From: The Subscriber Loop Revolution...What Will Work, and Who Will Work, to Make IT Work: The Public Will No Longer Tolerate the Call That is Occasionally Noisy, by Russ Gundrum, Telephone Engineer & Management, September 1, 1981
“Consider the following scenario: Mr. Jones has just purchased one of the latest telecommunication pieces of equipment at one of the local department stores. Not only could he just plug it into his telephone outlet at home and use it as an ordinary phone, he could have it record messages, remember previously stored numbers and automatically dial them at the punch of a button, and perform many other features normally associated with the electronic PABX at his office. However, this particular product could do much more. Tied into the home television, he could play a variety of dialed-up video games, receive stock quotations, conduct his personal banking, order items out of a catalog listing, and even conduct his business at home by calling up his office computer system. In fact, by doing his own inside wiring, he could connect his own burglar and fire alarm system and even certain appliances and environmental controls that could be remotely activated. (Remember, this was written in 1981, way before the PC revolution and even before the AT&T divestiture on 1/1/84!)
Now, of course, word spread around the neighborhood of all these marvelous things that Mr. Jones was able to do with this newly discovered technology. Not to be outdone by the Joneses, others decided they would shop around and find similar, if not better, devices that might even be less expensive and perform more functions. In fact, the whole community was actively involved with this endeavor, because it was becoming more of an economic necessity than just a fad.
Suddenly one hot afternoon, the equipment stopped working at Mr. Jones’s house. Luckily the telephone was still operable, so he could call the telephone company to see what had happened. The utility’s centralized remote testing center did a quick check of Mr. Jones’ line and said THEIR lines were OK—the fault was probably in his equipment. That sounded strange to him since everything worked fine during the morning and at night, but failed to operate in the afternoon. After deciding to pay for an expensive repair visit, which also proved that no troubles were found on the Jones’s telephone line, Mr. Jones decided to return his set to the store where he had purchased it. When tested at the downtown store location, everything appeared to be working fine. However, since the equipment was still under warranty, the store gave Mr. Jones a new one to try out.
We don’t have to guess what happened. The new set experienced the same problems as the old one. Mr. Jones soon discovered that some of his neighbors were also having similar problems—even though they had other manufacturer’s equipment. In fact, one neighbor had called one of the new private telephone repair shops, who made some tests and had placed a telephone line interference filter to correct the problem. This didn’t seem right to Mr. Jones, even though the expense and trouble he had already been through would have more than paid for the filter. He felt that the problem was obviously in the phone company’s lines and that they should go to the expense of correcting it. Besides, he reasoned, what if the filter didn’t work under all circumstances? Also, the telephone repair person had said there appeared to be a power company problem which he had no control over.
Mr. Jones finally decided he would lodge a complaint against the power company to see if they could help solve his problem. The power company repair person came out and said that since Mr. Jones lived out of town and was served by a long, single-phase power lead, he was not able to help him either. The power company was not going to spend the money for two more phase conductors to serve his area, and even if they did, there were no assurances that the effort would be a permanent solution.
By this time Mr. Jones had reached his limit of frustration! Although his phone worked, all the other features that his equipment was suppose to perform, and the ones he had grown to depend on, did not work at all times. Since his local telephone rates had dramatically increased over the past year, he felt he deserved better service from the utility. This time he decided to take his complaint to the local telephone company manager. The manager promptly informed Mr. Jones that his crews had been working on the cable route serving his area quite a bit in the past several years, but felt helpless during peak power load conditions. His people had dug up splices and checked their cable sheath bonds and had spent a small fortune in driving ground rods. In fact they, also, had been experiencing problems with their new electronic central office equipment.
After hearing all parties’ stories and still not having his problem resolved, Mr. Jones decided to try a last resort measure and write the public service commission about this unique predicament. Luckily, before the letter reached the attention of board officials, a regulatory staff engineer got involved and suggested to both the power and telephone utilities that they get their heads together and explore their mitigative alternatives for solving the problem. Hopefully they could come up with the best overall engineering solution, since the customer was obviously going to pay for it in the end anyway. Also, if they didn’t resolve it between themselves quickly, the commission might tell one of them to fix it, and that fact just might be brought up the next time a request for a rate increase was filed.”
