Test Equipment for Open Wire

Testing Equipment for Open Wire Systems

Early Electronic & Electrical Test Set Equipment for Linemen and C. O. Technicians



The “Butt Set” Used By Installers, Splicers and Communications Linemen

Late 1930s Butt Test Set rotary style with pins – Western Electric

Test (Butt phone) in use. Don’t “frog” the wires!


While the Lineman’s Test Set has changed a bit since the days of open wire, this extremely handly device not only allowed the lineworker to talk to “Central” but allowed simple analog tests to be accomplished while high atop a pole.

Newer versions exist, but this simple analog phone possessed a clip on the end for the lineman’s belt, two leads labeled for “tip” and “ring” so to properly attach the leads to the correct polarity binding posts at a terminal, a rotary dial with steel pins and a switch to enable the unit to be switched on and off while connected.

Later versions with which I worked, were digital units and used push-button selectors instead of dial, and were of a rugged plastic, instead of this early rubber style.  Colors too, were pretty limited: black was the only Western Electric choice color.

These early units were designed for voice frequency circuits and thus not equipped to handle data communications.  This particular set is too old to function properly on digital systems, but with standard open wire, this unit was highly economical and useful.

One of the most useful facts about this particular test set was its ruggedness.  I’ve been told that when a lineworker had to challenge a particularly vicious dog, throwing this test set at the offending beast would quickly subdue the animal if targeted properly–and still work after the attack!

In the digital era, the new test sets had a conference or speaker type phone, and much like today’s residential and business sets, had a “redial,” a “monitor” switch, muting, selection as to whether this unit would be plugged into a circuit using either pulse or tone dialing as well as a low pass filter.  This latter option guarded against any distruption of data circuits.

Western Electric Butt Test Set. Note the pin type rotary dial and switch to talk.

Alligator clips were attached to tip and ring binding posts at terminal.

The 87B Test Set by Western Electric




The Western Electric Type 107 Open Wire Fault Locator


Western Electric Type 107 Tester for locating faulted (shorted) lines. Shown here on a wooden bracket lead.  Drawing by D. G. Schema

The Western Electric Type 107 Test Unit for lineman use.  Above is a drawing showing the 107 Test Unit in service.  We’re using a wooden bracket service station lead for an example, although it could be used on any open wire leads.  The unit has a history of being used on a variety of different phone systems where power sources to energize the lines could be from: manual (crank) magneto phones, manual common battery power sets, and lines where composite telegraph was used in the field.  The unit could also be used in the central office (C. O.).  The dial on the unit allowed convenient C.O. use.

Let’s see how this little tool operated.  Above we have a faulted (shorted) open wire lead to a farm which has eight parties attached.  We’re trying to locate a “cross” somewhere along the lead, which might average several miles.  Let’s identify the unit’s components.

The first is the switch box.  The second is a lineman’s listening device, such as the Head Telephone Set.  The third comprise two long leads labeled by color: red and black.

Using the drawing above, the “A” lead was the black insulated alligator clip; the “B” lead had a special coil in series with the lead and was attached to the other wire of the pair.  It was colored red for this differentiation and purpose. The letter “C” denoted the position by lever to the various of three positions on the circular dial.  “D” was the actual unit which was furnished with a convenient leather strap on the left so it could be hung from the pole or ladder.  “E” was the tip and ring plug which allowed the lineman to hear an audible tone.

Lets find our “cross” in the line.  Before attaching the leads to the line wires a couple of things had to be done first.  The head set monitor switch needed to be turned to “off.”  Returning to the test unit, you located the switch lever.  Now, turn it to the second position.  When this was done, the line leads would be quiet upon attachment to the live line.  After connecting the two pairs properly, you listened to determine what kind of service the line carried.  In our simple example, we have a magneto phone system, like the old party lines without common battery service.  If this was not known prior to contact–as in a multi-armed exchange lead, it would become clear by the noise on the line.  The switch position is now turned to “3.”  Our example is an “exchange” line, not toll.  The numbers on the dial of the test unit would be set to the appropriate type of line designation.

Note that the unit on the left side has a crank.  This was the same thing as on a magneto phone.  By cranking the unit, a signalling current would be produced when it was attached to the line wire pairs.  The linemen whom I talked to informed me that on “toll” testing pairs, such as 20 or 135 cycle lines, proper placement of the dial on either “1” for 20 cycle and “2” for 135 cycle, would allow you to signal and talk from the unit.  When higher frequencies than these were used, my associates informed me that a “1A” whistle was used!

