A Brief History of Inductive Coordination
and Open Wire Systems
Illustration from a 1926 Ohio Brass Company Catalog depicting strain and double yoke insulator assemblies. Line voltage presumably 138-kV double circuit. Note the two open wire leads below the tower line. The four-arm lead in the lower right foreground is presumably an AT&T facility. The other line on the left is likely an exchange lead. Both d.c. telephone leads would have been subject to intense inductive currents of the double circuit a.c. transmission line.
Here is a an overview of a very complex subject, with which telephone and telegraph plant engineers grappled for many years to find resolution, as open wire facilities expanded and progressed in Canada and the United States.
All open wire lines, whether commercial telephone, rail or transportation styles, were susceptible to inductive currents from nearby power systems. Sketch by D. G.Schema
We all recognize that the leader in communications within the first fifty years of this development was the telegraph industry. Overland Telegraph, Postal Telegraph, various railway telegraph systems and Western Union, operated jointly, with little conflict in their engineering experiences together. Their various lines ran in competition to one another at various cities and towns, yet the physical act of placing poles, running conductors and operating them simultaneously electrically did not impact the whole appreciably. Very little interference was affected by their common operation. Occasionally, because many systems were grounded earth one wire telegraph systems, the aurora borealis could conflict with signal transmission. There were occasions reported in the past when this phenomena impacted the telegraph companies’ operations to a significant degree.
By the 1880s, the telephone systems in towns were expanding. Toll leads to connect larger cities and exchange circuits linked to nearby towns were also being built and expanding. Around this time the birth of the electric utility industry began and low voltage a.c. power distribution and some higher voltage a.c. transmission circuits began to affect the transmission qualities of telegraph and telephone lines–most of which were overhead and not buried or undergrounded.
Telephone sets operated by many farmer mutual companies and small towns were–like the early telegraph systems–grounded earth products. When electric trolleys, electrified railroads and other traction devices were added to the mix, the interference problem grew proportionally. Things were becoming severe and questions were asked as to how to fix this increasingly bad situation.
Equipment suppliers tried to mitigate the situation with improved conductor to counteract this inductive interference, whereas this “bleeding effect” of energy spewed across other electrical systems and caused serious problems. For example, in a book entitled, Beginnings of Telephony by F. L. Rhodes, a new wire in development was announced around 1883:
“…A new iron wire was made which…would be free from inductive disturbances. Its cross-section was shaped like a four-leaf clover and the grooves ran around the wire in a spiral. The company that was making and promoting this wire interested themselves in its use on more than a dozen experimental lines. They had a theory that the voice current would follow the spiral and in some way produce a beneficial effect. An officer of the spiral wire company said it was not understood by electricians.”
Typical armless rural distribution line with open wire communications arm below. United Telephone, Kansas.
Another unique occurance along long distance open wire communications lines was the action of “crossfire” on telegraph circuits. This was when two separate telegraph circuits interferred with one another. Sometimes this was due to insulated wires’ defects in the covering, or inductive or capacitance coupling between telegraph circuits. High voltages and operating currents occasioned this problem The amount of crossfire was wholely dependent upon the value of various factors.
Something as minor as the moving parts of relays and instruments could cause voltage problems as well. The inertia of the parts moving sideways and up and down slowly without drag, was another influence on the proper operation of long distance circuits. Telegraph instruments, dependent upon prompt operation at stations, could not be impared and friction was designed to be kept minimal. Dust could interfere with moving parts of relays, instruments and dirty bearings also added to the problem. Most important instruments were kept covered for protection and cleanliness. Also important, was minimalizing arcing between contacts. The quick break action of keys and sounders on telegraph systems with the armature travel as brief as possible was vitally important, too. Sometimes, on the down stroke, the armatures would become sluggish without adjustment. This was done at wire centers, depots and stationhouses, where regular maintenance was scheduled and performed.
