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Bonding of Distribution Lines and Communication Cables

A report developed with and to be used by North American Standards organizations, utilities and communications companies.

Project Background

​A telecommunication line (cable, TV, telephone) in parallel with an overhead or underground power distribution line is subject to electromagnetic induction from the power system under both normal loading and fault conditions. In order to minimize the possible hazards and damage caused by the electromagnetic coupling from the power system, bonding of an overhead telecommunication messenger or underground communication cable sheath to the electric utilities’ neutral is common practice. In some North American (CSA and IEEE) Standards, the interval between bonding locations has been specified as 300 m.

On overhead pole lines the neutral is accessible in most locations, but the interconnection of these two systems results in significant work during installation. Underground feeder circuits run uninterrupted for hundreds of meters, and such bonding is even more time consuming and expensive.
North American utilities desired a detailed modeling study to determine the technical rationage for the 300m bonding practice, and recommendations on whether revisions could be made to reduce the requirements. The stakeholders asked, "Is a 300 m bonding interval too conservative? Would hazards to equipment and personnel rsult if bonding was less frequent?"


Scope of Work

The Kinectrics project (conducted for CEATI) studied the electromagnetic coupling between the telecommunication and power distribution lines for joint-use overhead or underground construction.   A sophisticated COMSOL model was created to assess both overhead and underground power lines and their adjacent telecommunication conductors.  The model computed the voltages and currents induced on the communication cable sheaths during normal operating conditions and faults on the power lines, and assessed whether these values exceeded agreed upon threshold values.  Various scenarios were evaluated considering different line voltages, grounding resistances, and intervals between bonding locations.
The project produced quantitative data and rsiults that established the following findings:
  • Induced voltages and currents on the communication cable are dependent on the separation distance and orientation of  the power and communication cables, as well as their orientation;
  • It is necessary to ground the communication messenger\sheath for a communication cable running in parallel with distribution power lines over a long distance to limit the electromagnetic induced voltages on the communication messenger\cable sheath.
  • Under fault conditions on the power system, a Ground Potential Rise (GPR) occurs and voltage magnitude is distributed along the power system neutral. The GPR is dependent primarily on the grounding resistance. This GPR dominates the overvoltage that is imposed on the telecommunication sheath that is bonded to the neutral.
  • There is a Ground Potential Difference (GPD) between the power system neutral and telecommunication messenger\sheath at the same location that is greatly affected by the bonding interval;
  • With a 300 m bonding interval, the voltages on the communication cable sheath were maintained below the accepted limits for the cases in this study, where multiple 25Ω neutral-ground resistances were assumed. For the 300 m bonding interval, the maximum voltage difference between the neutral and communication sheath was less than 300 Vp, However, this parameter did exceed 300 Vp when the bonding interval was increased;
  • In the modelled scenarios with 300 m bonding intervals, where the neutral grounding was greater than 25Ω, the maximum GPR and GPR gradients were greater than those in 25Ω scenarios. With grounding resistances of greater than 25Ω, the GPR can exceed the 3000V criteria; 
The report explains how the bonding of the telecommunication overhead messenger or underground communication cable sheaths to the electric utilities’ neutral impacts the electromagnetic coupling.  It provides the rationale for the typical 300 m bonding interval and illustrates under what conditions intervals other than 300 m may be acceptable, and the GPR and GPD impacts if changes to the bonding interval are made.
The report will be used by (and was developed in conjunction with) North American Standards organizations, utilities and communication companies to validate and potentially modify the guidelines for bonding of power system neutrals and telecommunication sheaths.
Further investigations/simulations are required to study the bonding interval impacts on distribution and telecommunication designs in joint use installations that are more complex than those addressed in this report, i.e. varying pole ground rod resistances and multiple grounding sources of the power system.
The prevalence of equipment or personnel exposure to GPD voltages, and the ultimate limiting value on GPD required for specific installations should also be examined in further studies.
For the underground application, if both power neutral and communication sheath contact the soil directly, the current may present in the soil where the GPD appear.