving electric supply locations often require special high-voltage protection against the effects of fault-produced ground potential rise (GPR), induced voltages, or both.

Some of the telecommunication services are used for control and protective relaying purposes and may be called upon to perform critical operations at times of power-system faults. This requirement presents a major challenge in the design and protection of the telecommunication system because power-system faults can result in the introduction of interfering voltages and currents into the telecommunication circuit at the very time when the circuit is most urgently required to perform its function. Even when critical services are not involved, special high-voltage protection may be required for both personnel safety and plant protection at times of power-system faults. Effective protection of any wire-line telecommunication circuit requires coordinated protection on all circuits provided over the same telecommunication cable.

1

In general, “wire-line telecommunication” will be referred to throughout this document as “telecommunication.”

1

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

1.2 Scope This standard presents engineering design procedures for the electrical protection of metallic wire-line telecommunication facilities serving electric supply locations through the use of on-grid isolation equipment. Other telecommunication alternatives, such as radio and microwave systems, are excluded from this document.

1.3 Purpose This standard presents workable methods that shall be used with greater reliability to improve the electrical protection of metallic wire-line telecommunication facilities serving electric supply locations through the use of on-grid isolation equipment.

2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies.

IEEE Std 80™, IEEE Guide for Safety in AC Substation Grounding. 2, 3 IEEE Std 367™, IEEE Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault.

IEEE Std 487™, IEEE Recommended Practice for the Protection of Wire-line Communication Facilities Serving Electric Supply Locations.4 IEEE Std 789™, IEEE Standard Performance Requirements for Communications and Control Cables for Applications in High-Voltage Environments.

IEEE Std C37.93™, IEEE Guide for Power System Protective Relay Applications of Audio Tones Over Voice Grade Channels.

3. Definitions, acronyms, and abbreviations

3.1 Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 5

2

IEEE publications are available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08855, USA (http://standards.ieee.org/).

3 The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc. 4 There is an approved PAR to revise Std 487 and re-title it to “Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations - General Considerations.”

5 IEEE Standards Dictionary Online subscription is available at: http://www.ieee.org/portal/innovate/products/standard/standards_dictionary.html.

2

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

demarcation point: The point of interconnection between the telecommunications facilities of a provider of wire-line telecommunications service and the subscriber’s terminal equipment, protective apparatus, cables or wiring. drainage units or drainage reactors: Center-tapped inductive devices designed to relieve conductor-toconductor and conductor-to-ground voltage stress by draining extraneous currents to ground. These devices are also designed to serve the purpose of a mutual drainage reactor (MDR), forcing near-simultaneous surge protective device (SPD)-gap operation. electric supply locations: Any building, separate space, or site in which electric supply equipment is located that may be subjected to the effects of ground potential rise (GPR). These locations include generation, transformation, conversion, switching, and delivery facilities. gas-filled protector (gas discharge tube): A discharge gap between two or more electrodes hermetically sealed in a ceramic or glass envelope. ground potential rise (GPR): The electrical potential that a ground electrode (or grounding system) may attain relative to a distant grounding point.

NOTE 1— Under normal conditions, the grounded electrical equipment operates at near zero ground potential. That is, the potential of a grounded neutral conductor is nearly identical to the potential of remote earth. During a ground fault the portion of fault current that is conducted by an electric supply location grounding grid into the earth causes the rise of the grid potential with respect to remote earth.

NOTE 2— See IEEE Std 367 for the method of calculating GPR

high-dielectric cable: Cable that provides high-voltage insulation between conductors, between conductors and shield, and between shield and earth. high-voltage disconnect jack: A device used to disconnect cable pairs for testing purposes. Used to help safeguard personnel from remote ground potentials. high-voltage interface (HVI): Protective apparatus that provides electrical isolation of wire-line telecommunications conductive paths. isolation transformers: Transformers that provide longitudinal (common-mode) isolation of the telecommunication facility. They can be designed for use in a combined isolating-drainage transformer configuration and also can be designed for a low longitudinal to metallic conversion. messenger: See suspension strand. metallic member: A non-communications metallic cable component such as a shield, vapor barrier, or strength member. optic coupling device: An isolation device using an optical link to provide the longitudinal isolation.

Circuit arrangements on each side of the optical link convert the electrical signal into an optical signal for transmission through the optical link and back to an electrical signal. Various circuit arrangements provide one-way or two-way transmission and permit transmission to the various combinations of voice and/or dc signaling logic used by the power industry. Single-channel optic coupling devices may be used in conjunction with other isolation devices in protection systems. optical fiber cable: A telecommunications cable in which one or more optical fibers are used as the propagation medium. The optical fibers are surrounded by buffers, strength members, and jackets for protection, stiffness, and strength. An optical fiber cable may be an all-fiber cable or contain both optical fibers and metallic conductors.

3

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

reliability (power-system protective relaying): A combination of dependability (failure to deliver a valid trip signal) and security (delivery of a false trip or control signal.). solid-state surge protective device (SPD): A protective device that employs solid-state circuit elements that provide a combination of high-speed voltage and current sensing. These surge protective devices are a combination of voltage clamps (avalanche diodes) and crowbar devices [multilayer diodes similar to silicon-controlled rectifiers (SCRs)] designed to limit the voltage to a specific value and to reduce current flow to low values of milliamperes within nanoseconds. These devices are usually integrated into the terminal apparatus. spark gap: An air dielectric between two electrodes that may be a combination of several basic shapes that is used to protect telecommunication circuits from damage due to voltage stress in excess of their dielectric capabilities. This device may or may not be adjustable. surge arrester: A device that guards against dielectric failure of protection apparatus due to lightning or surge voltages in excess of their dielectric capabilities and serves to interrupt power follow current. suspension strand: A stranded group of wires supported above the ground at intervals by poles or other structures and used to furnish, within these intervals, frequent points of support for cables. wire-line: Describing a network that uses metallic (e.g., copper, steel, aluminum) wire conductors for telecommunications.

3.2 Acronyms and abbreviations ac

alternating current

AWG

American Wire Gauge

BIL

basic impulse insulation level

CO

central office

dc

direct current

GDT

gas discharge tube

GPR

ground potential rise

HVI

high-voltage interface

MDR

mutual drainage reactor

MGN

multi-grounded neutral

OGC

overhead ground conductor

PVC

polyvinyl chloride

RDL

remote drainage location

SCR

silicon-controlled rectifier

SPD

surge protective device 4

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

SPO

service performance objective

ZOI

zone of influence

4. High-voltage environment 4.1 Locations Two different electric supply location environments are considered, but the application of the standard is not limited to these: a) Electric power stations: These locations generally utilize an extensive ground grid designed, per IEEE Std 80, so that all grounded structures within the station can be connected to a common grid, thereby minimizing potential difference in the system during a power fault. However, the potential difference between this grounding system and other distant grounding systems may be substantial, and a significant portion of these currents may be redirected to any wire-line telecommunications cables entering these locations. Ground grids in these locations help reduce surges caused by lightning strikes. b) Power line transmission and distribution towers or poles: These locations generally utilize a smaller grounding electrode system with a common arrangement being one ground rod per tower leg incorporated into the grounding system of the equipment placed at the tower base. Thus, the grounding systems at towers or poles usually have a significantly higher resistance to ground than those at electric power stations. Transmission lines also have a higher probability of being struck by lightning, thereby increasing the chances of fault-producing surges. A significant portion of these surge currents may be redirected to any wire-line telecommunications cables entering these locations.

At these types of locations, when a power-system ground fault or lightning strike occurs, all or some of the current returns via the earth through the grounding electrode and produces a potential difference between the grounding electrode and remote earth. The fault current may be symmetrical or may have some degree of asymmetry, depending on such factors as voltage phase angle at fault initiation, location of the fault, impedance to ground and other power-system characteristics. The impedance to ground depends primarily on the geometry of the grounding electrode, the connections to it and the resistivity of the soil in the vicinity of the site.