Let’s say we had a dial system on this line–only one subscriber–not a multi-party service lead.  Then, we’d put the test dial on number “3”  If it were a common battery party line, it would be placed on “1”.

Several of these units evolved during the late 1950s and early 1960s.  One was called the 107A and it was superceded by the 107B unit. 

What was important about the unit was that even if there was accidental contact with overhead a.c. distribution circuits by the shorted line wires, both 107 units were protected up to 10-kV.

The use of these units would allow:

a) loud noise beckoned a “cross” or unintentional twist of the wires together causing a short somewhere along the lead’s travel.

b) the unit could also test for ground and in what direction by a loud and louder tone picked up by the 107 tester’s head telephone.  The red insulated alligator clip was attached to the wire which was grounded.  Taking the series coil alligator clip lead, the lineman attached this to the ground.  You turned the crank to create the signalling current while pushing the switch box’s push button off and on intermittantly.  When a louder tone is heard versus a softer tone, you have determined your ground.

c) We have a ten pin lead.  One of the line wires of a pair is broken or “open.”  Attach the leads from the 107 unit as shown in the drawing above to the line wires of each pair until the suspect pair is located.  Attach the leads as above and crank the generator on the unit.  Push the button off and on until a lower tone demonstrates the fault’s direction.  From the C.O. (to the field) or towards the C. O.  Sometimes, there is no difference in the tones.  This means that the open pair may be damaged at a point before the terminal open wire structure where cable and open wire meet or at the C. O.

I thank the retirees who helped me understand this very useful Western Electric product.


Bob Goodrich, a fan of this site and expert from his days in the telecom industry, has offered this corrected information: This is a Hewlett-Packard HP-200CD Wide Range Oscillator.  For further information, please see:



Westinghouse Watt Meter, portable; c. 1940s.

Westinghouse portable Power Factor Measurement Device, c. 1930s.

D.C Milliamperes Meter, portable; c. 1970s.

Thompson Alternating Ammeter, c.1910

Thompson Alternating Ammeter with wooden oak carrying case, c. 1913.

Portable Westinghouse 1913 period current transformer test set.

D.C. Resistivity Meter, c. 1960s.

D. C. Resistivity portable meter, c. 1960s.

Gas Tech test device for warning of hazardous manhole noxious gases, c. 1960s.

Biddle D. C. Capacitance Bridge Test Unit for shop use, c. 1970s.

Other Installer, Splicer and C. O. Test Apparatus of the Era

No matter how effectively open wire circuits were engineered and competently constructed, their day-to-day operation operation depended upon the “masters of the test board.”   Those who continually guaranteed quality service over these metallic circuits were a highly educated craft guild unto themselves no matter what phone company or railway communications system they serviced.


Even in the early days there were portable test equipments to allow for field testing, as well as those installed in the central offices or at repeater stations.  In our discussion here, we will explain some of the initial direct current test proceedures and then advance to the finer, more delicate tests, which defined signals impressed with early carrier systems.


Since many of the earlier Direct Current (d. c.) circuits were those applied to telegraph and low frequency voice communications, we will consider these first.

Measurements using the Wheatstone Bridge arrangement.  This basic instrument would allow for the measurement of single wire resistance measurements.  Also, with the use of this device, the “Varley” process  allowed for determining resistance unbalances in a wire pair when a third wire was missing.  A third and fourth varient of this instrument process allowed for locating grounds on a bridge circuit.  The latter was commonly referred to as a “Murray” bridge application.   Why would one want to know the resistances of these circuits when the resistance of the wire was already known?  Typically, poor splicing (of which there would be many in a long 50 mile toll lead pair) could lead to series resistance unbalances.  In the case of the Varley and Murray tests, trouble shooting could be undertaken when sending linemen out to find out why circuits were experiencing grounding.  A good test board operator, with significant experience, could tell a lineman nearly the pole number where this problem occurred. Let me cite an in-teresting example: in the mid-1930s, at a Northwestern Bell wire center in Norfolk, Nebraska, dead shorts were being experienced between poles 161 and 164 on a 20-mile exchange open wire line.  These were mysterious occurrances.  They were intermittant; the test board operator noticed that these rapidly punctuated the circuits generally around 6:00 am and 7:00 am.  After 7:30 am or so these effects disappeared when thereafter calm reigned on the circuits.  Then, like clockwork, between around 7:00 pm up to about 8:00 pm the grounds punctuated the circuits causing lots of problems.  Thereafter, the night was calm.  So . . . what was causing this mysterious problem?  Using Murray tests, the board operator sent a lineman out to poles 161-164.  The test set could tell the board operator that these were the specific locations along the line where this issue was appearing.  The lineman drove out to the location around 5:30 am and reviewed the line.  His inspection revealed nothing out of the ordinary.   He went back to his truck and began to write down his findings.  Suddenly, a flock of birds began to gather on the wires.  Soon it was like Alfred Hitchcock’s Daphnie DeMauier’s story “The Birds.”  A large black mass of crows decended upon the lines and soon the top wires were contacting the lower crossarm circuits and birds below that were contacting the fourth arm’s circuits.  For about 40 minutes, the birds rested, squawked and crowed.  After they had gathered, then they flew off in a sheet allowing the wires to bounce back and forth, contacting and re-contacting the wires in the span until finally peace reigned once again.  And . . . yes, the problem was between those exact pole numbers and the problem solved.  Later new structures with better tensioning elminated the morning and evening problems.