Whether or not this pecular conductor configuration was a success or not certainly was not reported publicly and the innovation quickly faded, as it was found that the use of metallic telephone circuits–instead of grounded types–significantly reduced interferences. A. G. Bell was one of the first to promote this more modern sequence of telephone design, but expense eliminated the mass introduction of the principle. It wasn’t until AT&T’s Chief Engineer, J. J. Carty promoted in several published papers the marked advantage of relieving interference by this metallic ground method. Furthermore, as experienced in practice by the major toll lead between Philadelphia and New York, the use of transposing wire pairs also reduced the problem when achieved with a properly engineered scheme. This came about in 1891 when the first transposition schemes were published in papers delivered by J. J. Carty, “Inductive Interference in Telephone Circuits” by the American Institute of Electrical Engineers.
Meanwhile power lines and the companies who built and operated them came in for the biggest bashing by the communications companies. It was here that the nadir of the conflict truly reached epic proportions by 1912. It was the Progressive Era. A more broad minded persuasion affected entrepreneural players. By researching the causes of these various conflicts, the immediate alternative of settling the problem in the courts was briefly considered but roundly dismissed. Clearly, this was less of a legal problem than a larger technical one. California, with its PG&E transmission lines reaching the upper limits of the power art at the time at Pitt River and simultaneously, Southern California Edison’s similar hydropower production voltage levels at Big Creek sites, became the place whereby the various parties experiencing inductive problems decided to amicably come to a mutual solution.
Open wire vertical dead-end on IOU power company joint facility. Such facilities were open targets for induction problems.
The parties met and determined that to best meet this problem head-on was to insist on forming an organization to meet, investigate the problem and then arrive at a fully agreed solution to the problem in 1912. It took about four years before all the information was collected. Much information formed the basis for published proceedings and with these preliminary results expanded to include the basis of theories which today are part of solving this problem.
In 1921, the old National Electric Light Association (N.E.L.A.) which became by the 1930s the National Electrical Manufacturers Association (N.E.M.A.) of today, joined with AT&T, to combat this situation. The Communications Section of the American Railway Association (A.R.A.)., now the Association of American Railroads (A.A.R.) also became part of this convocation and earnestly investigated how to mitigate the problem of inductive interference on open wire lines (and aerial cable to some degree).
Below is a diagram of the physical geometry which inflicted open wire lines with induced power line currents. The magnetic field of the power line cut through the pairs of the nearby open wire toll and exchange circuit arms. In this example, we have an early style TS-1 69.5-kV triangular configuration medium voltage transmission line in proximity to a typical telephone aerial wire lead. Other architecture for power transmission may have separated the energized conductors on either a horizontal or vertical plane, but the effects were no less damaging.
The telephone top arm pairs nearest the 69-kV/Y alternating current phase conductors (40-kV phase-to-ground) would suffer the greatest and have a greater voltage induced so that a difference of potential will exist between the various pairs on the arms. This is metallic circuit induction. When this occurs, the current induced in the telephone circuit by the power line source will transfer not only voltage but attenuation in the form of interference and noise. People using the circuits for conversation will hear a noticable 60-cycle hum. Telephone drainage, or induction drainage devices will be installed along the telephone pair route to eliminate much of this problem.
TS-1 REA 69.5-kV transmission structure and how its energized phase conductors affected nearby open wire toll and exchange circuits. Composed by D. G. Schema
Note how the increased a.c. power conductor’s proximity to the nearby open wire pairs negatively impacts the quality of phone conversations by interference and increased voltages. Magnetic fields around the center of the a.c. phase carrying transmission line conductor cuts through the telephone conductors causing severe effects.
Enough about the organizations committed to resolve the problem. Let’s see what their investigations concluded about the inductive interference problem. There are three factors combining to create the problem:
Combined with this are the actual components of each system. In power it is the generator (source) and the intervening equipment such as transformers and other inductive equipment, along with the client’s equipment (load). In telephone, it is the loading coils, repeaters, type of line construction, geography and geometry of the line design. In telegraph, similar situations compare to telephone’s challenges. These combined are called the “Influence Factors” for power utilties and “Susceptiveness Factor” for the communications companies. To include the proximity of lines to each other as features of geometry and geography, we introduce th “Coupling Factor.”
