The SAC Box & the Economy of Telephone Voice Pairs
A typical example of American Railway Association/Communications Section major depot-sited cross-connect cabinet. This location in north central South Dakota remains from linewrecking in the spring of 2017. Owner: Milwaukee Road Railway.
In this section, let’s review the basics of the Serving Area Concept, or a theory where economical dedication of subscriber pairs through the use of properly sized aerial cables and a Serving Area Interface is used. This large terminal is often called many things, commonly a SAC Box, SAI, X-Box or B or BD Box. However, all nomenclature speaks the same language: affording efficient utilization of subscriber pairs for each feeder cable. Telephone people know these impressive cabinets as “Cross-Connect Terminals.”
Now, you ask, “What does this have to do with open wire?” Hold on a minute. I can only rely on my good memories for this, for alas, memories are good but photos are scanty of these large terminals on open wire structures. None physically exist in the U. S. (to my knowledge) where they were applied to open wire application. However, I recall their extensive use at large terminus and junction/terminus structures where major open wire toll leads involving 60 or more pairs joined with central-office originated/departing toll cables. Since “cable” is simply the combining of many pairs in one sheath vs. spreading many pairs on crossarms, the use of this large terminal can be applicable to both circumstances for similar purposes.
When communications organizations began to experience burgeoning load/traffic expansion in the suburban areas of the 1950’s, the concept of sectionalizing or isolating large traffic into localized segments eased engineers’ future telephone planning worries. Now, Telephone Transmission and Local Loop Planners could exercise better judgment. Now were less cautious by predicting cable sizing growth as well as define area traffic allocations. This helped to ease congestion. Again, all engineering boils down to a social science: economics. Open wire, then cable, experienced the same degree of re-examination over each decade for cost savings. We’ll examine the technique how the Serving Area Concept caught on and how early use of the large terminal for open wire opened that door to the modern period. This, once again, is part of the beauty of studying this old open wire epoch: what started then is used today in slightly different form. We learn from the past.
A typical installation in Wichita Falls, Texas where the aerial mounted SAC terminal with splicer’s balcony can be seen. Original owner Southwestern Bell, now AT&T.
I recall as a child, walking beneath large toll terminal structures where open wire pairs converted to aerial or buried cable pairs by the use of the “BD” or “B” Box and inspecting this massive, bulky and impressive structure. Terminal poles for open wire toll were particularly awesome, but with the addition of a balcony and this BIG box, where all the little black wires wrestled each other in a race from the dead-end arms above to this glamorous silver-colored attention-getter, created a fundamental mystery to me as an eight-year old.
These cabinets or boxes took the same assignment, but received different treatment by whomever their owner might have been. These BD or B Boxes may have served as cross-connects, but classifying them per diverse operating telecommunications or railway companies, was slightly more complex by nomenclature. Not to pick favorites, but in our case here, we’ll sing Western Electric’s praises, to simplify our discussion where inspect the various styles these terminals presented to the outside world.
The railroads were very independent in their style of appellation; they used the term: “Distributing Box” or similar for their “B Box.” All were “manual” type connecting terminals with rows of dedicated binding posts assigned for each cable and open wire pair. We need to make this distinction of “manual” vs. “Digital,” as many telecom supply companies sell “Digital Cross-Connect” systems today, splicers are unnecessary in the traditional sense of being outdoors climbing a pole, opening the box and making physical pair connections with a ratchet and wrench. Digital Cross-connects are usually found in large Central Offices or Remote Terminals of significant import where fiber and multi-pair copper is used. However, for our brief discourse here, we’ll refer only to the open wire era, where cross-connect scheme of manual release or connection of terminals was done.
Typical aerial SAC in Topeka, Kansas by Southwestern Bell, now AT&T. The standard design of this terminal follows basic Western Electric design and is found throughout the U. S.. Many other telecom equipment manufactured similar enclosures to the same high quality. Today, aerial cross-connect boxes are rapidly being replaced by pad-mounted SAC boxes. This sight above will become very rare indeed within ten years.
Where large toll lead convergences from open wire to cable or vice versa occurred in the past, we’ll focus on those large terminals where these BD Boxes were supported on the cable entrance side of the pole rear, installed with a splicer’s balcony. Here, a literal “river” of bridle wire flowed from the white or silver-colored carrier filters and other cross-arm mounted equipment. Coursing down the pole through bridle wire rings it quickly rushed into the rear of the BD terminal.