“Actually the previous scenario is nothing new to those of us who get involved with these types of problems on a daily basis. The only difference today is the fact that we are now dealing with more of a consumer market than ever before. A forewarning of this
growing situation has already occurred in the interconnect industry, primarily because of all the electronic PBX, Key and other special circuit equipment applications. Obviously the scenario could have been painted much worse, because in many cases the telephone will not even function—much less all the other desired services. And now that we have the very real prospect of the homeowners doing their own inside wiring with a direct, hard-wired connection to the network, a real possibility of a potential electrical safety hazard will exist in some areas. It is not uncommon to deal with customers who have more than 100 volts rms to ground on their telephone cable pair, in addition to normal DC battery plus ringing voltages. These AC power-line induced voltages and the resulting alternating currents that could be in excess of several hundred milliamperes, well exceed generally accepted safety criteria.”
“Power induction problems are nothing new to the telephone industry. They have been around since the beginning of power and telephone service, and as long as both utilities continue to serve the same customers over metallic facilities in the same general right-of-way, these problems will continue to plague us and get worse before they get better. The future revolution in the subscriber loop has already been established. Advances in electronic technology, coupled with the competitive environment, promise a whole host of services to the consumer that will make his telephone pair and his own equipment an invaluable asset that will be utilized 24 hours a day. Presently, few people consider how really important their telephone service is, and how dependent they are upon it, until it fails. Then they become very concerned! The public will no longer tolerate the call that is occasionally noisy as an acceptable nuisance. This effect in grade of service expectations can already be witnessed in resort areas of rural communities when people from the cities try to use the local phone service and report it as unsatisfactory. No longer will we be able to tell our customers that rural line hum is not correctable. They just will not accept it!”
“This problem is not a small one that will go away. It is a major one that threatens to have a dramatic impact on everyone in the entire telecommunications industry all over the world: from the supplier to the telephone utility engineer and craftsperson and certainly to the customer, who ultimately will bear the expense of correcting these problems. (Or decide to choose a competitive service that doesn’t have these problems!) The most important thing is getting a handle on this situation and solving the problems before they arise. The real need is for a basic understanding and a perspective on what has to be done in the most timely, practical and economical manner possible.”
“There was much discussion at the International Conference on Communications (ICC’81) in Denver, Colorado last June 14-18 on the need to establish telecommunication systems performance standards in order to qualify, compare and classify the quality of service to the customer. For instance, does the problem lie with the customer’s equipment or is it in the network, or is it a combination of both when they are tied together? While the consensus of opinion was in agreement for such industry standards, it appeared obvious that this development would not occur immediately and that natural market forces will probably decide the resolution of these matters in the interim. The communication engineers’ focus of concern was more on transmission loss, echo and noise performance levels. Nothing was brought up about the fact that even if you have a good metallic circuit with adequate singing margin, low loss and noise, that the equipment connected to it may still not even work because the 60 Hz common mode rejection threshold had been exceeded. This is becoming a more potent problem today, because of the economic design limitations placed on the competitive terminal equipment in lieu of no established standards on the subject. It costs money to design and manufacture equipment to tolerate such a situation, and if the majority of loops don’t need it, why go to the expense?”
“Whether you call this disturbing voltage induction, foreign EMF, common mode, longitudinal voltage, or voltage-to-ground, by the time you have 30 volts of it and 5 ma of longitudinal alternating current flowing, personnel safety, equipment malfunction and noise become real problems, so these would probably be reasonable objectives from a design standpoint. However, there are presently numerous examples of equipment on the market today that will not even tolerate half of these values. Some people would like to see the design threshold raised to the Bell System tariff value of 50 volts rms to ground, which was established many years ago for a craftsperson’s safety. This value though, did not take into account the fact that newer electronic devices would come on the market and not be able to tolerate this much induction from a noise generation standpoint (due to saturation effects), or from an operational standpoint, as well as the electromechanical equipment it replaced. This arbitrary value of 50 volts AC rms also did not take into account the public having direct access to the wire conductors.”
