Rural Power Line Telephone Carrier

Rural Power Line Carrier for REA-financed Electric Distribution Systems

Open Wire Communications Merges with Power Systems: the M1 System

One of the late, most exciting final rushes of open wire technological innovation began in the post-war period in 1946, when commercial telephone companies advanced the idea of power line carrier as the medium of subscriber communications to the Rural Electrification Administration’s cooperative borrowers.  Bell Labs and the REA strove to build a system whereby the “open wire” was in effect, the rural distribution a.c. circuits’ conductors.  These typically carried 7.2-kV rating single phase. 

The immediate and fundamental advantage of this new system was existing single phase distribution circuits could be slightly modified to accept this carrier current system–and without building extensive and lightly loaded telephone open wire lines to parallel them costing much expense in both construction and maintenance by the telephone company.  In cases where ranches and farms were many miles distant, such a system made financial sense and was developed around these rural economies of scale.

The effect on the power system’s operation was negligable with the use of this system.  Furthermore, most equipment was provided by the telephone company, not from the the REA electric distribution cooperative’s budget.  However, it was essential that the power distribution scheme be closely coordinated aligned with the communications organizations plant facilities.  If any changes were planned in the power system, it was the obligation of the rural electric cooperative to inform the phone company, so that the electrical values might be maintained properly.

Early Partners in Communications: A brief history


The carrier system employed then was commercially similar to that of today’s transmission line carrier systems for command and control of the intricate network of power system facilities.  Ironically, stepping back a few decades in time, it was Bell Laboratories who had instigated and promoted the essential technical ingredients which led to the later successful development of power line carrier systems.  It all began with Western Electric/Bell Labs and PG&E, when that early technology, at the invitation of Pacific Gas & Electric in around 1922, to install experimental equipment at PG&E’s s Vaca Substation near San Francisco, California. 

While the effort was an experiment; much was learned during that time for both AT&T and PG&E.  The subsequent carrier technology installed did not have a long and eventful life but the legacy encouraged utilities and their communications suppliers to proceed along AT&T’s lines.  Indeed, Bell Labs taught the electric utilities many things about the operation of such a carrier system for inter-connected transmission systems, substations, switchyards and generating plants.  Bell Labs justified their work as an experimental sojourn into the power systems world, but as this task was beyond the scope of traditional Bell System commercial affairs, AT&T said “Good Luck” and retired from this electric-utility style communications technology.

However, both the electric utility and Bell advanced their mutual causes after the 220-kV Vaca Substation Project episode and began to independently progress in carrier development where it suited their uniquely sovereign, but important entrepreneural differing roles.

During the five years of war, between 1941 and 1945, the United States had promoted the war effort’s tasks over languishing, unimproved and worn facilities, whose tears and seams were beginning to disintegrate.  Roads, highways, canals, streets, utilities and other infrastructure had not seen any physical improvement  since the late 1930s.  Now, there were jobs to be had in bringing these neglected national features back up to snuff and prepare them for the 1950s national reimurgence.  Meanwhile, the war had advanced the development of electronics, radar and communications.  It was apparent that a new growth period was on the horizon.  Telephone companies with their power utility neighbors mutually began to see common merit in joining forces again.

While 16-channel open wire carrier was but a twinkle in a few researchers’ eyes at Bell Labs, a new opportunity revealed itself: the use of power distribution lines for rural subscriber carrier.

Each condition where the carrier was to be designed and installed was slightly different owing to the cooperative’s geographic location and required substantial engineering to accomplish a successful effort.  By using frequencies between 150 and 455 kHz, the band offered five carrier telephone channels

The usefulness of the particular selection of bandwidth offered common advantages as did–or better–than the traditional open wire party line bracket lead.  Using this system, the same number of subscribers served from an an open wire bracket lead would be the same: eight.  And furthermore, if the system were divided properly and if the circumstances warranted, about 40 subscribers could be served.  But this took careful engineering, manpower effort and time.

Above are displayed some of the specifications based upon the REA standards for  7.2-kV single phase distribution, using the A1 design parameter.  This system was not designed for 14.4-kV single phase systems, which were then coming into vogue.  Delta systems, and their multi-phase conductors, which were not permitted use in Rural Electric Cooperative systems, could also be applied to this system, if the phase to phase voltage was less than 8.7-kV.   This meant that muncipal systems might extend to some of their outlying areas with the system, should they wish to use it.

The other limitation on this system was the load capacities of single phase circuits.  The continuous load amperes could not exceed 15 amps with this system.  During short circuit conditions, a brief one second 500 ampere current, was the kiss of death and not permitted for use with this carrier.  Asymetrical short circuit currents simply would destroy the choke coils, and could fail violently, as these units in series were not designed to exceed this rating.