UPRR open wire construction in northeast Kansas.
How is energy induced into neighboring communication lines by high energy power systems? Again the holy trinity of inflicting this upon communications lines can be presented thusly:
Leakage between two or more circuits. This can be by transferance between the circuits by bad insulation, insulators, earth grounded communications lines. The solution to these issues is generally pretty simple, straightforward and preventable.
Magnetic fields convey energy into neighboring communications circuits on joint use poles, nearby R.O.W. facilities being shared by different systems and so forth. This bleeding over of energy from power a.c. currents to open wire circuits is particularly serious. In series, a pair can experience what is called “longitudinal-circuit induction.” More on that later.
Distributed capacitance between circuits. This problem is one of the easiest to understand and is capable of an immediate engineering solution.
Let’s talk for a moment about longitudinal circuit induction, as mentioned previously. We have a paralleling 34.5-kV low voltage distribution line along a highway. The Delta phase conductors are mounted about 45 feet above the land surface of the bar ditch easement. On the other side of the highway
Noise energy, which is simply the induced voltages onto the open wire toll and exchange aerial circuits, will be introduced by a number of unique factors according to the design of the various interfering power or communications line designs. Here there is not only the induced voltages but from the factor of capacitance, there will be changes distributed between the open wire pairs involved.
Let’s explain the diagram I’ve cooked up below. We have a triangular configuration TZ-1 REA “Z” Structure on the left; on the right, is a typical open wire toll with one arm of exchange (lowest arm) pairs.
Let’s just consider only two pairs. The figure “C1” represents the distributed capacitance between power wire and pairs 3-4 and 7-8.
“Cg1” is the capacitance of conductor number one to ground.
The effective voltage we call “E,” which is Phase “B,” will divide inversely proportional to the capacitances and thus we find in the calculation:
Eg1 = EC1/(C1 + Cg1)
The voltage from conductor number two to ground is determined in a similar instance, but this will be less than Eg1, since you will note the clearances between the wires are greater.
Thus, the two respective telephone pairs’ voltages are raised above ground. This is what we spoke prior as longitudinal circuit induction. However because the geometry of spacing and height is different, the two voltages will naturally be unequal. This means we also have an instance of metallic circuit induction, as the voltage between them is different. Listeners on a circuit speaking between toll centers will register interference between them. Noisy circuits will mean there is current flowing between them.
Below, is a diagram and pictorial of a nearby open wire communications line being affected by a 69-kV Z-structure transmission line. Each a.c. phase is carrying 40-kV phase-to-ground; the entire line is a 69.5-kVY circuit.
Note the “E” at mid-left. This is the power voltage to ground. The distributed capacitance raises the telephone conductor voltages unequally above ground. The voltage Eg1 exceeds Eg2. We can postulate the actual invisible, but highly radiating, electric field permeates the vicinity of the transmission line structure and each span between structures as shown here.
TZ-1 REA “Z” Structure for 69.5-kV triangular configuration with nearby open wire toll and exchange lines. Study composed by D. G. Schema.
Now I want to draw you a little diagram of what is happening electrically within the communications and power facilities. This is really…quite interesting when studying the interaction of the two systems. Note that we have a triangular configuration of a 34.5-kV shielded low voltage alternating current transmission line on the right. We can assume, since it is properly shielded, that it is also properly bonded and grounded at each structure with a butt ground, wrapped ground–or best, a copperweld shaft driven into the earth nearest the pole base.
On the right, we have a 50-wire toll lead (40-toll and 10-exchange wires). For clarity, I’ve excluded many communication line conductors and don’t necessarily have installed properly distanced transpositions, but we’ll assume this structure is properly grounded (at least at every other pole) and the pins and bolts bonded beneath the arms. This would be a good provision and consistant practice of a toll telephone company owner. The Long Lines’ Kansas City-Council Bluffs/Omaha lead was so bonded and balanced throughout its 176 mile length when in operation through the 1920s to the late 1970s.