The GPR at an electric supply location may be reduced substantially because of its physical connections to remote ground points by means of overhead ground (earth, static, or sky wires) conductors (OGCs), multigrounded neutrals (MGNs), counterpoises, cable shields, rail lines, etc. These connections affect the distribution of fault currents through the system grounding paths and also affect the total site impedance to remote earth. A rigorous analysis of GPR calculations at the power system fundamental frequency is presented in IEEE Std 367. 6

4.2 Practical implications When a fault to ground occurs on a transmission or distribution line terminating in a grounded-neutral transformer bank at an electric supply location or substation, fault current will flow from the electric supply location ground grid to the system neutral by way of the station grounding system. Since the grounding system has a finite impedance-to-remote earth, it will experience a rise in potential with respect to remote earth because of this ground return fault current. The magnitude of the GPR depends upon such factors as 6

Information on references can be found in Clause 2.

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

the impedance-to-ground of the electric power station grounding system, the magnitude and location of the fault, the fault impedance, the impedance of the transformers feeding the fault, the presence or absence of ground wires on the line (insulated or not), and other grounding structures in the area. Also, a fault occurring at an electric power station without a grounded-neutral transformer or at a tower location may produce a GPR as long as there is a ground source on the system and there exists a ground current path to the fault.

4.3 Earth potential profile The area surrounding the electric supply location that is raised in potential above a remote (or true) earthing point is referred to as the GPR zone of influence (ZOI). In practice, 300 V is often used as the boundary of this ZOI. For a more complete discussion on this topic, refer to IEEE Std 367 and IEEE Std 487.

The potential of the ground around the electric supply location, with respect to remote earth, falls off with distance from the electric power station grounding system as indicated by the equipotential lines in Figure 1. Excluding alternate return paths, this potential is roughly inversely proportional to the distance from the station grounding system. For simplicity, the equipotential lines are shown as concentric circles in Figure 1.

Due to the irregularity of the grounding system, variations in the earth resistivity around the station, and the presence of metallic underground structures such as pipes and cables, the equipotential lines will not be circular as shown.

Figure 1 —Theoretical illustration of electric supply location GPR with conventional protection on telecommunication circuits

A telecommunication circuit extending from the electric supply location to some remote point is also shown in Figure 1. In this example, the protective device is installed at the electric supply location end of the telecommunication circuit with the protective device ground terminal connected to the electric supply

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location grounding system. At some remote point on the telecommunication circuit, such as the far end or some intermediate point, another protective device may be installed. In the latter case, the surge protective device (SPD) ground terminal is connected to what might be regarded as remote ground. These SPDs are used to limit the voltage that may exist between the wire-line telecommunication circuit and local ground at the SPD location. Therefore, SPDs are installed to help safeguard personnel and preventing damage to property and equipment that might be caused by induction, lightning, GPR, or direct contact with power circuits.

When there is a GPR at the electric supply location, a potential difference that is equal to this rise will exist between the ground terminals of the SPDs at the two locations on the telecommunication circuit. This difference in potential will cause (if of sufficient magnitude) the SPDs on the telecommunication circuit to operate, possibly grounding the telecommunications circuits permanently or damage the telecommunication circuit, and create personnel hazards.

In order to prevent the ground-return current from circulating over the telecommunication circuit and its protective devices, methods have been devised that are discussed later in this standard. In applying these methods, determining the expected GPR as accurately as possible is necessary.

4.4 Cables in high-voltage environments Telecommunication cables are exposed to the effects of GPR when they are entering the GPR zone to serve the electric supply location, to serve subscribers within the ZOI, or are merely passing through the ZOI. The metallic shield-to-core and insulating outer jacket dielectric withstand (strength) shall be considered with respect to the expected GPR value at the cable location. Not placing SPD and cable shield grounds in this zone without consideration of the effects of GPR at the proposed grounding location is important.

The dedicated cable to the substation always enters the area of highest GPR, while the general-use cable may pass through the ZOI at some lower potential level. Refer to IEEE Std 789 for a detailed discussion of the specifications for such cables. Furthermore, SPD or arc noise shall be avoided on electric power station cables carrying protective relaying signals (see IEEE Std C37.93). If protective devices are used properly and metallic members of the cable can be assured of being insulated from substation ground, then possible hazards from GPR will be greatly minimized.

The protection scheme is not to be installed to cope merely with existing fault current possibilities.

Provision should also be made for future power-system expansion or an increase in fault current levels or change in station ground grid impedance.

5. Protection theory and philosophy 5.1 Background The protection of wire-line telecommunications facilities serving electric supply locations is a complex subject involving several disciplines. On one hand, there are the protection schemes and their hardware employed where the electric power utility alone is involved in protecting its own wire-line telecommunication circuits. On the other hand, there are the leased telecommunications wire-line facilities that involve additional protection problems. In the first case, a question of satisfying the operational, personnel safety and reliability needs of the power utility itself is essential. Personnel and public safety is of utmost importance to both utilities, but the power utility personnel are more accustomed to working on or near high-voltage circuits. The use of the leased telecommunications facility involves all the problems of the power utility owned services plus the problems associated with the possible impairment to the general-

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use telecommunications cables and wider exposure of non-power utility personnel. There are possible or even probable different treatments for the protection requirements for these two classes of facilities. Within the electric power utility industry itself, there are divergent opinions regarding protection schemes just as there are within the telecommunications industry. There are also different approaches around the world.

In the case of the electric power utility or user-owned circuits, the maximum permissible interfering voltage and time duration may be higher without the need for special protection than are permitted by many telecommunications utilities. Some telecommunications administrations may also permit higher interfering voltage levels without special protection.

No matter which utility or utilities are involved, as accurate a prediction as possible shall be made of the magnitude and time duration of the interfering voltage. The level of protection established and agreed upon shall be consistent with the SPO Class 7 of the circuits involved. Safety questions shall be considered in all cases.

Depending on the circumstances and configurations, there may be economic considerations as to basic protection schemes to be employed as well as to hardware specifications. In the case of leased (rented) telecommunications facilities, the use of a high dielectric dedicated cable from the electric supply location to a point outside the influence of the electric supply location ground grid shall also be considered and agreed upon if the dielectric value of a general-use type cable is determined to be inadequate. The effects on or from telecommunications subscribers’ protection equipment within the ZOI shall also be considered.

Both the telecommunications protection engineer and the power-system protection engineer agree that the basic objectives for the protection of wire-line telecommunication facilities serving electric supply locations are to maximize personnel safety, to protect the telecommunications plant and terminal equipment, to maintain reliability of service, and to accomplish these in the most economical way. In the design of a protection system to meet these objectives, however, the telecommunications protection engineer and the power protection engineer may differ in their design approaches due to differences in their network reliability standards and protection philosophies. The design of a protection system requires a blending of the philosophies of the engineers responsible for telecommunications protection and for protective relaying in order to affect a solution that meets the primary protection objectives of both industries and the mutual customer base they serve.

The leased telecommunication facility protection concepts and system designs described in this standard have been reviewed and proposed by the power and telecommunications industry representatives who produced this standard. Where divergent views exist, they are covered by notes and dashed-lined boxes in the circuit diagram figures. In these cases, the telecommunications protection engineer and the power protective relaying engineer should reach a mutual agreement regarding the design to be implemented. The protection designs for telecommunications facilities to towers (including transmission line, telecommunication, broadcast, etc.) or other locations subject to GPR are generally specified by the telecommunications protection engineers with mutual agreement with other parties.

Unfortunately, laying down hard and fast protection rules is very difficult. Therefore, this standard presents several options and recognizes that each system and alternative configuration may have different design criteria and bona fide safety objectives. The determination of the electric supply location GPR or induced voltage, or both, will usually involve several engineering departments within the power utility.

The maximum GPR and induced voltage calculations and any derating factors used in protection design are critical to the success of any wire-line protection scheme. These voltages are essential for the design of the protective systems and devices (see IEEE Std 367). The relay and telecommunication engineers will then design a protection system to suit their own service performance objectives (SPOs) and safety needs. Up to this point, there will have been a number of engineering judgments made. Where there is a leased telecommunications facility involved, however, the relay and telecommunication engineers from the power 7

SPO Class is defined in IEEE Std 487.

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utility should then come to a mutually acceptable agreement with the engineers from the telecommunications utility on a protection scheme and its hardware in order to achieve the desired results.

