Testing on Open Wire Telegraph Lines

by L.E. Trump

Since the ranks of the Open Wire Testboardmen are thinning, and Open Wire Telegraph lines are almost non-existent today, it may be of interest to describe some of the testing techniques employed by those who did such work.

Western Union, the railroads, AT&T and other companies had extensive open wire plants spanning the U.S. up until the 1960's. Technological advances have largely supplanted the open wire lines and over-come their vulnerability to disaster.

The old-time Testboardman or Wire Chief had an interesting job to do, especially when the weather was less than ideal and rain, sleet, snow and ice wreaked havoc on the lines. Then, too, Lightning often walked about, seeking arbitrary paths to Mother Earth, and what better place to go than a nice fat Telegraph wire strung high on wet poles. "Atmospheric" electricity was something to be reckoned with indeed, and it caused many a Wire Chief to tear his hair.

When the weather was fine over his district, the Wire Chief usually did routine tests once a day which were intended to determine the basic "health" of the wires traversing his territory. Routine measurements conducted at one end of the line, with a distant office assisting, were conducted to measure the leakage to ground or "insulation resistance" of each wire, continuity of same, and working current in the circuit assigned to the wire if any. These measurements were usually recorded at intervals perhaps daily, or weekly or even monthly depending on the particular Company's policy. These records were kept for reference, and could be referred to as a "standard" or benchmark when things went wrong.

This testing was service-interrupting, so it was normally done during the "low usage" part of the day when the wire was idle. If this was not possible, service would be maintained by "patching" the circuit carried by a given wire off to a known good spare wire while the testing was done.

The primary testing instrument on a typical telegraph switchboard was the ES7A Volt-Mil-ammeter (VMA). The Switchboard Volt-mil-ammeter was a special "zero center" Galvanometer (micro-ammeter) that was calibrated to 200 units each side of center. A special set of shunt resistances, along with a jack and plug arrangement allowed the meter to be quickly set up as either a voltmeter or a mil-ammeter, with a full scale reading of 200 volts, or 200 milliamperes either side of center, as desired. When set up as a voltmeter, special testing potentials of up to plus 200 volts, minus 200 volts, or ground, could be connected to one side of the meter, with the other side of the meter connected to a test cord with a plug that fit the line jacks on the board. A strap key was also provided that could reverse the testing battery potential applied to the meter. When set up as a mil-ammeter, both sides of the meter were connected to the test cord. Current value in milliamperes and direction could then be easily observed by plugging the test cord into a series jack in the switchboard jack circuit of any given wire.

Insulation resistance tests were carried out with the distant end of the wire under test known to be open at a given location. The switchboard meter was set up as a voltmeter, and connected to the wire under test at the line jack in such a manner as to open off the normally assigned office equipment and working battery.

First ground, then both potentials of testing battery were applied in turn and the meter readings observed. On a clear wire, on a dry day, with no crosses or grounds, and testing battery applied through the meter, the voltmeter reading would settle to a steady value, of only a few volts, indicating a high value of insulation resistance.

The voltmeter reading was recorded, and the value was used to calculate the leakage resistance of that section of line, using Ohm's Law. The relationship of testing battery voltage and Voltmeter resistance, both known values, aided in this calculation. Normally a leakage resistance of 100 megohms per mile of line was the target value. In practice, once this calculation was completed, and the value recorded, it was not often repeated unless major changes to the wire occurred, such as changes in length, or repair of major damage, or something of that nature that would materially change the characteristics of the wire.

If the wire was "crossed" with an adjacent wire, or grounded, the meter reading would indicate a higher value than normal, reverse polarity, or possibly would fluctuate in unison with the working of the other wire. The meter was observed while each of the other wires on the line was opened in turn until the "cross" was determined. In the case of a grounded wire, intermediate offices would be contacted to open the wire under test until the trouble was localized between two offices. This was also done to determine the location of a cross. Once a ground or a cross was so localized, a lineman was dispatched to investigate and clear the trouble. Similar tests would be repeated while the lineman was in the area of the trouble if necessary to assist him in locating the trouble. A competent Wire Chief familiar with his district and working with a good lineman could usually pin the problem down to within a span or two fairly quickly.

One other test performed with the wire open at the distant end was the "static kick test". The testing battery applied to the voltmeter was reversed with the strap key, and the action of the meter needle observed. On a clear wire, open at a distant point, the meter indication would swing to a value of several tens of volts, and settle back down to its normal reading. The peak value of this "kick" (due to the capacitance of the wire to earth), was noted and recorded. With the wire open at a known location, this meter reading was a coarse indication of the length of the wire. In the case of a wire being open at some intermediate point, the kick would be of less value, and so a rough estimate of how far out a wire was open could be obtained. For instance, if a wire known to be good, and open at the distant end gives a kick of 60 volts, a similar wire adjacent to it, also known to be open at the distant end, but suspected of being open at some nearer point, only gives a kick of 30 volts, it is a fair assumption that the location of the open is about half way between the testing office and the far end. This gave the Wire Chief a place to start the lineman out, at least.

One last test of the wire would be to have the distant office ground it, and verify that the voltmeter (with testing battery connected to it) indicated full testing battery potential. This indicated that the wire was at least continuous to the distant office.

Once the voltmeter testing was completed, the switchboard meter was set up as a milliammeter, and plugged into the wire with normal working battery applied. This gave a current indication of roughly half the normal working value to the distant ground, and anything significantly less than this could indicate a possible high resistance joint or splice somewhere in the wire. The distant office was then requested to "put it regular" or take off his ground and the normal working current measured. With this being satisfactory, the wire was returned to service.

In practice, on complaint of trouble on a wire, the switchboard VMA would be plugged in and cursory checks of current, voltage from each end etc, would be quickly done and then more thorough tests would be performed as the troubleshooting effort went forward, depending on what was found in the process.

During periods of wet weather, the insulation resistance of any wire decreased owing to imperfect insulation. The various leakage paths to ground along the line were accentuated by the moisture, and voltmeter tests as described above would be virtually useless because the increased leakage would cause a full scale meter deflection. Most wires would still be capable of working in this situation, but the amount of current change in the various instruments along the line would be less than that on a dry day. This would result in the operators along the line having to readjust their relay or sounder magnets away from the armatures so as to compensate for it. The leakage to ground of the working current tended to hold the relay armatures up, unless the magnets were backed away, allowing signals to be read. This is why most Morse relays and Mainline sounders have a quickly and easily variable magnetic gap adjustment.

This "working margin" on wet wires could not be detected with the VMA used as a voltmeter, but could be determined by setting it up as a milliammeter and having the distant office open the wire under test. The VMA was then plugged into the circuit with regular working battery applied. The resulting current measurement represented the leakage to ground and the difference between this value and that obtained with the wire closed up would be the available current to work the instruments.

Given the vulnerability of any open wire pole line to troubles caused by broken wires, insulators, crossarms, poles, and the likelihood of the wires on it coming in contact with foreign voltages, grounded surfaces, lightning, etc. you can see that the old-time Wire Chiefs had a challenging job. There was a never-ending variety of things that could and did go wrong. Wirechiefing is a skill that has followed the Landline Morse Telegraph operator into history.

73
L.E. Trump
Fairbanks Alaska

Return to the Telegraph Technology Index

Return to Telegraph Lore Homepage