The current in this a.c. power line will vary with demand but we can assume it to be 50 to 100 amps–or more carrying 34,500-volts (19.9-kV phase-to-phase) Grounded Y. The sheid wire on top will also furnish the common neutral return negative phase.
Here’s what is so very interesting about the phase conductors. The power line current and the resulting magnetism essentially act like little generators along the length of the line in series with the three phase conductors.
Our lower, but yet important neighboring communications lead to the left, has inflicted upon it higher induced voltages to the pairs by the power line voltages. This is due to the magetic field of the power line phases. These too, act as little generators connected between the telephone pairs and the ground underneath the lines.
What about noise? Good question. There will be LOTS of it, if either the series voltage induced or ground-induced voltages are unequal. This will be due to a difference of voltage between the two pairs and telephone receivers and speakers will notice the attenuation immediately–especially if it is over many miles of distance.
If you note the diagram closely, there is internal impedance to each little generator. That is due to the capacitance between the wire and the ground.
We’ve got a situation where three phase conductors are acting appreciably, or less appreciably depending upon their distance from the telephone pairs, upon the nearby telephone toll lead. The configuration of three-phase power is transmitted as economy best suits the electric transmission engineers. Single phase transmission, as well as V-Phase is not practical for long distance a.c. facilities. However, until High Phase Order arrives, three-phase power transmission will be the traditional norm.
A geometric study of induction induced on nearby open wire toll and exchange circuits by a neighboring 34.5-kV triangular (shielded) low voltage transmission line. Composed by D. G. Schema.
Let’s now digress slightly and explain how to defeat these problems by constructing an open wire line which is “balanced” or “equal” in its configuration. By doing this, we can eliminate much of the interference and unbalanced voltages and induced currents on the d.c. communications circuits.
The growth of power transmission lines to connect not only distant towns, but to reinforce the flow of power between generating stations by interconnecting them, brought this problem to a head. We know that currents and voltages on nearby lines can induce parallel disturbances from one line to another in its vicinity. Beyond the issue of parallel series-induced and phase-to-ground disturbances, we have a big problem where interference and noise denies the user of the telephone a quality call. This was the major drawback of such aerial lines and when loading coils and other transformers boosted the signal, they were also boosting the interference on to the listener. The secondary coils simply picked up the noise (and voltage causing them) and cast them down the line.
So here in effect, is what the open wire transmission engineers had to do: create a balanced system to prevent this voltage (noise) from hindering proper speech transmission.
Let’s take a pair, wires 1 and 2 on the top arm. We’ll raise the potential of these two wires to be equal. We will also guarantee that in series with the line, the impedance is equal. We’re also providing that the ground impedance is also exactly maintained as identical. We now have a balanced pair telephone line. It is considered balanced because no noise will be created in the pairs by any noise currents. However, any engineer will note that the physical world and the actual construction of an open wire line will not maintain such a perfect alliance. Even without open wire’s partners of the highway, a.c. distribution and transmission lines, a telephone line will over its length become imbalanced. Noise will occur along its length and heard in phones by each speaker and receiver.
One issue which was overcome by telephone engineers, previously creating some earlier imbalance problems, was found to be poorly made or existing telephone conductor splices. The former Western Union splice, for example, while physically secure and created without special tools, was found to be a creator of much series imbalance. The rolled sleeve types, compressed styles, too, combined to overrule the antique methods which while strong, contributed to poor connections between reel lengths of wire.
On the other hand, engineers tackled the “ground” imbalances cautiously because there seemed to be a number of nasty ways this condition might result. One of the first found was that of bad insulators. We’ve spoken about insulators’ quality and types elsewhere, but many early units were not much more than scrap glass collected from various vats at bottle manufacturers and dumped in a press at the end of the day. Priority was not given to glass quality, designs to improve transmission, mechanically pure content, nor physical strength. Furthermore, in the Age of Steam, railways poured tons of pollutants upon the surfaces of insulators along the rail route causing insulators to fail or simply function well intermittantly well.