In order to minimize difficulties, this cooperative consultation shall begin at a very early stage in the life of a project and such cooperation needs to be an ongoing process. Essentially, an engineering solution shall be employed that will result in the most appropriate solution to all concerned. Rigid positions, if taken by either utility without sound engineering evaluations, usually will not produce the most satisfactory solution.

Consultation and cooperation are, therefore, paramount.

The type, quality, and quantity of protective devices that would be used in any particular application shall be dictated by the nature, magnitude, and frequency of occurrence of the interference, the nature of the service requirements, considerations of personnel and plant safety, and by the general protective policies employed by the organizations concerned.

The operation of a protective device may result in a residual voltage between the telecommunication conductors and earth. The permissible magnitude of this residual voltage shall be such that the requirements for personnel and plant safety are not jeopardized.

IEEE Std 487 provides further details, including operating characteristics, on various types of protective apparatus that are generally available and/or in-use.

5.2 Concepts and concerns A fundamental concept regarding the protection of wire-line telecommunication facilities serving electric supply locations is that of a coordinated protection system design. This concept refers to a system of protection in which special protection measures are applied to SPO Class C services, as well as to SPO Class A and Class B services, that are provided in the same cable so that a circuit interruption or outage on an interruptible service will not cause a circuit failure or interruption on a non-interruptible service. The protection devices used on the various services shall, therefore, be coordinated with each other with respect to the environment and the SPO of the services on which they are employed. The objective of the coordination is to minimize the likelihood of cable failure, SPD operation, failure of special protection devices, failure of terminal equipment, or other similar occurrences that could create hazards to personnel and plant and result in interruptions or outages of critical and non-critical services alike. The various special protection systems described in Clause 6 of this standard are examples of coordinated systems of protection.

WARNING In situations in which the only telecommunication services at an electric supply location are of an interruptible type (SPO Class C) and in which the electrical environment is judged to be hazardous to personnel or equipment, responsible protection philosophy requires that special protection measures be taken.

This situation is of concern because of the need to provide special protection for personnel and plant safety when uninterrupted service performance may not be a requirement. Instead, only ordinary station protection is sometimes installed and this installation could result in an inadequate or potentially unsafe condition and appropriate protection is to be provided in these situations.

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5.3 Service performance objective (SPO) classifications 8 The SPO, with respect to the effects of power-system faults, fall into the following three classifications: Class A: Non-interruptible service performance (shall function before, during, and after the power fault condition.

Class B: Self-restoring interruptible service performance (shall function before and after the power fault condition) Class C: Interruptible service performance (can tolerate a station visit to restore service

5.4 Responsibilities for installation and maintenance Wire-line telecommunication circuits entering electric supply locations may be owned by the electric power utility or by the end user, or may be leased from a telecommunications service provider. In the case of leased telecommunications circuits, modular-type protective devices located at the high-voltage interface (HVI) are considered part of the network.

In some jurisdictions involving leased telecommunications facilities, the telecommunications service provider/user interface (demarcation point), by mutual agreement, would be at a point outside the ZOI; therefore, this point would not be the high-voltage protected interface. In this case, the entrance cables traversing the ZOI and the high-voltage protective interface (HVI) equipment would be owned either by the power utility or by the end user. For this arrangement, the telecommunications service provider/user interface shall be a protected terminal or SPD block outside the ZOI. Telecommunication circuits in this arrangement are of various types and have different SPOs, as described in IEEE Std 487.

5.5 General-use telecommunication cable in the electric supply location GPR ZOI When the general-use telecommunications cable, to which the dedicated cable is connected, passes through an area subject to GPR, dielectric breakdown in that cable may compromise the reliability of the electric supply location circuits. Additional protection may be required on the general-use cable to avoid such degradation. The case of a general-use cable passing through the ZOI beyond the junction with the dedicated cable is discussed in Annex B. Routing the electric supply location circuits through another electric supply location ZOI between the remote drainage location (RDL) and the central office (CO) is not desirable and should be avoided. If this routing cannot be avoided, then the reliability of service on cables that pass through the ZOI of another electric supply location is to be evaluated.

Telecommunications-type cables suitable for this type of installation are covered in IEEE Std 789.

The electric supply location GPR may be transferred by the MGN outside of the ZOI under certain conditions and especially in rural environments, see Rajotte et al. [B26]. This transformation may require alternate methods of cable isolation and shield grounding to be considered.

NOTE—A more detailed description of transferred potentials by the MGN may be found in 9.5 of IEEE Std 367.

8

More detailed information about SPO is in IEEE Std 487.

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5.6 Dedicated cable 5.6.1 Dedicated cable overview A common feature of Voltage Level II and Voltage Level III circuit configurations, discussed in 6.1, is the use of a high dielectric withstand, dedicated cable containing only those pairs serving the electric supply location. Depending upon local agreement or authoritative regulations, the dedicated cable may be owned by either the telecommunication service provider or the end user. Pairs to all other subscribers are excluded, minimizing pair-to-pair stress during a fault. A dedicated cable, if provided by a telecommunications company, may extend the entire distance from the electric supply location HVI location to a telecommunication center; however, it should extend at least to a point where the GPR profile has decreased to an agreed value. This point is to be located so as to protect the general-use cable plant and, in the case of remote drainage, to minimize SPD block operation and the resulting generation of noise, which could interrupt or interfere with critical services. See 8.3 for safety concerns related to installation and maintenance of this item.

If an RDL is not used, the dedicated cable extends only to a point on the GPR profile that is compatible with the assured dielectric of the installed, general-use cable. The reliability of the wire-line telecommunications facility is dependent, in part, upon the high-voltage integrity of the dedicated cable.

Refer to IEEE Std 789 for typical specifications for telecommunication cable serving electric supply locations.

Special considerations are to be observed when grounding the dedicated cable. Cable pairs and shields between the HVI location and the edge of the ZOI shall not contact the ground structure at the electric supply location. The cable shall be routed through a well-drained (i.e., dry) insulating polyvinyl chloride (PVC) conduit in the electric power station grid area, reducing the possibility of solid or incidental contact with the electric power station grid. If arresters are not used, the cable shield shall be cut and rendered inaccessible to prevent workers from erroneously connecting the cable shield to ground at the electric supply location.

Low-impedance grounding of the dedicated cable shield within the GPR ZOI is likewise not permitted.

Incidental, high-impedance grounding, however, can be tolerated. An incidental ground results from a small puncture or pinhole in the outer covering of the cable. Incidental grounds are assumed to have, and continue to have, sufficiently high contact impedance to limit shield current and internal cable stress.

The dedicated cable facilities are shown in Figure 3 through Figure 7. The telecommunications service provider/user interface is defined as a point at which the user and telecommunications service provider facilities meet and to which both user and telecommunications service provider personnel have access. The location of the service provider/user interface is determined by local agreements or authoritative regulations. The user should note that this point may or may not be at the same location as the high-voltage protective apparatus location referred to as the HVI. High voltages due to GPR and induction can appear between cables, protection hardware, and the local ground at various points of an installation. Understanding the potential dangers and observing the safety practices discussed in Clause 8 by personnel accessing the installation is essential.

5.6.2 Aerial cable installation Wire-line cable service to an electric supply location is to be accomplished utilizing a buried PVC conduit containing the dedicated cable. The use of an aerially-supported dedicated wire-line cable utilizing a metallic messenger is not recommended.

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If the dedicated cable is to be aerial it shall transverse the ZOI using a pole line exclusively for the dedicated cable. Down guys and messenger attachments within the ZOI are also to be isolated.

The vertical grounds and all down guys located within the ZOI impart a portion of the GPR onto the metallic messenger and subsequently the cable shield.

WARNING This situation creates a cable placement method that is difficult, at best, to protect or isolate from GPR events, and, at the same time, may endanger anyone working on this type of entrance facility.

The additional engineering methods and personnel safety measures required to help minimize safety risks associated with an aerial entrance installation may outweigh the choice of placing an aerial installation over a buried or underground installation.