“The real crux of the problem, of course, really depends on where the equipment is used and how much induction it will be exposed to. The longer the cable from the central office, the more likely will be troubles of an inductive interference nature. What may work fine in town, may not work out in the suburban or rural environment.” (Did someone say ADSL and Project Pronto?)
“Part of the reason we find ourselves faced with this frustrating dilemma is the fact that many people think that since they have the latest technology equipment and the best gas tube protector on the market, that they should not have a problem. There is a false sense of security that the primary protector, even if properly grounded, is going to furnish all the protection that the customer’s equipment needs from all types of power line disturbances. There are many case histories—especially in the rural environment (where noise and protection problems are the greatest) that this just isn’t true! It is not a coincidence that this environment just happens to be where the AC induction levels are high enough to aggravate these problems. The biasing of gas tube or other protectors by induction (since it is voltage to ground) is a large contributor to repair and maintenance problems.”
“Who really wants to see a protector operate anyway? (Since that momentarily disconnects their service.) Are the secondary protection devices on the market today in the form of MOV’s, zener diodes and other transient suppressors the answer to suppressing voltages below 350 volts rms? Do we really need all of this expensive protection, which is used only in the name of equipment reliability and is only designed for transient or surge conditions that sneak by the operational level of the primary protector? What happens when AC induction of a steady-state nature is high enough to operate these devices continually? Equipment fails to function and customers are irate, that’s what happens!”
“We all agree that basic telephone service is becoming better because of improved test equipment and newer plant facilities. But what happens, after investing all this time and money into new cable and equipment, if we still have a problem due to the effects of 60 Hz induction? WHO is going to answer the fundamental and overriding questions of HOW do you control the effects of AC induction and WHO is responsible for correcting them? Also, WHO will answer the embarrassing questions from the public when things don’t work, their own equipment is damaged, and all of this happened on a warm but beautiful day or, even worse, on a cold and miserable day?”
“It should come as no surprise that the basic policy of Part 68 of the FCC rules, which allowed for competition in the network, is to protect the telephone network from potential harms that might arise from the operation of telephone terminal equipment located on the customer’s premises—whether the equipment is provided by the telephone company or the customer. What is surprising, however, was the Commission’s decision last Fall that ‘there are no requirements in Part 68 to protect the terminal equipment from potential harms originating from the telephone company’s side of the demarcation point—such as lightning transients or induced hum. As a consequence, the registration program excludes devices such as induction neutralizing transformers (INTs), which are specially designed to suppress the effects of longitudinal AC induction.’ According to the FCC, ‘INTs are considered like carbon or gas tube protectors, repeaters or facility termination sets, which are normally associated with loop plant located on the telephone company’s side of the demarcation point.’ The FCC pointed out in its ruling, though, that ‘if customers wanted to use these mitigative devices, they would neither approve or disapprove of their use.’ This is basically the same policy the Commission declared in Public Notice #25923 on February 1, 1980 regarding the use of supplemental gas tube or MOV protectors that are used in addition to the telephone company provided protector.”
“Thus, the industry should not expect to look to the federal government for assistance in defining acceptable levels of AC induction or who is responsible for paying for its correction in the event a problem occurs. There are serious and legitimate concerns of the telephone utilities that say they should not be responsible for providing loop treatment devices when the longitudinal balance of customer owned and maintained equipment is poor and causes a noise problem to occur when connected to a line having generally acceptable levels of power influence. Even if the balance were good and noise was not a problem, the longitudinal voltage or current equipment thresholds could still be exceeded and cause a malfunction to occur, or at worse, fail to operate and even damage the equipment. Does it make more practical economic sense that in those areas which might be affected by such an occurrence, that a filter should be placed either in the cable or ahead of the equipment instead of having that line or equipment redesigned and recertified? In this particular problem area of power line interference, the customer will probably have to pay and install the mitigation device himself when the levels fall into the gray area of responsibilities—which for most telephone companies will probably be less than 50 volts rms to ground. Any 60 Hz voltage above that level will probably want to be corrected by the telephone utility, if they haven’t already had to do so in order to prevent problems with personnel and equipment.”