Another issue was terminal equipment. Early line terminal structures were wooden boxes with either porcelain or glass inserts for binding posts. Some used knob and cleat wiring devices seen in older homes to carry or transition circuits from open wire to a lead cable. Before 1910, it was common to see a lead cable simply terminated below a crossarm with the line wires using NO terminal what so ever. Rain, moisture and humidity entered the cable sheath and caused a multitude of problems.
To take this discussion further, these problems were of a “leakage” flaw which could be cable sheaths, bad insulators, poor outside wire conductor or physical problems with the line itself.
With the coming of aerial power lines, the induced voltage affected more than just quality of the phone call. These errant currents and voltages could exact a lethal dose of current posing a hazard to the telephone employees or subscribers. There were cases where an open wire telephone line of a commercial communications or railway company might carry hundreds or greater volts between conductor and ground.
Commercial telephone company engineers and railway communications engineers embarked on making their respective lines of higher quality to combat this problem. This problem’s source might be determined, but the remedy was expensive–but necessary for practical purposes of ensuring a good quality transmission medium. Removing the faulty wire joints on pairs, improved tree clearance and better insulator quality helped to alieviate the problems to some degree.
But what about the issue of equal series voltages, now that we’ve discussed the phase to ground problem? Let’s go back to our example for which I graphed out previously to make the series voltage problem clearer. Note the same tri-angular 34.5-kV line and the open wire lead immediately above. When the voltages appeared along the telephone line from the overhead power sources, would act as impedances to ground, requiring currents to take these routes. Whether or not the induced voltages were equal, the series and ground induced voltage unbalances caused an unequal flow of current. Noise appears on the telephone pair promptly because of the different voltages.
This is why telephone engineers took the greatest step in denying induced voltages a further welcome on their open wire facilities. The transposition scheme. When the commercial telephone, telegraph, railway and power people convocated to study the problems of induced voltages, one major solution offered itself. That was the interchange of pairs (in communications) and phases (in power distribution and transmission) on owned and operated utilities’ facilities.
By inserting a physical break in the configuration of each pair and by transferring one wire of the pair to the former position of the other at either a point, two spans or with other methods such as a rolling point in wooden bracket construction, then a series voltage equalization would be affected by each line wire suffering from magnetic coupling. Noise as well as induced currents, would be reduced substantially.
A properly disposed transposition pole eliminates the problem markedly by viewing it from an electrical theory angle. We have eliminated the OWG on the 35-kV class transmission line and extra open wire communications pairs for clarity, illustrating how a top toll pair and a bottom exchange pair group take different pin positions on the third pole.
How transpositions, depicted here with drop brackets, can eliminate the generated currents of non-transposed pairs. Other pairs omitted for clarity. Diagram by D. G. Schema.
Before we proceed, we need to explain the circuit diagrams I have added above. We have both an Actual and an Equivalent Circuit illustration. This is useful because with the aid of transpositions, the intended interchange of conductors at pre-selected intervals, we can break the inductive linkage between the pairs. By reducing the metallic circuit induction, we can eliminate the increased voltages being generated.
Our drawing above uses drop brackets, which are among many hardware and design techniques to eliminate the problem. Drop brackets allow two spans to re-establish pin positions of a pair and are not as effective as the point type transposition bracket or transposition pin insulator. The earliest systems used two-piece and single piece transposition insulators designed for the purpose. Double arming was used in order to facilitate double groove Pony insulators use for “point” type tramps. Point-type proceedures were much more effective electrically to alieviate the problem of mutual induction. The phantom bracket was developed early on (1890s) but later in more substantial testing, it was found to have hardly an effect on the problem, even after so many had been installed throughout the nation on many of the most important early open wire toll circuits.
Interestingly enough, while series voltage induction is largely mitigated by the use of a proper point-type transposition scheme, the voltages between the pairs and the ground induced by power distribution and transmission lines, are hardly assauged significantly.