If hard rock soil conditions preclude the placement of buried PVC conduit, then an aerial dedicated cable may, as a last choice, be placed on a separate pole line for the exclusive use of the dedicated cable. The facilities designer has to be aware that the use of an aerial supported wire-line cable utilizing a metallic messenger is not recommended for the following reasons (not necessarily in a particular order): ⎯

Difficulty of establishing and maintaining separate pole line to assure integrity of the protective design. A separate pole line for aerial dedicated cables may be precluded by any of the following: ⎯

Existing joint use agreements or contracts

⎯

Telecommunications Act of 1996 regulations on sharing of structural facilities

⎯

Federal and/or local regulations on sharing of structural facilities.

⎯

Difficulty in providing for personnel safety by minimizing touch voltages between metallic members (support messengers, cable shields, etc.).

⎯

Difficulty in assuring the integrity of the design during or after maintenance and repair activities.

⎯

Difficulty in maintaining the designed isolation of metallic strands (support messengers) and cable shields.

⎯

Meeting National Electrical Safety Code® (NESC®)9 (Accredited Standards Committee C2-2007) [B1] bonding and grounding requirements may conflict with the need to isolate metallic members.10

5.6.3 Conduit containing metallic members or tracing medium The cable entrance facility shall be in an all-dielectric raceway (such as PVC conduit) from the HVI location to a point at least 3 m (10 ft) on the CO side of the grid or perimeter fence, whichever is closer to the CO. Conduit containing any metallic members shall not be utilized for the cable entrance facility to the electric supply location.

9 National Electrical Safety Code and NESC are both registered trademarks and service marks of The Institute of Electrical and Electronics Engineers, Inc. 10 The NESC does not consider the effects of GPR on facilities.

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The metallic members (locating wires or metallic materials) violate the concept of an all-dielectric cable entrance conduit and are not be used for safety reasons.

5.6.4 Resistive balance Longitudinal current may result from an insulation breakdown of cable pairs or shields during the fault or if induced voltages are sufficient to operate the drainage mechanism provided on the pairs. This current will produce metallic voltages if the resistance of the tip and ring conductors or the drainage devices is not balanced. This longitudinal-to-metallic conversion can produce sufficient metallic noise levels to disrupt sensitive critical services. The effect of longitudinal-to-metallic conversion are to be minimized by checking the resistance balance of pairs for electric supply location use and assigning critical services to the pairs with the best resistive balance. Effects of capacitance unbalance are usually negligible when direct drainage longitudinal currents flow.

When determining resistive balance, the entire pair length between the electric supply location and the telecommunication center is to be considered. Where the dedicated cable extends only a short distance from the electric supply location and joins with the general-use plant, the general-use telecommunications cable may form an appreciable section of the cable run and will have the greatest contribution to resistive unbalance. If this cable is exposed to induction, strategically placed drainage devices along the cable route may be used to maintain the voltage within acceptable limits; however, drainage current will be converted to metallic noise on poorly balanced pairs. Presently, resistive balance is considered acceptable when the difference in resistance between tip and ring conductors meets the requirements of IEEE Std 789.

5.7 Special wire-line protection design requirements In order to design special protective systems for wire-line facilities serving electric supply locations, the following conditions need to be known. However, no single condition is to be used as the sole criterion for determining the need for special high-voltage protection. a)

The quantities, service types, and SPO classifications of all services at the electric supply location

b)

The transmission requirements of the terminal equipment (ac signals only, dc signals only, ac plus dc signals, signaling frequencies, transmission capabilities of the transmission facility, e.g., noise squelch levels versus expected noise performance of the facility)

c)

Factors such as the total available single phase-to-ground fault current and its distribution, maximum GPR (rms), X/R ratio, fault-produced longitudinal induction, and lightning exposure

d)

Electric supply location ground grid impedance to remote earth and the grid area

e)

The extent of the GPR ZOI

f)

Whether the transmission parameters and SPOs are compatible with the available or proposed facilities

g)

Anticipated future changes in any of the above data

h)

Whether lightning protection is required

i)

Any available past trouble report history for the electric supply location in question (see IEEE Std 367)

NOTE—IEEE Std 487 Annex I includes a form that may be used to provide this and other similar information.

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Refer to IEEE Std 487.5 [B21] if discrete isolation-type protection equipment (hard-wired) is to be used.

6. Protection configurations

6.1 Ground potential rise (GPR) plus induced voltage levels Voltage protection levels in this standard are given in terms of peak values due to telecommunication cable dielectric, bare spots on wires, and air gaps essentially break down close to the peak of the voltage waveform and because some degree of a dc offset may be superimposed on the sinusoidal waveform.

Voltage levels, discussed in detail in IEEE Std 487, are summarized below: ⎯

Voltage Level I – Less than 300 Vpeak

⎯

Voltage Level II – Greater than 300 Vpeak but less than 1000 Vpeak

⎯

Voltage Level III – Greater than 1000 Vpeak

Descriptions of applicable figures and options are discussed later and are included in Table 1 and Figure 8.

NOTE— Many administrations have chosen a value of 300 V, either rms or peak, as the upper limit for Voltage Level I. Other administrations have chosen values such as 420 V, 430 V, or 650 V (rms or peak). Some administrations have chosen even higher voltages on the basis of their higher cable and equipment dielectric withstand capabilities. The design configurations contained in Table 1 and Figure 8 shall be amended by the user to accommodate the higher voltages chosen.

6.2 Basic protection system Depending upon the SPO requirements and the GPR plus induced voltage level (see 6.1) requirements, basic wire-line telecommunication protection, as shown in Figure 2 and Figure 3, may be used. The basic protection illustrated in Figure 2 may be used on all electric supply location services, provided that the interfering voltage is calculated not to exceed the Voltage Level I range, as discussed previously.

Administrations, power utilities or telecommunications companies, and end users who so choose, will permit this limit to be higher. Above the chosen voltage, basic protection, as illustrated in Figure 3, may be used on certain services in the Voltage Level II range (see 6.1). Class A SPO requires special protection, i.e., isolation, in both Voltage Level II and Voltage Level III ranges.

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NOTE 1— See Table 1 and Figure 8 for applicable options.

NOTE 2— Some telecommunications administrations may require that shields of all general-use cables be grounded at electric supply locations. Current-carrying capacity of the shield under power-system fault conditions is to be considered.

Figure 2 —Basic protection for Voltage Level I all service classifications

NOTE 1— See Table 1 and Figure 8 for applicable options.

NOTE 2— Although this standard recommends that the dedicated cable shield be isolated from the electric supply location ground grid, some administrations may require that shields of all dedicated cables be grounded at electric supply locations. Current-carrying capacity of the shield under power-system fault conditions is to be considered.

Figure 3 —Basic protection for Voltage Level II range Class B or Class C services only

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6.3 Protection configurations employing isolation devices 6.3.1 Protection configuration overview Circuit configurations range from the simple, consisting basically of the isolation device at the electric supply location and dedicated cable to a remote location, to the more elaborate, when distance to the remote location or distance between the isolation device and terminal equipment at the electric supply location is increased. A remote location is defined as another electric supply location or dispatch office, telecommunications CO, or other remote telecommunications terminal. Figure 4 illustrates the simplest situation. Figure 5 and Figure 6 illustrate the more elaborate situations in which extended distance between the electric supply location and the remote location may make routing of dedicated cable for the entire distance impracticable. The dedicated and general-use cable plants are normally interconnected, and remote drainage protection may be required at the junction point. The decision to use remote drainage protection is to be by mutual agreement between the administrations involved. When the cable length between the isolation device and terminal equipment at the electric supply location becomes significant, protection is further complicated by the shielding required for the interconnecting cable. Where the remote location is another electric supply location, an HVI may also be provided at the remote location (see Figure 6).

6.3.2 Basic isolation protection configuration A very simple and effective protection system can be realized with high-dielectric isolation transformers or optical coupling isolators, high-voltage disconnect jacks, surge arresters, and high-dielectric dedicated cable. In all cases, the dielectric rating of the devices shall coordinate with GPR and the dielectric rating of the dedicated cable (see Figure 4).

NOTE 1—Use of drainage device and surge arrester to be by mutual agreement between protective relay and telecommunications engineer.

NOTE 2—PVC conduit, 3 m minimum from grid or fence, whichever is further.