“If the problems of inductive interference have been with us since the beginning of power and telephone service, then the solutions have been with us that long. Devices such as longitudinal chokes, neutralizing transformers and drainage reactors were developed long ago to suppress power line interference on open wire lines. Significant improvements in this field were made when the industry moved into twisted, paired conductors instead of ground return circuits (which incidentally, continue to be reinvented!) and enclosing the pairs in a metallic shielded cable. No system is perfect, however, and even well balanced and shielded cable pairs have their limitations when it comes to the brute force of 60 Hz induction; also perfectly balanced power systems are a practical impossibility (not withstanding the fact that unbalanced power systems don’t cause a problem to the power company). And guess who is a sitting duck for knowing when the power system experiences a problem? That long, paralleling telephone cable is a big antenna that is going to pick up interference whether it’s up on the power poles, or is buried underground, and it won’t help much even if it’s across the road!”
“In order to live and operate in this hostile electrical environment today, it is necessary to put back some of the metal we have generally been removing from the cable sheath and the equipment. However, it is just not economically feasible or practical to place our cables in iron pipe conduits. Instead, we can lump this iron at one or more locations in the form of an INT. This type of transformer is often confused with the large, oil-filled high voltage neutralizing transformers that are specially designed for critical communication circuits into power stations that can’t tolerate even a momentary protector operation. The principle of operation of how the INT works is the same, but the costs can be as little as $12.00/pair. Alternative non-metallic communication systems such as fiber optics, microwave or satellite fill certain needs, but have not graduated to become competitive against properly engineered and maintained metallic cable systems (since they have active components that require backup power), especially if the only reason these alternatives are being considered is because of their immunity to power interference problems. We simply can’t yet afford to junk our investment in copper plant.”
“It is often hard to distinguish where noise mitigation ends and protection engineering begins, since the two overlap. But looked at from a total operating system’s viewpoint, it is usually most cost effective to neutralize the longer loops, since they will have the greater exposures to power and are the bulk of our noise and electrical protection problem areas. When such cable routes are mitigated with an INT, a dramatic decrease in trouble reports, equipment failures and noise problems results, because 90 to 95% of the AC induction on the cable is divided and collected across the transformer instead of appearing at the customer’s premises. This same effect also occurs during power line surge conditions, even those occurring during lightning storms. Many of our damages and protector operations come from lightning-initiated power arc follow-through, even though we usually just blame the lightning and assume there is little we can do about it. Since harmonic noise is part of the 60 Hz frequency, any reduction in 60 Hz energy by an INT will also cause a reduction of its harmonics. This helps explain why power influence reductions of 20 to 30 dBrnc are often reported, along with impulse noise reductions that override on the 60 Hz and its harmonic wavefronts.”
“Telephone company inductive interference problems and solutions are somewhat analogous to the situation of electronic computer equipment failures on the AC supply side due to common mode (longitudinal) and transverse (metallic) voltage surges. What’s one of the solutions for these problems? The age old isolation transformer; only nowadays, it’s a super or ultra isolation transformer for suppressing noise coming in directly over the AC line. This technique is also used in the telephone industry on dry, tone-type transmission circuits. However, the bulk of telecommunication circuits still require DC continuity. This is one of the main advantages with neutralization over isolation, besides the fact that isolation transformers are only for single-circuit applications. The analogy to power system filters such as isolation transformers is more than just technical. Who pays for this device when it’s needed?”
“Now we understand that what seemed to be an unsolvable and complex problem, does have a practical and economical solution. We can lay to rest our fears and decide that we can make anything work with the proper equipment and attention of all responsible parties. Yes, there is an expense involved with everything we do in this business, but the basic goal has been and should always be providing the best possible service at the lowest possible price. Any service or product's results are a measure toward meeting that objective.”
“Although this subject matter may be glamorless, it is the substance upon which the present and future telecommunication services depend. The factors contributing to inductive interference problems are increasing significantly, but the progress in accepting the techniques required to control them seems slow. The mitigation technology has been around for sometime now and has been standardized by many operating companies and accepted by the REA. However, many still have the attitude that something will surely come along to solve the problem and that although the mitigation device works great, the equipment is a black-box technique and only a band-aide approach that masks the true solution. Of course there are no funny tricks or black magic being performed here. The physics of the problems and solutions have been the same all these many years. The only thing that has changed have been the personnel, and that’s the reason there is such an important need in the area for educating all telecommunication people that become involved with these problems.”