And also so mysteriously, snakes of large polycarbonate black sheathed or grey lead-sheathed cables emerged from the box above or below this enormous terminal. Here, cables scampered off in an aerial route towards the city or were devoured by the ground beneath the sidewalks and streets where central offices or other exchanges became its target. Such was the case of a 60-wire open wire Northwestern Bell toll lead following North Broadway in Council Bluffs, Iowa, which as originally constructed, began its sojourn to Sioux City, Iowa along old U. S. 75. where U. S. 6 joined the old Lincoln Highway Extension (by-pass which would in the 1920s become part of the new U. S. 75) at East Broadway. Here, I recall a 600-pair box and balcony. The former seven arm lead left the scene in the spring of 1963. NWB removed the open wire along the highway and assembled an “elephant fence” of aerial cables to take its place. Later, these were further converted to underground facilities in the ’70s. These aerial cables then met what remained of the 98-mile open wire link to Sioux City. There, a new cable to open wire terminus was built at Coit Road (on the city limits line). In 1973, ten years later, this longer, remaining 1920’s lead was line-wrecked all the way to Sioux City and finally, the Council Bluffs-Sioux City, Iowa Lead bit the dust and remains but a memory.
An interesting aside. When in 1973, this mammoth toll lead was being demolished, certain incorrigibles recognized the financial value of these 70 wires where 109, 128, and 165 gauge copper conductors were seemingly hanging idle and clearly appeared “theirs for the taking.” Even with this non-working wire, it was yet possible to “alarm” several of the existing open wire pairs on each of the seven arms. There was still a continuous circuit back to and between the various intermediate exchanges. When these nocturnal miscreants embarked on their escapades targeting these resources, they only needed cut an “alarmed” pair. Within minutes, the Pottawattamie or Harrison, or Monona or ultimately . . . the Woodbury County Sheriff’s Department, would come ‘a-runnin.’ Quick justice was meted out to some very surprised individuals whose criminal pride was suddenly vanquished. Copper . . . does have a value . . . both ways.
As a child, I would inspect this amazing scene above where a terminal balcony jutted out just beneath the large cabinet, where with ten or twelve pole steps below, a splicer could climb onto the wood seat and get comfortable where splicing was required for service demands. I recall quite vividly, when a large tan canvass tent was erected over both the balcony and SAC Box, where inside, the splicer was busy re-arranging pairs, in a driving rain or sleet storm.
We’ve discussed this previously, but just to review, let’s talk about two types of terminals within the copper cable and open wire world:
- Fixed terminal
- Ready-access terminal
The first is the “fixed count” type–a permanently mounted (usually to the pole or side of a building) with a required number of binding posts within–version. “Ready access” types realized a fundamental type of “instant” terminal configuration which could be cut into a cable anywhere along its route. These could be added by a splicer or installer along a cable, C-Rural Wire or Multiple Wire Cable. They could be supported on the strand of a non-self supporting cable or self-supported cable. These are and were installed inside pre-existing splice cases, could be placed near poles or in mid-span. From any of these points on a cable, such installations allowed taps, such as drop wires, bridling runs, from any point within the span to a subscriber, an open wire bracket lead or business. Blocks with binding posts can be ordered of various sizes. Most common are four, ten, twenty-five or fifty capacity sizes. These could be jointed together to make from a single to multiple terminal array. Removal of an existing “fixed” terminal was not necessarily required. If the count required a larger capacity at the ready-access site, then whatever requisite number of binding post blocks necessary to be configured inside the plastic case could be installed. I’m sure most of you have noted mid-span drops to your home or office. These are called “string by” drops or their application noted by similar nicknames. Sometimes new service occurred where no fixed count cable terminal was available–such as a long span cable near a construction site of a new building. Ready-access terminals were very convenient.
With ready-access terminals, hence, all pairs on that cable were available for use. A service drop might be connected to any of these pairs. Ready access applied only to cable construction–not to open wire. To my knowledge, and experience, all open wire terminals identified have been “fixed count” types.
Let’s call attention to an important point: when fixed units are installed, the lineman knows each cable pair is connected to a binding post corresponding to the originally installed cable and pair at the central office. No installer can simply jump pairs (“frog”) or combine any to his/her liking, because they may be appear to be available. New service or removal of service is based upon that “dedicated” pair only! A large printout of each terminal on a cable run specifies the number of working pairs, defective pairs, spares and special circuit assignments.