Figure 4 —Basic isolation protection configuration

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6.3.3 Protection at the electric supply location Protection at the interface between the electric supply location and the incoming telecommunications cable (i.e., the isolation device) is intended to keep the GPR from appearing on the incoming cable. The electric supply location telecommunications cable interface will, therefore, be referred to as the HVI. This point is not necessarily the telecommunications service provider/user interface or demarcation point.

The HVI could be located at either the edge of the station ground grid or in the control building. Wiring between the HVI and terminal equipment is to be short to minimize exposure to inductive interference, switching transients, or differential ground grid voltages; or measures as shown in Figure 5 and Figure 6 need to be taken to protect against such interference. Drainage to the electric supply location ground is provided on the station side of the isolation device.

NOTE 1— Use of drainage device, spark gap, and surge arrester to be by mutual agreement between protective relay and telecommunications engineer.

NOTE 2— PVC conduit, 3 m minimum from grid or fence, whichever is further.

Figure 5 —General isolation protection configuration

For a totally ac class of service, direct drainage may be applied. When ac and dc signals will both be present on the pair, a drainage reactor with a gap SPD in each leg, termed a mutual drainage reactor (MDR), is to be used because the MDR presents a low bridging impedance to dc signals. Blocking capacitors could be used in place of the gap SPDs; however, resonant conditions are to be considered.

Drainage provided on pairs assigned to Class C SPOs may consist only of gas discharge tubes (GDTs), or solid-state SPDs. Pairs assigned to Class A SPOs shall be equipped with an MDR arrangement to minimize noise interference and prevent signal loss.

The isolation device may also be an isolation transformer with a well-balanced center tap serving the dual function of isolation and drainage. The center tap on the electric supply location side may be connected to the ground grid to provide direct drainage, as shown in Figure 7, as long as a ground loop is not created.

The center tap of the line or CO side winding shall have a specified minimum drainage capability.

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NOTE 1— Use of drainage device, spark gap, and surge arrester to be by mutual agreement between protective relay and telecommunications engineer.

NOTE 2— PVC conduit, 3 m minimum from grid or fence, whichever is further.

Figure 6 —General isolation protection between two electric supply locations

If the dielectric of the isolation devices and cable may be exceeded, cable protection can be provided on the remote location side of the isolation device to the dedicated cable shield through a spark gap intended to limit pair-to-shield stress. This cable protection will not be effective for longitudinally induced voltages, as both the pair and shield are in the same field. When isolation devices other than well-balanced, centertapped isolation and drainage transformers are used, separate drainage coils with direct, capacitor-blocked, or SPD-drainage connections should be provided for Class A SPOs and may be provided for Class B SPOs. Class C SPOs utilize only GDTs or equivalents. The spark gap does not normally operate except as a safety measure to prevent cable damage in the event that an isolation device fails or the dedicated cable shield contacts the station ground.

The surge arrester shown in Figure 7 protects the isolation device in the event of a lightning stroke to the electric supply location ground structure or telecommunications facility that exceeds the isolation device’s basic impulse insulation level (BIL). The arrester may be eliminated if the dielectric BIL of the isolation device is capable of withstanding the voltage from a lightning stroke.

The dedicated cable shall be routed in a well-drained (i.e., dry) non-metallic conduit (PVC), within the electric power station ground grid area.

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

NOTE 1—Use of drainage device, spark gap, and surge arrester to be by mutual agreement between protective relay and telecommunications engineer.

NOTE 2—PVC Conduit, 3 m minimum from grid or fence, whichever is further out.

Figure 7 —Composite protection system 6.3.4 Protection at the CO or non-electric supply location remote location The basic configuration of Figure 4 shows the dedicated cable extending the entire distance to the remote location where the shield is grounded. Drainage is applied to all pairs at the point of entry to the remote location to ensure that voltages from telecommunication line-to-ground are within prescribed safety limits.

On Class A SPO circuits, direct, capacitor-blocked, or SPD-blocked MDRs are used, depending upon whether or not dc is present. On Class B SPO circuits, the use of an MDR is optional. GDTs or solid-state SPDs are used for Class C SPO circuits such as exchange telecommunications service. If drainage is required along the cable route (outside the GPR zone) to mitigate the effects of longitudinal induction, the equipment is to be applied to all cable pairs to preclude possible arcing between in-service and unassigned or unused pairs.

6.4 General isolation protection configuration 6.4.1 Dedicated cable In many situations, using dedicated cable facilities for the entire distance from the electric supply location to the serving location may not be feasible or economically practical. Dedicated cable facilities may be

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merged with a general-use telecommunications plant at a location outside the ZOI of the station GPR. As an alternative, the dedicated cable could be merged with general-use cable at a point where the station GPR coordinates with the dielectric strength of the general-use cable. Protection at the electric supply location and at the remote location is identical to that provided in the basic configuration of Figure 4.

6.4.2 Remote drainage protection Remote drainage protection may be added at the point at which the high dielectric dedicated cable facilities and the low dielectric general-use plant merge, as shown in Figure 5 and Figure 6, so that voltages are maintained within the capabilities of the low dielectric cable in the event of failure of the isolation devices or the dedicated cable insulation. For types of drainage units, see IEEE Std 487.

A suitable location for remote drainage protection may be determined by using the information given in Clause 4 and in IEEE Std 487. The site shall be chosen to help ensure that local GPR does not exceed Voltage Level I. If a higher GPR point were chosen, the dielectric strength of the general-use cable jacket might be exceeded and personnel safety might be jeopardized. In addition, circuit noise could be produced due to an unbalance of the drainage system.

If parallel routing of power and telecommunication cables exists, then remote drainage protection shall be located at the point at which the combination of longitudinally induced voltage (on the remote side of the point) and GPR does not exceed Voltage Level I. Consideration is also to be given to local GPR due to ties or couplings with local power line grounds.

Two grounds are established at the remote drainage protection location as shown in Figure 5 and Figure 6: a local ground associated with the general-use cable and drainage, and a remote ground associated with the dedicated cable shield. This standard recommends that these grounds be established a minimum of 6-m (20-ft) apart.

Additional lightning protection is provided by the spark gap connected between dedicated and general-use cable shields. When there is assurance that the isolation devices at the HVI have been properly engineered and installed, the ground electrode on the dedicated cable and general-use cable are bonded at the splice.

6.4.3 Local terminal When the HVI is located sufficiently far from the terminal equipment to expose the interconnecting cable to inductive interference, the cable between the two locations is to be placed along with a 2/0 AWG bare copper conductor. The cable shield and the 2/0 AWG conductor shall be bonded together and then bonded to the electric supply location ground at the location where the cable enters the conduit and leaves the conduit.

The circuit balance is to be maintained by using balanced, twisted-pair conductors within this cable and in any wiring extending to the terminating equipment. Interconnecting cable length is not critical, provided that proper shielding has been applied.

6.4.4 Composite protection system Figure 7 shows a protection system utilizing various types of isolation devices at the HVI using the general protection configuration of Figure 5. Direct drainage, SPD drainage, or GDTs are provided at the various protection locations, depending upon the type of service provided, over the wire pair. Figure 7 shows how the various drainage techniques are used with the different isolating protection devices.

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6.5 Protection configuration employing modular high dielectric and optic isolators High dielectric and optic isolators may be used either as a single-channel isolation device in the protection system described in 6.3, or in a cabinet specifically designed to mount a multiplicity of transformer and optic isolators, which may be used to replace the entire HVI cabinet of such a system. In either case, the design criteria for placement of the cabinet, choice and treatment of the dedicated cable, choice of surge arrester, etc., is applied as described in 6.3. A location shall be selected for the surge arrester to minimize the arrester-to-grid grounding conductor length.

Factory-designed and constructed protection system cabinets are available that accept plug-in modular optic and transformer devices for various numbers of telecommunication channels. Available optic and transformer devices can handle many forms of telecommunication signals used by the power and telecommunications industries including regular telecommunications service, telemetry service, tone relaying service, dc relaying service, analog data service, high-speed digital data (DS-1) service, etc. The major advantages of using a modular system based on high dielectric properties are the reduction of field engineering required to provide a reliable system with maximized safety and the flexibility of being able to change telecommunication service types and add or remove channels readily.

