From: Mitigation of Interference From Power Line Disturbances. Part 1 of a Two-part Series on Effects of Power Line Disturbances on Nearby Metallic Telecommunications Facilities, Focuses on the Major Factors Involved in the Problem, by Russ Gundrum, Telephony, August 11, 1980
“Most telecommunication power-line induced voltage and current problems are the result of long, paralleling facilities. Traditional approaches such as improved cable shielding (including sheath continuity and grounding) and the application of spark-gap protectors (such as carbon blocks and gas tubes) or energy-absorbing suppressors (such as zener diodes and varistors) may not represent the most practical, effective or economical solution to the problem. Alternative solutions may need to be considered, particularly if service improvements are to be maximized and maintenance expenditures are to be minimized.”
“No longer will the simple, infrequent telephone call that may be occasionally noisy and a nuisance be looked upon as unimportant; the increasing use of voice circuits for data transmission will demand that the interfering effects of power line disturbances are controlled for error-free service.”
“One thing can be said about the power influence parameter—it is mainly under the control of the power utility. It is one of the most difficult interference parameters to control, because even with the best inductive coordination efforts between the power and telecommunication industries, there is only so much that can be reasonably accomplished. Many factors that go into the complex nature of power interference are highly variable and even if seemingly solved at one point in time, the same problem or another may return, so the solution may have only been temporary at best.”
“The coupling parameter in the interference interaction model...is part of the environment the telcos must operate in, and thus, if you are to be successful in mitigating power influence via this parameter, you must do your homework in the planning and design stages of the initial cable route, or you must live with the consequences and mitigate the interference by some alternative method later.”
“The coupling...phenomenon is best described by a set of mathematical equations (involving 23 different variables!) developed in the 1920’s by John Carson of Bell Laboratories. A graphical representation of Carson’s equations for determining the mutual impedance between the power and telephone systems is shown ... for 60 Hz, since this is the frequency from which the greatest amount of induction occurs. It can be seen from this diagram that the highest mutual impedance occurs when the facilities are closest together at the highest earth resistivity. Thus, it is reasonable to conclude that one of the worst inductive environments will be in an area of granite earth formations (10,000 meter ohms) on a long, paralleling, joint-buried cable route. So in the initial design, based upon practical realities, one would strive for maximum separation with a slanting or perpendicular exposure. However, with telephone and power utilities striving to serve the same customers, in all probability, a close paralleling situation will exist and roadway separation might not be enough, because the induced voltage is only cut in half by going from 25 to 300 feet away! Also, merely burying the cable does not significantly improve the situation. This only helps to increase the separation distance, as the earth is not an effective barrier of electromagnetic interference.”
“Standard aluminum and copper cable sheaths, while effective in reducing harmonic interference, if properly bonded and grounded, are NOT effective in reducing 60 Hz induction. One must place heavy, ferromagnetic shielding material (remember lead?) if effective 60 Hz reduction is desired. However, the cost and construction and maintenance difficulties of applying heavy steel tapes to cables or placing cables in iron pipe conduit, may not represent the best engineering solution compared to other mitigative techniques.”
“If the cable route happens to be in a ‘built-up’ area where there are substantial metallic shielding conductors such as water and gas pipes, other metallic cables, railroad tracks, or steel reinforcing used in buildings and concrete highways, then additional shielding reductions may be obtained. However, more plastic pipe is being used today in water and gas systems, and the farther away from a metropolitan area the cable is routed, the less of a built-up area will be encountered.” (i.e.—suburban and rural areas!)
“The third and final parameter of the interference interaction model is the susceptibility aspect of telecommunication facilities. In other words, how sensitive are the equipment and cable pairs in converting longitudinal interference into metallic noise or into causing malfunctions, damages or a personnel safety hazard? Probably the most common problem area in the telecommunication industry from a metallic noise standpoint is from unbalanced cable pairs. However, there is a practical and economic limit to how well balanced the cable facility should be maintained in solving a noise problem. For instance, if the power influence parameter is exceptionally high, it is as honorable a procedure to try and mitigate that influence, even if you have unbalanced facilities, because you can at least immediately improve subscriber service and buy some time while making plans to rehabilitate or replace some of the cable plant, which may not affect the power influence levels at all.”
“However, there is a practical and economic limit to how well balanced the cable facility should be maintained in solving a noise problem. For instance, if the power influence parameter is exceptionally high, it is as honorable a procedure to try and mitigate that influence, even if you have unbalanced facilities, because you can at least immediately improve subscriber service and buy some time while making plans to rehabilitate or replace some of the cable plant, which may not affect the power influence levels at all.”
“Equipment sensitivities have taken on a greater significance today with the advent of solid-state electronics that will not tolerate as much induced voltage and current as equipment of the older electromechanical design. Placing mitigative devices in front of this newer equipment to reduce such interference is the price that must be paid for taking the metal out of the design. Longitudinal induced AC voltages as low as 10 to 20 volts to ground and longitudinal induced currents of 3 to 4 milliamperes can cause false equipment operations and even take some equipment out of service.”
“The government, in OSHA Rule 2207 subpart K 1926.405(k), defines a ‘shock hazard is considered to exist at an accessible part in a circuit between the part and ground, or other accessible parts if the potential is more than 42.4 volts peak (30 volts rms) and the current through a 1500 ohm load is more than 5 milliamperes.’ Under conditions of negligible contact resistance, which could occur with a salt solution on the skin due to perspiration or from cuts and abrasions, a minimum body resistance of only 200 – 500 ohms instead of the normal 1500 ohms might exist. Thus, a telephone craftsperson working on a hot, muggy day could be very susceptible to an electrical safety hazard.”