One other thing we’ll briefly note here, before we move on. In our collection we have many terminal types used by Independent telephone companies, railways and Bell organizations. Some have an embossed descriptor on the front access cover denoting whether these are “protected” and whether they are “unprotected” types. What this means is that should some accidental power line contact or lightning strike occur, the excess voltage and current follow-through is mitigated. A common business alleyway terminal you’ll find is the NC-25 type. Our collection’s variety of “NC-25” terminals (50 wires; 25 pairs) are noted as protected terminals. The “N-25” terminals are not protected. The majority of building terminals are protected for customers’ premise equipment. Similarly to the terminal on your home, it acts as a “station protector” terminal. It is protected. Whether protected or not, they do what all terminals are designed to do: connect an external service to a neighboring cable or open wire pair. The Tyndall, South Dakota terminal/junction pole rescued (See Chapter on National Register Nomination) had two NC-25 terminals connected by two 25-pair cables. Those two 25-pair lead cables then entered a splice where a 50-pair buried cable encapsulated splice was found. These demanded protection, as in the rural areas which spans of open wire traveled above ground, made them excellent targets of potential lightning attack and inadvertent power line contact. The filters used with the O-Carrier and similar electronic high frequency carriers contained over current protection, too. There are many stories of personnel climbing this particular pole after a storm to replace fuses with those little white boxes hanging on the BDE crossarms.
We’re containing our exploration of these terminals to the open wire era, where aerial-mounted units were used in abundance and considered the norm.
Internal Features of the SAC Box
Let’s begin by describing how these modern units appear in detail. Later, we’ll re-visit the historical lineage of these apparatus. Briefly, the very early units were small, wooden boxes with external hinges and metal roofs, with two or more 5/8″ bolts drilled through the pole to support the unit. Once mounted, the cables were then brought to the access points on the terminal enclosure. These wooden boxes swiftly gave way to completely galvanized steel–and later aluminum–terminal housings by the late 1950’s. GTE, AT&T Companies as well as Independent Companies, affixed aerial boxes similarly to the photos above and to my above description. Most large boxes possessed these specific components:
- An aluminum external and internal housing
- Top eye bolts with two handles on the top case for installation
- A stub cable from the top or bottom of the metal case
- Two latches for access to the interior of the unit with hinges on dual doors
- Two latch handles (upper & lower) incorporating in the lower, a means to securely padlock the unit from unauthorized personnel.
- Mounting brackets (detachable) on the rear for hanging this heavy device on poles.
- Interior contains three rows of connector blocks for required number of F-1 and F-2 pairs.
- Holes in bottom or top (sometimes sides) for grommets to shield entering bridle, drop or other distributing wire from weather.
To my knowledge, and from other reports elsewhere, I’ve never encountered these terminals mounted on multiple crossarm assemblies or hung on arms, but rather mounted to the pole directly with 5/8″ galvanized bolts and washers.
Ameritech in the Chicago area possesses an abundance of these aerially mounted BD SAC boxes. You can find and view them while driving throughout the older neighborhoods in the central, southeast and northwest suburbs.
Within The Electric Orphanage of North America’s collection, we boast two major aerial types, both Western Electric. A “BK-300” 1970’s style as well as an earlier 1950’s BL-300 style. The “B” designates the style of cabinet and the number, total pair capacity of the cabinet. Rest assured, both cabinets are very, very heavy to lift. From inspection, BL-600 type features 600 quantity 24-gauge copper pairs enclosed in a plastic internal wrap with an external lead sheath cable. This PIC cable is factory-furnished so a “pig tail” allows splicers to make connections to existing aerial cable. Weight on both units is considerable and a boom truck is necessary to winch the terminal to its installed pole height. The units in our collection are barely able to be moved by one person. The pad-mounted units below are 1200-pair or greater units and are installed in parts at site.
As you can see, these enormous fixed terminals were mounted at a low height, correspondingly to the aerial cables (or open wire dead-end [BD- or BDE] arms above). I’m sure the splicer was happy with a less arduous climb to reach his perch allowing access to the unit’s double doors.
Internal Features of the SAC Terminal
Now, things get interesting when the doors are opened. Each door has internal high strength steel springs and keepers located at the top and bottom hinge area. When doors are extended fully, these snap to lock securely in full open 90-degree angle position–to prevent wind gusts from knocking the heck out of the splicer’s arms when such delicate work is being done on fine gauge wires inside. To unlock, there is a tab with high-strength steel spring to pull down at top and pull up at bottom releasing doors, which then can be closed, then latched, locked. This maintains a weatherproof enclosure. There is also an option for a padlock–which is a wise idea for owners.