Measurements made to determine leakage or insulation resistance.  The basis for successful insulation between the conductor and the ground was due to several factors.  First, if the line were using insulated conductors, with a sheath or plastic jacket around them, was secure and unbroken.  Secondly, if the conductors were bare, then the ability of the insulators to prevent leakage to ground from the current carrying conductors; Thirdly, there was the problem of leakage currents between both conductors and ground as with a pair or pairs on the same arm.  The various methods used to detemine these faults and locate them could be done with the Voltmeter Method.  To test a pair, a voltmeter was placed in series with a battery connected to the conductors under test.  The connection would detemine leakage between the wires and the ground connection.  High voltages aligned with a high resistance voltmeter would provide higher accuracy since the leakage measurement would presumably be quite high.  This test would allow for the location of broken insulators, entanglement of conductors with tree branches, loose tie wires, broken metal hardware contacts, such as a steel brace unfastened from its arm carriage bolt contacting a lower conductor, for example.  Toll circuits were commonly and frequently tested in this manner.  If trees were the culprit, then a tree-trimming crew could be dispatched and eliminate the wet tree branch contact problem.  Additionally, this test could impress a 200-volt test voltage on the line to check central office and subscriber line insulation values.  Hence, to use the apparatus, the voltage would be equal to the current/resistance drop over the voltmeter plus the current/resistance drop or the drop due to leakage over the end section of the circuit back to the battery.  These measurements would allow the reader to find the number of megohm values in one mile of wire, for example.

Earth resistance tester with case, c. 1950s.

Megger Test methods.  While we are working in this website mainly with open wire, some comment must be made to cables–since toll entrance cables were used as the convenient means to eliminate aerial wire facilities in crowded suburban or urban areas prior to entering or exiting the central office.  Voltmeters, such as described above, were not satisfactory in measuring resistance for cables, since the need for insulation in such a confined space was far greater than that of open wire construction.   500 megaohms per mile, is not considered an excessive value for a compact multi-pair lead-sheathed cable.  In this case, the Megger Test was devised.   The Megger unit could be portable, confined to a small hand-carried wooden box, this test could be carried out anywhere: manholes, ground terminal boxes, aerial BD boxes and other aerial locations when required.   The unit has a crank handle on the side.  This moves the high voltage direct current generator to produce up to 400 volts, which then allows cable testing.  Inside the Megger unit are two coils: one in series with a fixed resistance and the other, a coil in series with the test conductor connection.    If the test circuit is not connected to the Megger unit, “Infinite” will be shown by the indicating guage pointer.  After connecting a circuit, where the “line” and “Earth” are contacted, current flows through this circuit due to its leakage and through the second coil of the indicating device.  Hence, the pointer will move to “Zero” position showing megohm resisistance values.

Earth resistance tester c. 1950s.

Measurements using telephone line voltage and current.  Telephone circuits carrying direct current signals involve measurements conducted with a simple ammeter or milli-ammeter in series with a battery–or generator.  This test, if carried out at the test desk in a C. O., involves the understanding that a central office battery system voltage is supplied through a regulating rheostat, cord circuit and meter.  To find the direct current resistance of these, one shorts the distant end of the subscriber’s loop or trunk conductors, and then reading the meter which is connected.  Because each subscriber’s loop is slightly different from another’s, some adjustment of current is necessary to import resistance value measurements correctly.  The rheostat in the test allows for these different lengths of loop length.  What is extremely important in this dicussion here is the implementation of carrier systems to the local loop.  When you add analog repeaters and loading coils to the cables and open wire, then amplification of the signal–and the interference can make these tests more complicated.  The telephone transmission engineer requires the system values to be highly regulated to close tolerances and limits.  Here, the test board operator has to rely on many more complicated in-house test features than the portable Megger device.