“The National Association of Corrosion Engineers (NACE, headquartered in Houston) has included in their standard practices that facilities (pipelines!) measuring over 30 volts rms open circuit and 5 milliamperes are unsafe and should be treated as live conductors. However, there is a controversy over what steady-state level is considered unsafe and it can range from the 50 volts-to-ground criteria for the Bell System to the 7.5 volts-to-ground criteria, which the railroad industry is considering.”
“One thing can be said in summary for the susceptibility parameter of magnetic interference: this is definitely under the control and responsibility of the telecommunications operator, and measures can be taken to mitigate the problems.”
“From a longitudinal-to-metallic noise conversion standpoint, a noise problem would be more likely with a resistive unbalance near the central office (when the original grounded battery configurations existed), since the longitudinal current would usually be the highest at this location. Conversely, metallic noise would be more likely with capacitive unbalances near the subscriber end, since the longitudinal voltage (and power influence) is usually the highest at this point in the circuit.”
“An interesting study of the distribution of longitudinal voltage and current was conducted by Bell Labs in 1971 as part of a noisy-loop survey. It is important to note that 90% of the noisy loops measured had less than 28 volts. However, the remaining 10% of the loops had up to 115 volts. The survey also noted that 90% of the loops had induced longitudinal currents less than 2.4 milliamperes, with 10% of them having up to 10 milliamperes. An important concept known as the center-of-exposure was developed from this survey. This concept states that based on the 1971 results, 90% of the loops measured had the electrical halfway point of 60 Hz induced longitudinal voltage 0.88 of the distance away from the central office.” (This is good, since this means that the cables with the bulk of the induction problems, probably due to single-phase power exposures, are out near the ends of the exchange area boundaries, which are mostly small-paired facilities. This makes mitigation costs much less expensive, and prevents the worse-case induction conditions from coming back into larger, non-exposed feeder cables and causing interference problems.)
“In some cases, as little as 20 amperes of ground-return current will be enough to cause 20 volts-to-ground for a two mile exposure or up to 50 volts-to-ground for a five mile exposure.”
“Many believe that any mitigation device is nothing but a ‘Band-Aid’ that ‘masks’ the true cause of the problem, whether it is added to the telecommunication or power company facilities. It is interesting to note that these same people tend to overlook the fact that mitigative techniques are commonly used in both industries, but they may not be as obvious as adding a piece of highly visible hardware ‘after the fact’. If the mitigation device had been built into the plant in the initial planning and design stages and was not as obvious as it is when applied in a maintenance effort, then it probably would go unnoticed or would be considered a standard part of the operation.”
“Usually for the overall cost per pair involved, it is much less expensive to mitigate the power influence on the telecommunications facility and you may have the added benefits of reducing equipment malfunction or damage problems and removing a personnel safety hazard.”
“Many believe that any mitigation device is nothing but a ‘Band-Aid’ that ‘masks’ the true cause of the problem, whether it is added to the telecommunication or power company facilities. It is interesting to note that these same people tend to overlook the fact that mitigative techniques are commonly used in both industries, but they may not be as obvious as adding a piece of highly visible hardware ‘after the fact’. If the mitigation device had been built into the plant in the initial planning and design stages and was not as obvious as it is when applied in a maintenance effort, then it probably would go unnoticed or would be considered a standard part of the operation.”
“The only feasible alternative left is the induction neutralizing transformer (INT), which is a specially designed inductor to provide the most effective mitigation for these types of circuits.”
“Another similar inductor (from the standpoint of how it is spliced into the cable) that is commonly used in the telecommunication plant is applied for transmission mitigation reasons and is known as the load coil. These devices are generally placed at 6,000 foot intervals on all voice-frequency telephone circuits exceeding 18,000 feet in length in order to mitigate the capacitive effects of employing fine gauge copper wire (enclosed in shielded cables) that would normally attenuate transmission signals in the voice band (providing passive gain). No one ever really considers the cost per pair for load coils, especially when weighed against the alternative approaches.” (Now look at the cost per pair to REMOVE load coils for DSL circuits! It’s also interesting to note that the typical loops that needed mitigation from an inductive interference standpoint were those long loops requiring load coils!)
“Many have the attitude—especially when considering the use of INTs—that mitigative approaches can only be applied as a ‘last resort’ when all else fails. It is always interesting and somewhat ironic that an approach or device that will work so well under that severe test will receive such a critical analysis before it’s applied. However, there are those who have carefully weighed the engineering and economic considerations of reduced repair man-hours, reduced customer trouble reports, improved quality of service, the prevention of lost toll revenue (from T-carrier cut-offs and busied-out noisy trunks), and the elimination or delay of expensive plant additions or rearrangements, and have designed an INT into the original job. They know that inductive coordination efforts with the power company alone will not be enough to solve all of the aforementioned problems or in eliminating a potential safety hazard.”
“Typical telecommunication problem areas that will require mitigation treatment with INTs:
*Areas of rapid growth and resort communities (Pinehurst/Tomball; Gulf coast beaches)
*Cables that are predominantly aluminum or copper sheaths
*Areas of high earth resistivity (Pinehurst/Tomball; Arkansas and Missouri)
*Areas where cut-over plans are being made to connect cables known to have induction
problems with electronic central office, electronic PBX’s or key equipment or digital
carrier systems for trunking or subscriber use (SLC’s and Project Pronto)
*Metallic cables that are paralleling and are close to or in a utility corridor or fee strip
containing many different transmission lines (or even little single-phase lines!)
*Metallic cables that are paralleling electrified railroads”
“It has been said about the mitigation philosophy that it is great to be able to do the marvelous things that can be done over a pair of wires, but there will come a time when the piper will have to be paid for all of these advantages. Also, the more metal that is taken away from the cable sheath, its surrounding environment and the equipment, the more likely it will have to be put back in some form or another.”