As with most terminals, they incorporate binding posts. A lot of them. For their duties, the unit works in thirds (in most circumstances, although some exceptions exist). The entrance cable (or open wire) from the central office or a remote terminal enters the cabinet by means of bridling runs or small lead cables. These pairs are called F-1 or “feeder pairs.” The other two thirds capacity of the box are designated F-2 or “distribution pairs.”
For example, in our own collection’s BL-300 unit from the 1950’s, 100 quantity pairs are considered “feeder pairs.” Extending out of the box are 200 quantity pair capacity binding posts for at maximum a 200-pair distribution cable. You do not require multiple, external cables entering the cabinet. Utilizing one 300-pair cable allows 100-pairs in and using within the same sheath, 200-pair out for distribution and these can be spliced into your existing and new cable outlays either above the ground or in a hand hole or manhole nearby. Also, with a 70-wire toll lead, 100 pairs offer 30 idle–potentially useful–pairs for a small underbuild lead cable or extra exchange wire, if needed. Growth potential is important.
Imagine these row upon row upon row of binding posts, all arranged in a very symmetrical order, from top to bottom. Each epoxy coated binding posts would be, and are, numbered left to right, top to bottom for the cable pairs. At the very bottom are binding posts separate for drop wire (which in many instances might be distributed at the SAC box pole to neighbors below). The bottom on of the Box has holes, filled with black colored rubber grommets for connector block or drop wire access. This is important. With open wire terminations, these allowed the bridle wire runs to enter the box without water entering as each drop included a drip loop as well. No mess. No fuss.
Okay, we’ve got a LOT of binding posts. And, a lot of pairs, whether they are cable or open wire. What do we do with them? Why is only one third capacity entering and two thirds coming out of the box?
The ground mounted, low profile SAC terminal with adjacent F-1 and F2 cable pair entrances. Location is Wichita Falls, Texas, where I often met this terminal’s interior in person. Date of photo is 1998.
What is a Cross-connect?
With some historical narrative out of the way, let’s ask the important question! Their use forms a marvelous concept based upon a cable’s (or open wire–but much less so) capacity and which pairs are energized and which ones sit idle. Not all distribution-side cable pairs are utilized by customers. Many variables combine, on occasion, to delineate between both permanent and temporary service. This explains why there may be more pairs exiting the unit than are supplied to it.
In the early days, a cross-connect cabinet performed a valuable service initially as a point where two differing sections of media intersected: most frequently, open wire media to cable, for example. Because open wire transmission rarely had idle wire, nearly all conductors were “doing something” and had assignments. Most people are unaware that open wire transmitted more than just voice frequency facilities. The Council Bluffs-Sioux City lead which I spoke about earlier, carried many carrier circuits, those added special circuit designs. FM/AM radio network program circuits were often carried by open wire. FAA circuits between airports maintained communications with different runways and information regarding landing and approach of aircraft; flight traffic controllers in the early years used these open wire circuits. Additionally, much data, both encrypted and non-encrypted was sent over these wires. Think of what the Transcon in Pennsylvania, or Iowa or Nevada carried in World War II telephone traffic! Imagine what interesting Manhattan Project information traveled over these circuits and others like them, such as the Denver-El Paso lead and so forth? Then, there was governmental open wire, where fire, police, and sheriff departments used teletypes, Telex and other open wire facilities. Fire and police alarms, messages for different military branches and so on, were part of the duties of these early and mid-1950s open wire leads.
In these more modern times, where aerial wire is no longer terminated in such boxes, instead, a split is made between central office “feeder cables,” known as F-1 (Facilities One) entering the terminal and feeding the “subscriber pair” cable pairs. Special assignment circuits yet exist, doing the same as in yesteryear–or more, with Internet access added as well. But . . . so what? Just connecting one to the other is a simple, practical way of connection, but what is accomplished?
Telephone planners, even in the early days, made an attempt to predict the number of subscribers using present existing service facilities and what possibly might be expected should the area grow in the future. Predictions are always difficult. Despite whomever owned the outside plant, accompanying angst arose when cables were found to be underused. With much investment in outside plant, especially when large cables (or in the case of large open wire leads) contained “idle” or unused pairs, this was wasted investment.
What the cross-connect accomplished, without any moving machinery of any kind–except the nimble fingers of the telephone splicer–was a margin of projected use by future subscribers with current facilities. Essentially, it allowed communications planners to look at a town’s growth, new subdivisions or industrial tracts, and project what the future expansion might portend by specifying higher density distribution cables with many pairs while eliminating a feeder cable with many vacant pairs.