Digital Ohm Meter, c. 1960s.

Capacitance Measurements.  These will be discussed briefly, however in the early days of open wire toll lines, little need was seen to seek information relative to the direct current capacitance.  However, since each subscriber had a condenser (capacitor) in his telephone, a simple d.c. test was conceived to look for these values on their loop circuits.  A 100,000-ohm voltmeter and 100-volt battery were connected (the battery was grounded) to the subscriber’s loop pair and the other conductor grounded.  A current would flow shortly in the circuit charging the condenser–which in turn–would pronounce a significant bounce in the voltmeter indicator.  The amount of movement would depend upon the capacity of the condenser and the capacity between the conductors.  And, if the tip and ring connections were reversed, the instrument would register an indicator movement in the opposing direction.  This would allow for a determination of whether the capacitors (condensers) in the sets were functioning at full efficiencies.

We next will consider what Alternating Voltage (a.c.) test processes could be explored in the world of open wire toll systems.  The d.c. tests were very effective in deriving the conditions of the telephone systems’ physical and electrical values.  Speech transmission is also fundamental to the consideration of a.c. test proceedures on the telephone open wire plant.  The operating conditions of a major (or exchange) toll wire facility dictated testing: a) inductance, b) capacity, c) resistance and d) leakage of the circuits’ properties. 

Measurements using an Alternating Current Bridge. This setup is not all that different from the d.c. Wheatstone bridge process; in this type of test, the d.c. source of power is removed and an a.c. power source is implemented.  To measure the outcome of the test requires an ordinary telephone receiver, which is the monitor.  Here are several open wire toll line test techniques applied with this instrument:  a) Test of Alternating Current Capacity.  This is simply a bridge circuit with two arms of equal resistances, firmly connected and static.  A condenser is placed between the fixed resistances and an oscillator is then connected to the two resistance arms.  There is a contact terminal where the fourth arm connects with the line to be tested.  The oscillator generates 800 or 1000 cycles for the test.  The telephone receiver then is applied to the “slide wire contactor” for the test.  The unit is portable.  What is so important about this particular test unit is when capacity is measured in short multi-pair copper cable lengths, non-loaded toll entrance cables, bridle wire runs of appreciable length, switchboard and frame wires and such other similar applications.  When repeaters are applied to the toll plant, their installation is impossible without knowing the values of building-out condensers to maintain line voice quality and balancing open wire circuits.  b) Capacity Unbalance Testing.  We’ve talked previously in the transpositioning portion of this website about balanced and unbalanced pairs–with the latter’s poor operation resulting in crosstalk susceptibility.  There is a test for that.  It is called the Capacity Unbalance test where measurement of electrostatic coupling can be made between the the aerial wires and the ground.  In cables, unbalances can create very disturbing effects: side-to-side and phantom (when used) side-to-side crosstalk.  In a quaded cable, circuits can be easily affected by this if not great consideration is given to the quad pairs’ splicing in consecutively designed lengths.   When the first quaded (25-pairs) of cable were designed, a special portable bridge test device was conceived.  It is called the Capacity Unbalance Test Set.  This unit could test unbalancd conditions in the cable in either direction; from the C. O. and to the field.  We consider it here, because toll entrance as well as cables meeting open wire junctions and terminations made it necessary to use for such different transmission media.

Tests of Impedance.  If a telephone circuit were a simple, predictable length with no loading, no special equipment for amplification, and the like, this test would be unnecessary.  However, when open wire was “loaded” and transformers were built into the circuit, retardation coils and other apparatus applied, a major factor emerges: What are the combined effects of resistance, inductance, and what we consider “impedance” upon an open wire line?  We talked about capacity bridges, but other “bridges” were introduced to test devices whereby the former standard condensers in the two arms were instituted for standard inductances and resistances.    We know that loading coils were used in the early adoptation of open wire long distance transmission as were such miracles in the use of long distance cable application.  Such devices were inductance-type apparatus.