Our classic example (and simple) pictorial above with the toll lead using drop brackets mid-length was actually tried–as the only point within a long toll lead–to combat the inductive problem. It failed miserably on the New York-Philadelphia Long Lines’ lead, one of the earliest grand open wire facilities of the nation in the 1880s. The hope was that one telephone line transposition of all pairs simply placed in the middle of the entire line would mitigate the problem. It did not. What was not understood was that telephone line transpositions tend to equalize the induced voltages in each wire pair by magnetic coupling.
Pioneering telephone engineers encountered a host of problems with this transmission technology early on by noting line interference was bad in some locations but not in others. In combination, the transmission quality was bad and in some cases terrifically so. The noisy effects were due to unequal voltages encountered throughout its length.
There was another issue at work and early engineers began to understand the relationships between phases, or phase relations. Current and voltage propagates along the length of the lead. They might have predictable magnitudes with low interference, but a failure to understand phase relationships discounted the possibility that induced voltages tended to add to the existing problem. With telephone’s higher frequencies and short wavelengths, clearly this was not fully understood.
With the advent of electric power distribution, transmission, electric traction (trollycar lines) and electric railroads, the impact of these neighboring facilities to telephone, telegraph and railroad communications was increasingly felt. Essentially, now adding to the burden was the problem of power line harmonics. This added further noise and interference to communications circuits.
When the various bodies of utilities, communications companies, railways and interurban systems’ engineering people met to resolve the problem, instead of using the courts, they proceeded to examine solutions. The communications people examined the problem on their end and added transpositions identifying the need to consistently and frequently install them on their lines. However, the electrics needed to improve their power system harmonics by making transpositions of their own distribution and transmission facilities. By keeping the disturbing circuit’s conductors close together, the influence factor, would reduce the stray fields on the lines. This process also unwittingly reduced the other condition: the susceptiveness factor.
How an Equivalent Circuit is diagrammed.
“Ghosts” and “Super Phantoms”
Transposition schemes are very complex. I address this issue in the section “Transpositions” in detail. What made the earlier toll leads so intricate in their specifying proper transposition design were the use of pole pairs and other arm pairs for phantom circuits. These “side circuits” were also susceptible to crosstalk, mutual induction issues. Phantom circuits would just as adequately carry the interference as the physical circuit arrangements. In keeping with this special consideration of phantom groups arrayed on each arm and pole pair group, the new bracket, called a “phantom transposition bracket” was devised. While hundreds of thousands of these were deployed on major toll leads beginning in the late 1890s, later tests in the 1950s found them to be of doubtful influence and some cast doubt on their continued use at all as drop and lift brackets were more adequate to the job.
When properly designed open wire toll leads were designed and constructed, at first it was assumed that every line section would be of custom design. Every transposition scheme would differ beyond that particular toll lead. Clearly, in practice, it was found this was not necessarily the case.
Once the standard 1885 ten-pin arm was devised with its standard 12-inch spacing between each pair and a 16-inch separation between the pole pairs, a standard could be approached on four levels–literally! A typical toll lead of the 1920s and 1930s comprised four arms carrying the most important toll services at the top reaches of the pole head. Any arms below the first 40 wires, such as in an eighty-pair lead, could repeat the initial pattern of the arm transpositions. Thus, after arm 1, 2, 3, and 4 were transposed, arm five would repeat arm 1’s same tramp pattern, arm six restore the same pattern as arm two, and so forth.
First, it must be acknowledged that the premier designer of open wire transposition systems was a telephone pioneer named Henry S. Osborne. In his paper, before the American Institute of Electrical Engineers in 1918, he enlarged upon previous work by J. C. Carty of AT&T in 1891. Carty identified the problem of mutual inductance and clarified how the first steps might be taken to abate the problem. Osborne went further with highly structured transposition patterns based on calculus he had solved to combat the problem once and for all. By practicing Osborne’s suggested transposition proceedures, “super phantoms” and “ghosts,” were widely eliminated. Before, failure to meet transposition requirements caused a phantom circuit to “phantom” itself one degree further, causing much havoc along the earlier aerial toll facilities.