It goes without saying, any neighboring telephone plant were not justified to parallel this system on distribution lines, and if suitable additional circuits could be garnered from non-heated cable pairs or if open wire were available.

Each rural subscriber was required to have electric a.c. secondary service.  This allowed the homeowner to power up his interface with the system and plug in a phone set.  With a special terminal installation, the subscriber’s service beyond the carrier terminal could be extended with a two-wire voice frequency circuit–if necessary.

How The System Worked

Because power distribution systems consisted of feeders, radials and loops, a high degree of attention was paid to the staking sheet work in order to maintain continuity of the carrier signal.  No one wanted a diminished or quenched signal through normal actions of a distribution system’s protective apparatus.  These typically included electric distribution equipment operation (reclosers and sectionalizers), air break switching, vacuum break switching, disconnects, follow through on voltage regulators and capacitor installations.

The carrier system was employed initially on feeder distribution systems.  Inspection of all sectionalizing devices, including disconnect switches and air break switching, required the placement of a loop supply to bypass these potential interruptions in the carrier system operation.  Continuity was the key.

As with power transmission lines in the early years, only one phase of the multi-phase distribution REA lines was employed for the successful implementation of this M1 Carrier System signal.  All subscribers on that division would be served from only one phase of the live feeder or radial branch circuit.

This system offered the opportunity to use magneto, common battery or dial systems, the last technology of late, were slowly being introduced into rural areas.

 Equipment used to apply voice frequency carrier to subscribers over REA power distribution systems was mainly the responsibility of the telephone company, although the actual installation was done by linemen of the electric cooperative in careful coordiation with the communications company furnishing the telephone service.  Here we account for some of the items required to specify for such a system:

Series Choke Coils, Coupling Capacitors, Fuses, Line Coupling Units, Power Line taps,118A Protectors,Mounting Brackets for the above. Common subscriber carrier terminals. Telephone instruments and sets. A power supply located interior of the subscriber’s home 120-volt a.c.


 Installing The System and Making It Work

Installing the system.  These procedures were undertaken to install the necessary equipment, maintain and test the completed facility for subscriber use. 

Just as a power transmission line carrier is balanced on energized 69-kV, 161-kV or 345-kV transmission lines, smaller representatives of the same equipment was placed to operate this lower voltage application.

The Choke Coil, which was an isolating device, prevented follow-through carrier signals from entering induction devices, such as transformers, regulators and similar apparatus which would normally be found on the distribution lines.  They work like a “line trap” on transmission lines at substations.  This device limits the flow of carrier current through an induction device.  There were two of these devices used:

Isolating Choke Coils – limits the flow of carrier current at higher power loads than a Tap Choke Coil.

Tap Choke Coil – lower power load operation.

Transmission Choke Coil – these were installed in series with the phase conductor of branch lines where the carrier is to be used.  The TCC has just enough impedance to allow the carrier current to pass through, but not more to reduce carrier current on the main circuit.

The isolating and tap chokes blocked the carrier frequency circuits.  The important consideration here was that the isolating choke had an inductance of 10 mh while the tap unit was rated at 2.5 mh.  The point of this was a tap choke–at a tap line–was suitable for use when isolating two different eight-subscriber carrier feeds.  The transmission chokes were rated between 0.3 and 0.6 mh used on branch lateral lines.

An important consideration for this equipment, unlike the standard open wire carrier systems, was that all equipment had to be rated for a.c. line voltage and then some.  But here was the rub: what we recognized as “low voltage” telephone equipment now found itself confronted with the same demands as its high-powered utility equipment.  Short circuits with higher currents, load temperature rises and lightning-induced surges. 

The challenge was to build equipment which operated carrier at high frequencies, yet was in the vicinity of equipment “hard charging” to disrupt the goal of equipment with low winding capacitance.  This created considerable engineering work.  Westinghouse was a leader in this technical area.  Much of the equipment employed for this system was of Westinghouse make.

Series Choke Coils were required to be placed on poles where no other equipment was located; perhaps a double dead-end structure on a tangent or 90-degree angle double dead-end line feature.  One reason for avoiding reclosers–despite the obvious operation of interruption of the carrier circuit during their abnormal line condition operation–was the high reactance of the trip coil.

At the homeowner’s transformer service pole, the cooperative personnel have installed a “coupling capacitor,” similar to their big brothers in power substations and on transmission lines, in order to serve as a common carrier terminal to subscriber sets.

As with any carrier system, the impedance must match at each end; hence the ends of the carrier system where this system was employed it was necessary to observe this so that “reflections” might occur–attenuation at specific frequencies.  Bell Labs conducted some early field studies in 1947, and found that they could predict the kinds of reflection losses which might affect the telephone user.  By using these test results, a simple table of losses, including approximate inductances, could be enlisted.