High dielectric and optic isolator devices may require power for the circuitry on both the CO and the electric supply location sides of the device. Power for the CO side may be provided over the CO cable or from local power through a power-distribution transformer. If a local distribution transformer is used, the transformer is to have a dielectric strength between the primary and secondary windings (including the neutral) at least equal to the voltage for which the HVI cabinet is designed, and the same precautions are to be observed for the transformer wiring and for the associated power supply for ensuring that safety precautions and voltage isolation as are specified for the telecommunications pairs. A power transformer and power supply for the CO side power may be supplied as part of an optic-coupling protection cabinet.

6.6 Protection practices for electric supply locations services 6.6.1 Protection practices overview Table 1 and Figure 8 are to be used together to determine the various protection options available for typical service types. As mentioned elsewhere in this standard, views on protection of electric supply locations services may vary, not only between power and telecommunications utility people, but also within their respective industries. Some of the options shown in Table 1 and Figure 8 are more common to leased services while other options are more common to services provided over user-owned facilities. This standard emphasizes that, in the case of leased facilities, mutual agreement upon the protection options selected is required between all responsible engineers of the owners and users of the facility. In most cases, the options showing no protection at the electric supply locations (i.e., hard-wired) are not recommended.

There may be other possible options than those indicated in Table 1 and Figure 8.

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Table 1 —Typical protection table for electric supply services Typeb

SPOc

Basic telephone services

1

B or C

Critical telephone or dispatch

1

B

1

B

2

B

2

B

Service description

Alarms, dc control, dc telemetry, voice with dc signaling or control DC telemetry (with automatic load control), remote or other dc tripping Pilot wire protective relaying (60 Hz with dc supervision or signaling) SCADA load and supervisory control (using FSKe with DC functions), FM mobile radio with FSKe or dc signaling, operational voice with dc FSKe or dc signaling) Pilot wire protective relaying (60 Hz without dc supervision or signaling) Critical SCADA load and supervisory control and other critical VF services (using FSKe without dc functions) Audio tone protective relaying (using FSKe without dc functions) Digital services T-1, DS-1, ISDN, xDSL (with station side dc powering) Idle and unassignable pairs

2

B

3

A or B

3 or 4

A or B

a

Permissible configuration and options for GPR plus induced voltage levels (see 6.1) Level I Level II Level III Config.

Optionsd Config.

Optionsd Config.

Optionsd W2-3, X1-3, W2-3, X1-3, Y1, Z2-3 or W2-3, X1-3, 1 1, 2, 3, 5 2, 3, 5 Y1, Z2-3 Level III Y1, Z2-3 requirements W2-3, X1-3, W2-3, X1-3, Y1, Z2-3 or W2-3, X1-3, 1 1, 3, 5 3, 5 Y1, Z2-3 Level III Y1, Z2-3 requirements W2-3, X1-3, W2-3, X1-3, Y1, Z2-3 or W2-3, X1-3, Y1-3, Z2-3 1 1, 3, 5 3, 5 Level III Y1, Z2-3 and or Y7, Y8 requirements W2-3, X1-3, W2-3, X1-3, Y1, Z2-3 or Y1-3, Z2-3 Protect per 1 3, 5 3, 5 Level III and or Y7, Y8 Level III requirements W2-3, X1-3, W2-3, X1-3, Y1, Z2-3 or Y1-3, Z2-3 Protect per 1 3, 5 3, 5 Level III and or Y7, Y8 Level III requirements

1

W2-3, X1-3, Y1, Z2-3 or Level III requirements

1

W2-3, X1-3, Y1, Z2-3 or Level III requirements

1

W2-3, X1-3, Y1, Z2-3 or Level III requirements

4

A

1

5

B or C

1

W2-3, X1-3, Y1, Z2-3 or Level III requirements W2-3, X1-3, Y1, Z2-3 or Level III requirements

3, 5

3, 5

W2-3, X1-3, Y1, Z2-3 or Level III requirements

Protect per Level III

3, 5

W2-3, X1-3, Y1-3, Z2-3 and or (FSKe) W5, 75, Y10, (dc) W7, Y8.

Y10

3–5

W2-3, X1-3, Y1-3, Z2-3 and or W5, Y5, Y10

Protect per Level III 3–5

3–5

3, 4

3–5

Protect per Level III

W2-3, X1-3, Y1, Z2-3 and or X6, Y6

W2-3, X1-3, Y1-3, Z2-3 and or W5, Y5, Y10, Z10

3–5

W2-3, X1-3, Y1-3, Z2-3 and or W5, Y5, Y10, Z10

3, 4

W2-3, X1, Y1, Z2-3 and or X6, Y6

See 6.6.6

a

Table 1 is to be used in relation to Figure 8 b Service types are defined in IEEE Std 487 c SPOs are defined in IEEE Std 487 d Options 1 through 10 represent protection options (see Figure 8) e FSK, frequency shift keying, also known as tone transmission and/or signaling, within the VF bandwidth.

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Figure 8 —Simplified protection schematic chart for an electric supply location 6.6.2 Description of protection table and protection schematic chart The design configurations contained in Table 1 and Figure 8 are based on the voltage levels listed in 6.1 and reproduced below: 

Voltage Level I – Less than 300 Vpeak



Voltage Level II – Greater than 300 Vpeak but less than 1000 Vpeak



Voltage Level III – Greater than 1000 Vpeak

NOTE—Table 1 and Figure 8 shall be amended by the user to accommodate other voltage levels selected.

6.6.3 Protection table for electric supply location services (Table 1) Table 1 is to be used in conjunction with the simplified protection schematic chart (Figure 8) to determine the various protection options available for any particular service type. A description of the various columns is given as follows: a)

Service description. Examples of the more common types of electric supply location services are listed in this column, grouped by transmission type (dc, ac, audio tone, etc.) and reliability importance.

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

b) Service types. Service types shown in this column are described and classified in IEEE Std 487. c)

Reliability or SPO classification. SPO classification or reliability is described in IEEE Std 487.

The responsible power utility engineer shall specify the SPO classification regardless of whether the service is leased, power-company owned, or user owned. Typical SPO classifications are shown in this column opposite each service type grouping. These could be specified differently, however, by the power company, depending on the circumstances.

d) GPR plus induced voltage level columns. These columns list the various figures and options that are permissible for each voltage level for each particular type of service listed. Instructions on how to use Table 1 are given in 6.6.5. As indicated by the general heading, a voltage level is established by adding the GPR voltage to that of any induced voltage that may appear during a fault on the power system. (See IEEE Std 367).

6.6.4 Simplified protection schematic chart (Figure 8) The chart in Figure 8 is intended to show the typical protective devices to be used with various service types depending upon the SPO class of service and the voltage levels. There are five basic protection configurations. Configuration 1 involves no special protection (i.e., no isolating devices). Configurations 2, 3, and 4 are various isolating arrangements. Configuration 5 is a neutralizing arrangement (see IEEE Std 487.4 [B20]). These configurations are not intended to show the complete system protection layout, which is covered in detailed drawings elsewhere in this clause.

The chart shows four different protection locations: a)

The electric supply location building

b) HVI location (normally near the fence line of the electric supply location or at the control building) c)

RDL

d) Non-electric supply location The protection options for the cable between the electric supply location building and the HVI are shown in the electric supply location building in this chart. This protection could be placed alternatively on the station side of the HVI as shown in Figure 5, Figure 6, and Figure 7; however, the question of safety at the terminal location is to be addressed.

The protection options in the electric supply location building and at the non-electric supply location are the same, unless otherwise indicated.

If the cable between the electric supply location building and the HVI is classified as exposed, then similar protection options are required at both locations, with consideration being given to ground loop currents.

In Figure 8 (bottom), various SPD arrangements (i.e., drainage or GDTs) are shown. These will vary depending upon service type and voltage level.

6.6.5 Use of protection table and schematic chart (Table 1 and Figure 8) From the protection table, the protection options are identified in the appropriate voltage level column opposite the particular service type and SPO class.