“Whole Line” Transpositions
By late 1919, the American Railway Association, embarked on their study of how the railroads themselves, with their myriad open wire transmission voice and signal circuits, might coordinate among themselves. This was especially important in large rail yards where multiple companies’ lines intermixed.
Not only by 1926, would the railroads protect their important open wire circuits from exposed a.c. power distribution, but institute with traditional transposition patterns a new feature, called “Whole Line Transposition Units.” These would eliminate the irregular exposures to other rail lines’ circuits as well as the power companies’ line ionization patterns.
What is a “Whole Line Transposition Unit?” These were locations along a railroad’s open wire route where special transpositions were cut into the “whole line.” That is, not only obeying the demands of the specific Osborne transposition pattern, but transposition poles which cut into every pair on the lead at that point. These would eliminate crosstalk, errant voltages and guarantee trouble free problems from the standpoint of other railroads’ signal circuits.
When phantom groups were used, the whole line transpositions, might be moved accordingly to a “non-regular” transposition pole, where engineers specified. However, all phantom groups would be transposed at that point, where ever necessary in their staking plans. There were serious restrictions and cautions to be observed. This was noted because the pin positions of the side circuits and other circuits on the lead would be unnecessarily influenced negatively.
So, to eliminate this, a Number One Phantom type transposition would be cut into the circuit at every phantom group at these points. When accomplished, each side circuit pair of wires would occupy exactly the opposite pair of pin positions beyond from those normally occupied in the regular transposition system.
However, in doing this, a degree of interference would be created beyond this point. The railroad communications engineers realized this. They solved the problem by installing a second set of whole line tranpositions (two-wire for non-phantomed circuits and No. 1 type transpositons for phantom circuits). These were cut in at a second point beyond the first point, as nearly adjacent as practicable. The crosstalk problem was then confined to the section of line between the two points where the two sets of whole line transpositions were installed.
To give you an idea of how complex introducing transpositions to phantom groups and regular signal and voice frequency circuits could be, let’s suppose we have a railroad communications route where typical transpositions might be installed. Keeping in mind that of those circuits:
Non-phantomed circuit was untransposed at this point
Non-phantomed circuit was transposed at this point
No transposition in phantomed circuit and no transposition in either side circuit
No transposition in phantomed circuit and one side circuit transposed
No transposition in phantomed circuit and both side circuits transposed
Phantom group transposed to Type 1
Phantom group transposed to Type 2
Phantom group transposed to Type 3
Phantom group transposed to Type 4
Note that in the fourth, seventh and eighth instance above, in one forth of a mile route, causes each side circuit to occupy an opposite pin pair position from those in a regular Osborne pattern beyond this point. One side circuit should actually be untransposed and the adjacent circuit untransposed should be transposed. In doing this, crosstalk would then be prevented from continuing beyond this point.
Simple, huh? This was called the one half mile unit transposition and was placed an equal distance from the whole point transposition pole. The one fourth mile unit was placed equidistant between these poles in separation.
There were two railway communications transposition systems. The “A-B-C System” and the “Exposed Line System.” Designers had to impliment these designs when practicable and necessary and where their lines were under specific disturbing influences.
Here is how the electric utilities contributed to solve the inductive coordination problem from their experience. We see a typical flat-top configuration 34.5-kV line as it begins phase order maneuvers to accomplish a rolling point power transposition.
Here is transposition pole number two. Note the lower arm allows phase conductor “A” to move beneath B and C phases. Third eight-foot arm carries Delta 12.47-kV distribution circuit.
Transposition pole number three: here the lower phase conductor prepares to move to the opposite pin position to complete the rolling point maneuver.
Pole number four: here the flat top configuration re-emerges and instead of A, B & C phases, we have B, C & A phases. Several miles from here, another maneuver is accomplished, and another after that. Finally, A, B & C phases complete a full rotation.