As density outgrew the feeder cable capacity, an outside plant engineer or local loop engineer only expected to replace or “rehabilitate” the existing feeder cable–not the entire network of feeder and distribution pairs if necessary. Rehabilitating cables meant adding an extra cable similarly sized or larger adjacent to the existing feeder cable. It’s size based upon “per lines per square foot” as in business or several lines per home, such as in a subdivision or existing re-sized suburban rehabilitation project. Most commonly, with land-based phones being installed in the 1940s and through the early ’60s, was two lines per house maximum. Should this be exceeded, as in the 1990s, the SAI or SAC Box might expect rehabilitation and replaced with a larger terminal. No distribution cables and drops to customers needed to be changed out.
There were situations, where to save money, telecom companies in times of tight budgets had foregone placement of a SAC terminal. Hence, you would find central office pairs directly to customers. I personally, came in contact with an area of Richardson, Texas which was in a strongly dedicated “Dot Com” Internet-saturated companies in the late 1990’s. My job was to place a new cross-connect and convert pairs from central office to customer to central office F-2 to cross-connect and then furnish F-1 pair service. Sound simple? It was far from that. The engineering was particularly challenging as so many special circuit designs were involved. Imagine not only voice-frequency circuits, but alarm circuits for fire and police, FM and AM Program circuits, FAA communications and a large number of possible “specials” for the Internet providers. Splicers were confronted with having to do many changeovers in my job where service continuity was paramount. Think about continuity of service when heavy 24-hour data circuits were supplying service, and service interruptions were not appreciated. Ultimately, this particular job was accomplished. However, it my suggestion to be well taken, that engineers, when confronted by budget cuts, should target other expenses rather than omitting a very functional and cost-saving cross-connect system.
In some of my work, SAI cabinets were planted where remote terminals, combined with T-carrier and Pairgain systems, punctuated with special circuits (fire alarms, police communications, FAA communications, cellular, Internet and data circuits of varying types), repeaters, and other electronic components were also installed. These isolated systems, especially in conjunction with services to RSTs (Remote Subscriber Terminals) which are and were, little “sub” central offices of and by themselves. They were typically fed by fiber, with the optical signal being converted at an Optical Line Terminating Multiplexer (OLTM) to an electrical one for the copper electrical signal. The fiber OLTM fed the RST. Copper paired cables then furnished F-1 service to other smaller cross-connects which then offered subscribers F-2 pairs for expansion.
How big can a Serving Area Concept Box be? You’ve probably driven by these installations every day and paid little notice or simply did not recognize these cabinets. They perform somewhat as a distribution substation in the power field delineates sub-transmission from feeder distribution. The outgoing pairs are typically smaller gauges in suburban areas–especially when not far from the RT or RST and SAC box. I worked with 19, 22, 24 and 26 gauge copper pairs. The size of the box depends upon the boundaries of your “sectionalized” Service Area. Typically, a box was a “300,” “600,” “900,” or “1200” pair capacity installation. There are some much larger, but less common. Remember, the idea is to break up the locations where subscribers are served. When problems occur, hence, fewer customers experience problems. It is better to have more “boxes” than gigantic installations where trouble might end up being experienced by more subscribers.
How small can they be? This is a funny question with a fascinating answer. When working near Gainesville, Texas on some ranch-serving buried cables up where the old POW site was located, there were some very ingenious installations. Where new residents were moving into an acreage near the Red River, I was required to open a small pedestal/terminal about two miles away from the subscribers. This terminal was inspected to determine what service connections might be made to these new subscribers without laying further cable. At a small terminal where the junction of two eleven (11!) pair buried cables connected, I found a cross-connect made from a two ten pair terminal strips. A Brown Recluse spider came out to greet me when I pried loose the terminal door with my can wrench. A little jump there! However, after some close inspection it appeared that the installer had jerry-rigged a cross-connect from two terminal strips. A very effective job. Must have been done twenty or thirty years ago. So . . . there you have it! Enormous to micro, the cross-connect was and is a very effective device for pair economics.
Another interesting story. East of Gainesville, Texas, if one paid attention on U. S. 82, a modestly sized SAC box peacefully abided its time next to a highway junction corner site. Complimenting it was an RST (Remote Subscriber Terminal), repeaters and stub mounted loading coils. Much to the regret of the driver of the truck who lost control one night and plowed through the midst of this roadside technical extravaganza of highly prized telephone equipment, scattering it to oblivion, damage was . . . considerable. His aim was amazing. He had to have been drunk–and indeed he probably was! My job involved replacing the existing (destroyed) pad-mounted box with a larger one and to re-design and assist in planning to rebuild the roadside site. And . . . by the way . . . the driver’s insurance company paid a very healthy sum to resurrect new equipment at this site and for Southwestern Bell to get it working once again.