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The configuration specified and the options to be used at each location shall first be selected. For example, Configuration 1 with W3 and X1 and Y1. a)

Refer to Configuration 1 on the simplified protection schematic chart (Figure 8).

b) The protection [W3] at an electric supply location building and non-electric supply location will be 3-element gas tube or gas tube and solid state combinations designed for the specific application. c)

X1 and Y1 mean no protection at the HVI or RDL. This means that there is no HVI and no RDL.

d) For the sake of brevity, possible options for Configuration 2 with W2 and (X1 or X2) and (Y1 or Y2 ) may be shown as follows: 1) Configuration 2 with W2 and X1 and Y1 2) Configuration 2 with W2 and X1 and Y2 3) Configuration 2 with W2 and X2 and Y1 4) Configuration 2 with W2 and X2 and Y2 6.6.6 Treatment of idle and unassignable or spare pairs and/or equipment Idle and unassignable or spare pairs and/or equipment generally fall into one of the following categories: a)

Idle cable pairs. These pairs are electrically continuous from the CO to the electric supply location with all required equipment installed at the HVI. Additional protection devices particular to a specific service may be required at the CO and/or the electric supply location terminal but the pairs are otherwise ready for assignment and use. All installations shall conform with Figure 2 through Figure 7.

b) Unassignable or spare cable pairs. These cable pairs are unassignable because they do not have electrical continuity from the CO to the electric supply location terminal and are usually unterminated in at least one location: at the dedicated/general-use cable interface, at either side of the HVI or at the inside terminal. One example would be a 50-pair dedicated cable brought to a cabinet designed to mount 12 modular transformer and/or optic isolators. Remaining unassigned or idle pairs are to be cleared and capped.

If the unassignable cable pairs are in the customer’s cable that is located wholly on the electric supply location’s ground grid and the HVI is located on the same ground grid, then the pairs could be “cleared and capped,” terminated at both ends, grounded at both ends, or any combination of these based on local conditions. If the HVI is located outside the electric supply location’s fence, distant from the ground grid, then the customer’s cable is to be treated as if it were dedicated cable.

For the case of unassignable cable pairs in a dedicated cable, whether or not they are spliced to pairs in a general-use cable, the unassignable cable pairs and the metallic cable shield shall be totally isolated from local ground at the HVI. This requirement applies regardless of the location of the HVI. Isolation shall also be arranged such that accidental contact cannot be made across the HVI. c)

Spare HVI equipment capacity. HVI equipment capacity that is considered as spare is that capacity that cannot be readily accessed or that is not connected to assignable cable pairs. As an example, 25

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consider a cabinet with a capacity for 24 modular isolators with only 12 cable pairs terminated, or connected, i.e., 12-pair spare capacity. Clear and cap the 12 other pairs. d) Unassignable cable pairs and spare HVI capacity. In general, this scenario has two options:  Connect through all facilities and equipment from the CO to the inside terminal on the electric supply location ground grid and treat as assignable or spare cable pairs.  Treat the cable(s) and HVI spare capacity separately, as outlined previously.

7. Periodic inspection considerations The protection techniques used for wire-line facilities serving electric supply locations are vastly different from those employed for general, business locations. This application is particularly true with respect to location, separation and isolation of ground points and conductors, and to the provision of higher-thanusual insulation levels in cables, transformers, and other protection hardware.

WARNING Telecommunications or power company technicians may inadvertently do things that could negate special protective measures. A common example of this activity includes the connection of a dedicated cable shield to station ground. It is possible for connections to become loose or burn open, transformer windings to fail, insulation to fail or become faulty, etc. Since a protection system is quiescent, such situations may not become apparent until the protection system fails to function properly under fault conditions, with the potential for failure or damage, or both, to critical electric supply location telecommunication equipment, and, possibly, injury or loss of life.

Periodic, usually annual, detailed inspections by both power utility and telecommunications company personnel of all aspects of protection facilities in and around electric supply locations and remote ground points are necessary to help ensure that special protection requirements have not been changed by conditions such as those mentioned above.

The period within which such inspection is to be conducted should be worked out mutually by the power and telecommunications utilities in the case of leased facilities. In addition to planned, periodic tests, this standard recommends that a very thorough inspection of protection facilities be made following each case of faulty or questionable operation of such facilities, particularly if damage has resulted or false relaying has occurred.

Inspection of protection facilities shall include all cables within the GPR zone, plus all transformers, isolators, remote ground circuits, SPDs, and wiring at the electric supply location and at all non-powerstation locations,.

Tests (continuity, polarity, insulation withstand, etc.) on major protection system components such as isolation and neutralizing transformers may also be considered. Such tests would be indicated at any location that has a history of equipment failures or damage. In the case of power-utility-owned facilities, the power utility usually has a standard inspection procedure available.

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

8. Safety 8.1 General safety considerations WARNING Hazardous voltages may appear suddenly and without warning on cables (shields and pairs) and on associated protection hardware during fault conditions.

The basic safety objective is to protect personnel from coming into contact with both remote and local grounds simultaneously. Therefore, safety consideration will be directed toward the following two goals: a)

Educating personnel regarding the special hazards of working on telecommunication facilities serving electric supply locations

b) Minimizing the possibility of simultaneous contact with both remote and local grounds and reducing the length of time that personnel are required to work under conditions that may expose them to danger This standard emphasizes that these exposed conditions include telecommunications plants external to the electric supply location itself (including, in some cases, a plant not serving electric supply locations, when such a plant is within the ZOI of an electric supply location). In the event of dielectric failure of cables or other components within the HVI, the exposed conditions may extend even further.

Appropriate measures are to be taken to separate the station and remote side terminals and hardware such that physical contact cannot be made simultaneously with both. Separation can be achieved either through distance or dielectric barriers. All exposed HVI metallic components shall be bonded to the electric supply location or local ground, as appropriate. The HVI installation or maintenance activities are to be completed following applicable safety precautions using rubber gloves and/or insulating blankets 11 to maintain separation between local and remote grounds. No work is to be undertaken on an HVI during an electrical storm. At all times, close cooperation between the power and telecommunications companies is required to maximize personnel safety.

Periodic inspection (see Clause 7) is also a vital component of overall safety considerations. All components of the special protection system, including non-HVI location items such as remote ground, shall be verified periodically for proper connection and/or operation.

8.2 Safety considerations in equipment design In the design of protective equipment for a telecommunications plant serving electric supply locations, consideration of the following features or precautions is recommended: a)

An isolated or dead-front concept is to be used for transformer cases or equipment cabinets, so that the external casing always remains either adequately isolated from all conductors or at the potential of the local ground.

11 Use a rubber blanket, (and/or insulated footwear) and rubber gloves of at least 20 kV dielectric strength rating (Class 2). Refer to OSHA 1910 Subpart I - Personal Protective Equipment [B25] for information on voltage ratings. In Canada, refer to CSA Z259.4 for gloves [B7] and Z259.6 for blankets [B8]. Other jurisdictions may have equivalent standards.

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b) The physical design should protect against inadvertent simultaneous contact with electric supply location and remote location connections on protective apparatus. Barrier covers or other types of insulated closures are to be used over all open terminals and exposed, non-grounded metallic parts of protection apparatus and its associated hardware and wiring. This standard highly recommends that as much protection apparatus as possible be housed in nonmetallic cabinets or grounded metallic cabinets that can be securely closed and locked. c)

A sign warning technicians of the hazards shall be prominently displayed. A typical sign might read as follows: WARNING There is a possible 15 kV or more potential difference between local and remote ground conductors. Do not interconnect local and remote grounds. Refer questions to the protection engineer.

d) On-site wiring shall be done prior to connecting telecommunication and signal cable pairs and shields. Pre-wiring of protective equipment cabinets would minimize on-site activity. e)

If arresters, gaps, and drainage units are not used, the cable shield is to be cut and isolated at a point removed from local ground to prevent technicians from inadvertently interconnecting local and remote grounds at the electric supply location.

f)

Properly insulated wire and plastic shields are to be used on the CO side of protection apparatus at the HVI, or on the electric supply location side at the remote drainage protection location, to protect technicians from contact with remote potential.