As with any protection of telephone open wire facilities, the outside plant engineer would also insist upon the application of telephone protectors on the carrier-engineered REA line.

Along with these protectors, all bonding and grounding had to be attached to the power system grounded neutral firmly.  This was not only necessary at locations where the equipment for operation of the carrier system was employed, but at subscriber’s premises as well, due to the high voltages encountered with this application.

Each coupling capacitor was designed so that its installation included an isolating fuse cutout.  If the coupling capacitor failed, the fuse would blow, disabling the defective equipment and hence any other equipment or persons connected with the line.

On a dead-end pole, where the last subscriber to the carrier system was connected, you will note that there were special conditions to protect the equipment and subscriber terminal.  In the early years of REA-financed cooperative systems, most distribution transformers were conventional units.  That is, a “conventional” transformer is one without internal protection, either found in the tank on the primary connection or the secondaries radiating out of the tank.  When the CSP, or “Completely Self Protected” pole transformer was introduced by Westinghouse in 1936, cooperatives began to introduce these to their system.

In the late 1940s, most of the rural systems still had low capacity load 3-kVA and 5-kVA conventional units.  These were protected on the primary side by a lightning arrester (for high voltage conditions, such as lightning) and a current limiting device, an external fuse cutout, both mounted in series with the transformer.

Completely Self Protected units eliminated the use of external protection equipment.  The fuse was located in the primary bushing, internal to the unit.  The lightning arrester was typically a valve, granulon or pellet-type, mounted next to the primary bushing on the side of the tank proper.  Additionally, the secondaries inside the transformer were prevented from harm with the use of a circuit breaker.  Larger units above 10-kVA were equipped with a thermostat and signal lamp, so that if the capacity of the unit were exceeded, or the unit was tripped by an abnormal condition, the light would turn on and require re-setting by the cooperative personnel after an inspection.

When the system was determined to be employed in a specific rural area, considerable engineering was necessary to make the system work properly.  This included the complexities of system coordination of overcurrent devices.  This would include the CSP transformers’ internal silver-sand fuse, any external cutouts, sectionalizers and reclosers on the system.

Both interaction by the electric cooperative engineering and telephone engineering personnel would benefit by mutual meetings to review the system and the location of subscribers who would most  benefit from this carrier system.

Looking at the layout of the system, the map provided personnel of both parties with the necessary information to select specific poles for the new equipment, and in some cases, field inspections were made in order to deduce whether existing poles were of sufficient height and clearance, or having proper youth to support communications equipment.

The engineers furnished the telephone engineering staff information the ratings of fuse cutouts, reclosers, sectionalizers and the load of each three phase, V-phase and lateral feeders.  Calculations of short circuit current usually had been made when the coordination of the system had been made for recloser and sectionalizer installations, so this up-to-date information was essential for the design work.

The telephone personnel and power people left their meetings, scheduled visits to subscribers and scouted locations of the plan in hand, so they could independently cover the completion of the project in phases.  Typically, the chokes, coupling capacitors, and fuses with appropriate grounding and bonding, were installed by the cooperatives’ lineworkers.

The telephone organization’s I & M (Installation & Maintenance) personnel would see to it that the drops were placed to premises, special cabinets placed for the premises-based equipment and furnish the telephone sets.   Since telephone repair was required to have access to the coupler enclosures mounted on the joint pole, they were installed safely below power system-associated equipment.  For most installations that was 40 inches or below the nearest energized apparatus.

After the system was established, tested and released for customer use, the on-going issue of long-term maintenance was necessary to continually operate the system on a day-by-day basis.  This meant that the necessary storm damage restoration component of service was expected.  So that both the cooperative and phone company communicated with each other relative to the replacement of blown fuses, coordination of planned outages for new equipment installation by the electric cooperative, defective equipment replacement and other necessary items of attention.

The choke coils were manufactured by Western Electric but the coupling capacitor units were built by Sprague Electric.

While this system was a marginal contribution and did not achieve vast service nationwide, it did deliver what was promised to electric distribution clients: effective communications to homes in far distant outreaches of the rural landscape. 

I have not included it within the topic “Carrier Systems” on this website, because of the novel and unique blending of power systems and telephony to a departure beyond impressing pilot wire carrier on existing lines designed for low voltage d.c. and a.c.  telecom.

This was by far, a unique theory, put into practice by the late 1940s, for which during a brief period of its popularity, was one of the last great achievements in this aerial wire culture.  Therefore, because it again stretched the techology of open wire one substantial step further, we believe strongly that this application deserves mention here.