“Except in certain circles, noise has been considered taboo by many in the telephone industry and no one wants to admit they have it. Noise problems are what everyone can easily relate to since they all can hear it, as opposed to the less than obvious effects of raw induction that can’t be heard, but can be creating other seemingly unrelated havoc on the entire system. Even though noise can be a reflection on the quality of workmanship and can reflect the abilities and commitments of management to control it, it should be emphasized that the best efforts in trying to utilize the plant facilities as they were originally engineered can still fall far short and the problem still remains.”
“A properly trained craftsperson with the appropriate test equipment can do an excellent job in controlling cable pair unbalances and sheath continuity problems, thus obtaining a reasonable amount of reduction in some of the effects of induction. However, he exercises little to no control over the type of cable plant that is selected, the environment it must operate in, or the type of central office, line repeater or terminal equipment that is connected to that cable pair. He has about as much control over his system’s ability to tolerate all the harmful effects of inductive interference as the power system repair personnel have in controlling load balances and sources of harmonic generation! Power companies have been in the interconnection game long before the telephone companies, and as long as any device has Underwriter’s Laboratories approval, it can be plugged into their network!” (Nowadays, of course, the power companies are going to experience the joy of deregulation, something the phone companies have been dealing with for quite some time.)
“The real need is for both telephone and power utility engineers to work together as if both plant facilities were under the control of the same management. (At least the power companies understand metallic facilities and transformers!) Cooperation is the key to obtaining successful results and reaching the objective of the best overall engineering solution. Along with the power equipment manufacturer, the power system engineer should be interested in keeping his system’s influence as low as possible and the telephone engineer should try to design and maintain his system’s susceptibility to induction as low as possible. The responsibility for action, however, should come from the telephone engineer, since he’s the person with the problem!”
“The telephone engineer should be able to take his craftsperson’s power influence and AC voltage-to-ground readings and with a couple of further simplified tests with a spectrum analyzer, and a few quick field observations by riding out the cable route, should give him a good idea of what his course of action should be in correcting the problem. Hopefully, he could have predicted the situation in the early design and planning stages to minimize the impact of induction problems by knowing the coupling considerations of the environment. But more than likely he will be faced with an after-the-fact situation which he must learn to deal with. Depending on the circumstances he may be confronted with, he may opt for a temporary mitigative solution because of time constraints. This may turn out to be a permanent solution, especially if the power utility agrees to share part of the expense of the measure, since they are partially responsible for the situation in the first place.”
“Just because a special mitigative technique has been applied and has yielded very successful results, doesn’t mean conditions may change warranting further mitigative and/or inductive coordination measures. There is no substitute for good, basic cable maintenance procedures after the installation of a mitigative device, or that further coordination with the power company has to stop. For instance, a capacitor bank may be added by the power company in the future that could increase the 540 Hz noise tremendously. Tests would have to be jointly conducted by both parties to see what the best possible solution might be. This could involve relocating the capacitor bank, resizing it, floating the ground or neutral connection, physically or electrically with a thyristor-type switch, or detuning the resonant condition with a neutral reactor. Load balance on a three phase power line might be looked at as a temporary solution, but future growth in a developing area many miles from the substation provides no guarantees that the problem won’t come back, especially in times of peak air conditioning or electrical heating loads. Besides, if there is a long telephone cable exposure in high earth resistivity soil, the only course of action is for the telephone company to mitigate their plant. This also provides the best protection or insurance policy under future, unforeseen power problems and many other hard to control factors, such as the weather.”
“It should be remembered that even with all the testing you can think of, you still can’t measure noise or induction out of the system! And all of the sophisticated analysis in the world will not make the problem disappear. We all spend a lot of time, money and frustration fighting these problems on a daily basis. Time may now be running out due to circumstances beyond our control. We should start doing what we know will work to get rid of induction, so our customers will have good telephone service. Let’s get on with the job at hand before we have to be told by somebody else how to run our business.”