Let’s get back to our lesson. Think of our one thirds ratio: one pair into the box from the F-1 feeder cable to two pairs out the F-2 distribution side. All pairs are terminated on binding post terminals. More modern units, built by Western Electric, 3M, Reliable, and other fine major suppliers, would apply connector blocks where you simply only had to take a little colored conductor and “punch” down the wire, stripping the insulation on both sides to make a firm connection. This made electrical contact secure and obviously, no soldering was unnecessary–as in the old days.
The point of these large, vertical running connector blocks divided into three major sectors inside the SAC box, allows a jumper to be run between the incoming feeder pairs and the distribution pairs. This can be very complex when a splicer is confronted with over 300 pairs or more! That jumper can be installed and removed per orders of the outside plant engineering people.
In electric power we think “source-to-load.” In telecom, we think “feeder-to-field,” as railroads signalmen applied the term, “East-to-West.” That’s also the concept at work in cross-connect work. A further important feature of an SAI terminal relates to when trouble occurs in a system, it is possible to differentiate problems as a boundary between either one or the other side of the box. Splicer’s test equipment quickly concludes a local or regional issue; where from the SAC Box a cable fault may exist (upstream or downstream) and where remedies might be swiftly applied. With no problem on the distribution side, then the feeder cable has a fault or defective pairs; if reverse, a similar conclusion is afforded. Work is concentrated on the actual segment causing the problem. In buried cable work, this is a particularly important advantage.
And finally, a comment should be made about “defective pairs.” It is widely recognized (although to a novice such a surprising notion!) that all cables manufactured have at least 1%-2% defective pairs. Occasionally in very large cables, 3% to 4%, or many more. Brand new cable and problems already? Well . . . yes. When you are laying, splicing and bending large, twisted multi-pair conductors within a tight sheath, under heavy pressure through under-street conduit, manholes, and handholes; pulled many thousands of feet by a bullwheel and pulleys, and manhandled by laborers, frequently stretched pairs have broken and simply will not work. Sometimes, these can be corrected with splicing. An installer or splicer may remedy the problem by trying to mend defective pairs and get them to “heat up.” I’ve had cases where splicers made this magic happen. However, mostly, this is very difficult. The cross-connect also can help this problem by disabling any defective pairs found or activating any which can be made to work again. Good feature.
One other thing: even in the fiber world, the cross-connect still exists. When I worked with various telecom companies, beginning in the 1980’s, Digital Cross Connects were appearing. NEC America, Inc, Fujitsu, 3M and others were optimizing the potential of making “electronic” jumpers instead of planting splicers facing cabinet interiors manually accomplishing them. Someone in a central office could easily make a change in a jumper using these electronic devices in a far-off location. So . . . the cross-connect did not disappear with open wire. Far from it. Open wire, copper cable and then fiber. The Greeks once said, “Nothing is original.”
Note the low profile of the unit in comparison to photographer’s standing height. The concrete foundation is located adjacent to the joint use pole with the attendant entering and exiting multi-pair cables. As with all terminals, its identity is determined by the street address. In this case “R-(rear) 2715 Kell West Drive,” for example.
Here’s my portrayal of an early, “B” Box for large order cross-connections. Note the mounting handles on the sides of the wooden cabinet. The top roof was tin or later, galvanized sheet metal. Use of this style would have been early 1900’s to mid-1930’s. A padlock could be attached to the handles on later models.
Historical Perspectives on Open Wire Terminated at Large Terminals
Obviously, the open wire terminal structure–or pole–is our alpha and omega of large terminal placement. At the turn of the last century, and right up until World War I’s end, certain practices were followed to improve the operation of aerial long distance toll and exchange plant through better terminal construction.
In the very early years, the use of urban open wire was considered acceptable and commonplace; in essence, in the early days the public welcomed the arrival of this innovative and novel technology. It’s appearance said something about the progress and modernity of the town–a welcoming advertisement to town visitors, of civic pride and technical efficiency. Negative responses to open wire were infrequent, thus most open wire from the country would come directly into a city or town as near as possible to the central office. “Proximity” was considered a major rationale for efficient voice telephone traffic operation.