Apparatus Balance Test Equipment.  We know that impedance unbalances create crosstalk.  However, when certain specialized equipment is operated on such open wire lines, these would also create unbalances, as well, introducing further crosstalk problems to the pairs.  Such items in the early days were phantom repeating coils, composite sets and certain cord circuits applied to cable and open wire.  Lots of noice, interference and other issues from this application of equipment led to the further application and use of the capacity bridge and impedance bridge as referred to above.  D. C. telegraph circuits were widely superimposed on voice channel circuit pairs.  The means to do this was through “compositing.”  These Composite sets would superimpose these telegraph circuts on voice channel circuits but in so doing, would create further crosstalk potential.  This test would quickly ascertain the balance conditions of the pair throught a special bridging arrangement in the test unit.  It is known as a Composite Set Bridge.  Tests would be simultaneously be made on the balance of telegraph circuit branches in their entirity or just tests of the values of their condensers and coils specifically. 

Noise & Crosstalk Measurement Test Sets.  Imagine a telephone line with its myriad potential for crosstalk on its 60 wire transposed line conductors between towns spaced 30 miles apart.  Now, imagine the introduction of foreign electrical values by electric utility a.c. power distribution and transmission lines, railway a.c. power centralized track control systems, signalling and communications circuits along much of the same right-of-way easement or coinciding at irregular intervals along the toll lead’s route.  Here we introduce further crosstalk and interference created by these foreign intruders lines’ effects.  We mentioned earlier that basic crosstalk testing can offer definition of unbalanced conditions–however, it is of little use in seeking specific locale of the problem for the plant engineer to correct.  After one’s suspicion’s are confirmed, then a further procedure is then to locate the cause and correct it.  This little test device is called a “Crosstalk Meter.”  This is how it is used.    Inside the meter is an a.c. generator to produce current which simulates the human voice.  Here a switch separates the introduction of this voltage to either a “disturbing circuit” or “disturbed circuit” connection.  The other side of the shunt is connected through the Wheatstone Bridge scheme to a second telephone circuit we called the “disturbed circuit.”  To achieve proper impedance in respect to variouis circuits’ qualities which might be tested, shielded transformers separate the Bridge portion and the A. C. generator from the disturbed and disturbing connections.  Hooked to the Wheatstone Bridge are two telephone receivers which allow the tester to hear noise currents prevailiing on the line.  When lines are tested, the alternating current tone is impressed on the “disturbing circuit.”  The shunt is adjusted so that the tone on the “disturbed circuit” is similar to the “disturbing circuit.”  The crosstalk meter registers the ratio of 1,000,000th “crosstalk units” between the currents of the terminals of either circuits and are measured.  So between throwing the switch to “line” or “shunt” the proper impedance relationship is illustrated.  Moving the shunt and line switch back and forth, the shunt is adjusted to noise equality.

Tests of Circuit Balance on Open Wire Lines or the “21-Circuit Test.  The name came about because the testing device is actually the “21 or 22 Type” Bell System repeater.  Otherwise, to Independent Phone folks, the test set was known as a Circuit Balance Test.   It’s pretty ingenious.  Here’s how it worked.  We mentioned previously the “bridge” for finding splice problems and other anomalies in impedance,  but expressed some caution because on lines typically repeaters and loading coils would be placed in the circuit.  Here is where the “ingenious” comes into play: good thinking focused on not building a piece of test equipment, but rather, by using the target of the test: the repeater itself,  to act as the actual test machinery.  Let’s propose a simple problem with a repeater.  There is a two wire pair.   To the left, or in the old telephone and railway signal parlance “West” direction, we had the C. O. In the center was the repeater unit.  Beyond the repeater to the right, or in the “East” direction–or “To the Field,” we had the circuit portion to be tested for a “balanced” network.  The line to be tested as the “East” portion was connected for normal repeater application.  On the “West” C. O. Side, the pair was physically opened.  The terminals connected to “West” were shorted.  A transformer with three windings comprised the repeater.  By connection as a normally wired repeater on the “East” portion of line acts as a Wheatstone Bridge.  The input of the “West” side of the amplifier being connected to the “balanced” points of the bridge.  The proportion of the current sent to the windings of the transformer from the East which is then inducted to the West side of the feed depends upon the amount of bridge balance furnished by the line being tested.  Open the three-winding transformer on the “line” side with the feed terminals shorted or with the network connected, the electrical result is that as a repeating coil.  The currents then “sing” or produce a specific audible tone if the gain of the two amplifiers is greater than the losses within the circuit of the repeater.  Since no line is perfectly balanced, there would be no tone, however, in our example.  To affect the best service conditions, the lowest value of singing is attempted to be achieved, and this is called “poling” of the repeater circuit.  Conversely, if we wanted to test the C. O. side of the apparatus network, then we simply reverse the instructions above.

Measuring Transmission Efficacy. 

Relay Protector Testing Associated With Open Wire Low Frequency Induction

More added soon!