8.3 Safety considerations related to installation and maintenance All safety precautions, detailed in applicable safety practices, are to be observed when installing or maintaining protective devices at, or in the vicinity of, electric supply locations, or when placing cables within the ZOI of the GPR. The following precautions are of particular importance: a)

When installing a new cable, the station end is to be connected first, while isolating the field end from ground. If used, high-voltage disconnect plugs at the HVI shall then be removed to isolate the station end from station ground while connecting the field end to the cable going to the remote location. High-voltage disconnect plugs are reinserted after all pairs and shield connections have been made. The use of Class 2 rubber gloves and insulating blankets is mandatory when working on or near protective equipment or telecommunication cables serving the electric supply location.

Refer to OSHA 1910 Subpart I - Personal Protective Equipment [B25] for information on voltage ratings. 12

b) In wiring protective devices, wiring and equipment associated with the electric supply location side of the protective device shall be adequately separated from the CO side to withstand impulses up to the BIL of the protection apparatus. c)

12

Bonding, grounding, and isolation procedures in installing protective devices are extremely important. Faulty grounds, bonds, and or isolation procedures can make an expensive installation inoperative. These procedures have been clearly defined in this standard and shall be meticulously

In Canada, refer to CSA Z259.4 for gloves [B7] and Z259.6 for blankets [B8]. Other jurisdictions may have equivalent standards.

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followed. For the most part, the bonding, grounding, and isolation procedures described in this standard are different than those observed in the general plant environment. d) Test sets and tools used for either installation or maintenance activities shall be of the batteryoperated type. The use of ac power test tools shall be avoided. e)

Consideration is to be given to including telecommunications cable work at electric supply locations in the permit and tagging system used to protect personnel working on power circuits.

f)

Work shall not be performed on telecommunication circuits when electrical storms are occurring in the area through which the circuits pass. Furthermore, work is not to be performed on equipment that has become wet from rain or other causes.

g) When both telecommunications and power company personnel are involved in an installation, close cooperation between the companies is required to maximize personnel safety.

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Annex A (informative) Bibliography Bibliographical references are resources that provide additional or helpful material but do not need to be understood or used to implement this standard. Reference to these resources is made for informational use only.

[B1] Accredited Standards Committee C2-2007, National Electrical Safety Code® (NESC®)13, 14.

[B2] AIEE Special Publication S117, Application and Protection of Pilot Wire Circuits for Protective Relaying. July 1960.

[B3] Anderson, R. B. and Eriksson, A. J., “Lightning parameters for engineering application,” Electra, no.

69, pp. 65–102, Mar. 1980.

[B4] ANSI C62.61-1993, American National Standard for Gas Tube Surge Arrestors on Wire Line Telephone Circuits.15 [B5] Bendito, E., A. Carmona, A. M. Encinas, and J. J. Jimenex, “The external charges method in grounding grid design.” IEEE Transactions on Power Delivery, vol. 19, no. 1, pp. 118–123, Jan. 2004.16 [B6] Blume, Steven W., High Voltage Protection for Telecommunications, IEEE Press, 2011.

[B7] CSA Z259.4-M 1979, Rubber Insulating Gloves and Mitts.17 [B8] CSA Z259.6 1981, Rubber Insulating Blankets.

[B9] del Alamo, J .L., “A powerful tool for grounding design in high voltage substations,” 6th Mediterranean Electrotechnical Conference Proceedings, vol. 2, pp. 1440–1444, May 1991.

[B10] Electrical Transmission and Distribution Reference Book, Westinghouse Electric Corporation, 1964.

[B11] Geer, E. W., Jr., and J. R. Whatmough, “Staged fault and laboratory tests on a cable pair protection system designed to serve power stations,” IEEE Transactions on Communications, vol. 22, no. 2, pp. 193– 199, Feb. 1974.

[B12] GR-974 (1999) Generic Requirements for Telecommunications Line Protector Units (TLPUS), Telcordia.18 [B13] IEEE Committee Report, “Lightning protection in multi-line stations,” IEEE Transactions on Power Apparatus and Systems, vol. 87, no. 6, pp. 1514–1521, Jun. 1968.

[B14] IEEE Interim Committee Report, “The isolation concept for protection of wire-line facilities entering electric power stations,” IEEE Transactions on Power Apparatus and Systems, vol. 95, no. 4, pp. 12161229, July/Aug. 1976.

13

The NESC is available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/).

National Electrical Safety Code and NESC are both registered trademarks and service marks of The Institute of Electrical and Electronics Engineers, Inc. 15 ANSI publications are available from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/).

16 IEEE publications are available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/).

17 CSA publications are available from the Canadian Standards Association, 5060 Spectrum Way, Suite 100, Mississauga, Ontario, Canada, L4W 5N6 (http://www.csa.ca/).

18 Telecordia General Requirements are available at: http://telecom-info.telcordia.com/site-cgi/ido/docs2.pl?ID=050179604&page=docs_doc_center 14

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

[B15] IEEE Std 4™-1995, IEEE Standard for High-Voltage Testing Techniques.19 [B16] IEEE Std 81™, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System.

[B17] IEEE Std 81.2™, IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended, or Interconnected Grounding Systems.

[B18] IEEE Std 487.2™, IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of Optical Fiber Systems.

[B19] IEEE Std 487.3™, IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of Hybrid Facilities.

[B20] IEEE Std 487.4™, IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of Neutralizing Transformers.

[B21] IEEE Std 487.5™, IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of Isolation Transformers.

[B22] IEEE Std 776™, IEEE Recommended Practice for Inductive Coordination of Electric Supply and Communications Lines.

[B23] IEEE Std C37.90.1™-2012, IEEE Standard Surge Withstand Capability (SWC) Tests for Relays and Relay Systems Associated with Electric Power Apparatus.

[B24] Nahman, J., “Cable models for grounding system analysis,” IEEE Transactions on Power Delivery, vol. 19, no. 2, Apr. 2004, pp. 841–845.

[B25] OSHA 29CFR1910 Occupational Safety and Health Standards.20 [B26] Rajotte, Y., J. De Seve, J. Fortin, R. Lehoux, and G. Simard, “Earth potential rise influence near HV substations in rural areas,” CIRED 18th International Conference on Electricity Distribution Turin, 6–9 June 2005, Session No. 2.

[B27] Sunde, E. D., Earth Conduction Effects in Transmission Systems, Dover Publications, 1968.

[B28] Trueblood, H. M. and E. D. Sunde, “Lightning current observations in buried cable,” Bell System Technical Journal, vol. 28, pp. 728–302, Apr. 1949.21

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The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc. 20 OSHA documents are available at https://www.osha.gov 21 Available at: http://www3.alcatel-lucent.com/bstj/vol28-1949/articles/bstj28-2-278.pdf

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IEEE Std 487.1-2014 IEEE Standard for the Electrical Protection of Communication Facilities Serving Electric Supply Locations Through the Use of On-Grid Isolation Equipment

Annex B (informative) Telecommunications cable in the electric supply location GPR ZOI A telecommunications cable that has its metallic shield grounded in a GPR ZOI will pick up current during the duration of the fault. The current will flow along the shield to remote ground and will cause a voltage on the shield. This phenomenon results in a transfer of potential from the GPR to the metallic shield.

When a dedicated cable carrying circuits to an electric supply location connects to a general-use telecommunication cable that continues through an area subject to a GPR, there are two general areas of concern. The first is a reduction in reliability of the circuits serving the electric supply location and the second is safety, service, and damage aspects of the general-use cable and the customers served by that cable.

The dielectric strength of cable plant depends on the type of cable and condition of splicing locations.

Many companies select several hundred volts as a minimum for cable dielectric in field applications.

If calculations, or experience, indicate that pair-to-shield voltages are excessive, full-count protection (all pairs protected) using GDTs may be applied at an appropriate point. If protection is placed at the RDL, MDRs with GDTs should be used on Class A circuits serving the electric supply locations.

Cable conductor and shield voltage and current should be evaluated to determine possible cable damage and the need for special protective apparatus. The impedance of customer drops (entrance wiring) can have a significant effect on cable pair and shield potentials. A further consideration is the effect that current flow on the shield of the general-use cable might have on the electric supply location telecommunication services. Because shield and cable pair potentials can differ from that of surrounding earth in the ZOI, safe working practices are required when contacting these cable pairs or the cable shield.

Refer to IEEE Std 487 for a detailed example in which the circuits to the electric supply location are in a general-use cable that continues through the ZOI.

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