By 1911, the rule followed by AT&T, required their lines to enter the central office on Number 14 Birmingham Wire Gauge copper bridle wire (often referred to as “distributing wire”) pairs in ring construction from the open wire terminal pole to the protected terminals at the central office. This was only when the number of toll circuits was less than five.
If more than five toll circuits entered a central office from open wire, it had to be through the form of a sheathed cable. And–that cable must not be more than 200 feet between the open wire terminal pole and the protected central office terminals.
If physical property or urban zones prohibited these prior two methods, a lead-sheathed, paper insulated, dry core cable with 19-gauge pairs be installed between longer sections spanning the distance between open wire and the C. O. This also applied to all underground entrance cables as well at the time.
Back in 1909, these connection points were spanned by aerial facilities, open to the vagaries of weather and climate. Lightning and accidental power contact with trolley wires, alarm circuits and arc lighting, by these toll circuits required a large terminal box “provided with all the protection equipment required in accordance with standard instructions.” Furthermore, the technical opinion at the time stated, where such aerial toll leads terminated at the C. O., “fuses, [lightning] arresters, and heat coils shall be provided, and where entrance is made through an underground cable, [lightning] arresters and heat coils shall be provided in accordance with standard practice and instructions.” [Specifications No. 806-P, June 14, 1909, New England Telephone & Telegraph Company].
By 1915, the necessity for pole mounted protection equipment became more acute, owing to more facilities installed for toll service. A practice of the New England Telephone & Telegraph Company ordered:
Wherever practicable, Long Distance Cable Boxes shall be placed wherever it becomes necessary to terminate aerial, underground or submarine cable conductors used for toll line circuits, such as cable boxes to be of sufficient standard size only s to care for the ultimate number of toll circuits likely to be used in the cables at that point. Long Distance (L.D.) Cable Boxes will be furnished in the following sizes: 10, 20, 30 40 and 60 pair.
Once the boxes became mandatory, each was shipped with No. 46 Protector Mountings, Fanning Strips and L. D. Cable Box Binding Posts. On open wire facilities where the box was utilized only as a major terminal, often the protectors were eliminated from being specified and required only binding posts. Here is where it gets very interesting:
In either case there are facilities provided for cross wiring so that under no circumstances will it ever become necessary to change a pothead or bridle wire from the terminal to which it was first properly connected.
By February 2, 1915, the specifications included, “cross connections” in the requirement for large open wire terminals, stating:
In order to allow for changes in assignment without changing pothead or bridle wires, biding posts are provided between the inc-coming and out-going conductors to allow facilities for cross-connections. In all cases the cross-connecting bridle wires shall be soldered to the clips [provided in the binding post strips. Connections shall be also soldered to the protector lugs. The cross-connections shall, as far as possible, be kept horizontal and regular, but if, through change of assignment or loss of conductors, this cannot be done, the cross-connecting wires shall be carried through bridle rings conveniently placed, as indicted in the drawings.
That same year, it was ordained the numbering of each protector should be numbered, “consecutively from the top beginning in upper left hand corner of the box as seen from the front. These numbers shall be painted on the German silver fuse plates,” according to New England Telephone. Additionally, “sheets of asbestos four (4) inches wide, shall be attached to the door of the box in front of the protector fuses to guard against fire when the fuses blow.”
The old rule: whenever a Distributing Box, BD Box or SAI was pole mounted, which had a capacity of over ten pairs (!) a balcony would be specified for the splicer’s access and comfort. This rule ran typical of railways, telegraph and telephone companies of the early part of the Twentieth Century.
Where open wire toll was converted to toll entrance cables, not only the potential of using the terminal as a cross-connect box was necessary, but typically the enclosure of wood and later metal, needed to be tough, resilient and have sufficient large interior space requirements. This space might contain mounted lightning arresters (over-voltage protection), fuses (over-current protection), and other potential equipment might be placed. Mica stone was used to separate the various terminal/binding post pair blocks. Mica was resistant to fire as fuses potentially might ignite the wooden interior of the enclosure.
As you can see by the specification above, dated around 1911, sections of crossarm were used to steady the box against the pole. Gains were made in the pole itself, as one were affixing crossarms for open wire. These were usually one half inch deep and the whole outfit was secured with 5/8″ crossarm bolts. Two and one fourth square washers secured both the box and the balcony. The balcony could be handmade with hardware store parts. One-half inch iron or galvanized water pipe with elbows, some wooden planks, crossarm braces for the back support and shortened alley arm braces furnished with the required number of carriage bolts and lag screws, would build you a pretty respectable balcony. Later, pre-built standardized styles were made available through respective telephone ordering houses.
The use of asbestos was quite common in these old early wooden boxes, owing to the proper and necessary fireproofing of wooden encasements such as these 1914 period Distributing Terminal Boxes. The important aspect of these terminal blocks was that separation existed between the toll entrance cable and the open wire bridling runs to the open wire pairs above. If there was undue physical strain or pressure from the cable weight or the open wire on arms, sufficient distance allowed flexibility between connections. Furthermore, when a splicer was required to make changes between dedicated pairs from one customer to another, or from different train stations, test stations and depots, it was never expected (nor necessary) to change a pothead (entrance cable location) or entering open wire bridle wire from the original terminal to which terminal it was originally connected.
Potheads are (in both telecom and power) the means by which an outdoor cable or conductor is terminated between different media. For example, in power aerial phase conductors my be dead-ended on arms and after being protected with fuses (cutouts) and arresters, is attached to the insulated terminal of the pothead. Within the pothead, in which small porcelain bushings are attached, the conductor enters a weather-proof insulated underground/buried cable to begin its sojourn to a customer’s premise or system. Telephone lacks the magnificence of the large porcelain bushings extruding from the metal pothead body, but has similar means to enter from and to the Distributing Box as shown above. The old rule was pretty simple on cable/conductor sizes entering these old Distributing Boxes: where the pairs of the cables were no larger than 19-gauge A.W.G. were brought into the Big Box, number 18 or 19-gauge potheads were used. Otherwise, if larger than 19-gauge, potheads able to accommodate no. 16-gauge rubber insulated wires were used.
In those days, the splicer was a solderer. Pairs brought into the box from the exposed cable, would be cut back from their insulation and other pairs beginning with pairs one and finally ending with pair 40, for instance. Cross-connections made it possible to change pair connections between entering and exiting conductors without changing pothead and bridle runs to terminal blocks. Typically, between these differentiated blocks, AWG gauge 18 conductor was used. The splicer would use these 18-gauge splicing wires (jumpers) to connect between binding posts and protectors. These were soldered to clips furnished with binding post strips. The whole idea was for the splicer to do a neat and clear job of running these circuits: as you can see by the diagrams below, the cross-connections were horizontal, regular and bridle wire rings were used to offset irregular crossed wire pairs.
Cables could enter from bottom center to either side of the box. Above, we see the center mounting of a lead cable with collar.
We’ve talked elsewhere on this site about repeater coils and loading coils. However, sometimes the location of these equipments combined within the Distributing Box and with the balcony addition, thus easing installation and maintenance of Phantom Repeating Coils where toll cables entered the enclosure. Usually, there were connection strips designed to access the eleven conductors joining the cable box and phantom repeating coil box within the Distributing Box. This allowed one phantom circuit to be connected one above the other. Inside the box, pair notations were made so that a special order of connecting phantom coil box connections from line side to office side could be made. The eleventh conductor was for the battery connection. Here’s what the railroads suggested:
Conductor 1 & 2: Line Side of Coil Number One
3 & 4: Office Side of Coil Number One
5 & 6: Line Side of Coil Number Two
7 & 8: Office Side of Coil Number Two
9 & 10: Phantom Circuit to Office
These last illustrations, typical of railways and their counterparts, the telephone and telegraph companies, portray different types of wiring schemes. Most of these boxes were used for toll line construction. This was not exclusively the case for many smaller offices where exchange open wire was run from small town to small town or county seat. Sometimes the cable was split into different conductors (two conductors of different sizes within the same sheath was not uncommon) which sizes were dedicated for different purposes: toll and exchange circuits. Occasionally, not all the conductors were “heated up” and spent a brief time as “spares” or reserved for future service. There could be exchange circuits within the same cable–which is not unknown to open wire configurations. We have often recalled the 40-wire toll crossarms above and then a few extra arms beneath carrying “exchange” circuits. Cable was expected to perform the same function, except within a smaller cross-sectional area.
Let me leave you with a contemporary facilities’ rule, which in 1915 was typical of most companies: “Where a cable is . . . used for both toll and local lines, the conductors used for the toll lines must not be spliced to any cable conductors extending beyond the point where the toll lines enter nor should these cable conductors be multiplied with any side cable leads, the object always being to have the least possible length of cable connected to the toll circuit.”*
Engineering Department, Specifications, New England Telephone & Telegraph Company, 1915.