Underground Cable Locating: yesterday and today 

Cable fault location is a tough challenge but we’ve come a long way in helping technicians through technology and by learning from past mistakes. Assuming you have current maps (a big assumption, I’m told) and some good equipment, your job is a lot easier than that of the early pioneers of cable fault location (CFL).

A Bit of History on CFL Test Sets
A need for accurate underground cable location was recognized when the first telephone cables were placed underground in 1883. It has been said that the first underground cable was placed on November 5th and cut on the same day, around 4:30 p.m., on a Friday. The first cable locator was sent home for the day on the following Monday, when his manager found out about it.

Unfortunately, the early practice was to assign a member of the sick, lame, or lazy (he was the fellow who had trouble understanding air pressure) to this difficult task. Untutored and untrained, this person was the chief defender of the underground environment. The complexity of the task was soon recognized, however, and better heads were assigned.

Telephone men discovered that conductive telephone cables disrupted the natural magnetic flux of the earth, and a pair of wires could be used to find this disruption.

The technician held a rod in each hand, pointed straight ahead, carefully balanced so the slightest change would affect the rods. As the technician worked the area, the rods would swing either in or out, parallel with the cable path, as the cable was crossed.

In dry soil, where sensitivity was not good, the more experienced technicians found it more effective to remove their shoes in order to become more conductive and increase wire sensitivity.

It was necessary to “believe” in the process, but it seemed to work better than a visit to the local gypsy or astrologer, though with training they might have done quite well. Some people still scoff at the procedure (Susan, my lovely wife), but it does work (me, Donald). Keep in mind that the rods will react to anything conductive. Old beer cans, garbage can lids, or anything metallic will have an effect on the procedure.

As more and more cable was placed underground, and as the sheathing techniques improved, a more reliable location procedure was needed. Early cable sheath was lead, sometimes with a jute-covered-tape armored protection. This made the cable more difficult to damage inadvertently, but it also made it hard to locate because the armor protection absorbed the tone. Technicians were forced to locate the exact position of the cable by exposing it with a shovel, which is not a bad idea still today, whenever there is any doubt on a locate.

Eventually, technicians discovered that an amplifier and exploring coil, held in the proper attitude, would pick up any AC voltage induced into the natural magnetic field of the cables. Not all areas had usable induced AC, however, so a method of inducing tracing tone into the cable was sought.

The Western Electric 20C test set provided the first reliable AC tone source. The tone was derived from the same AC ringing current used by technicians to locate solid resistance faults. This set used a Ford Model T coil and an interrupter to produce a low frequency 20 cycles per second tone with a peak voltage of 900 volts. Four 1.5 volt dry cell batteries powered an identifiable tone as far as 1,500 feet on direct-buried, common grounded cables.

The 20C could still provide the most accurate locates on all cables, but the problems caused by the audible tone (the frequency of which was in the range of human hearing), led to its demise. Low frequency tone interfered with subscriber service. Trouble reports suspected everything from Martians invading Earth to someone tapping their lines. Cable test centers spent a large amount of time explaining to customers why the tone was on the line.

The breakdown set was also used for cable location. A 93-type receiving coil was used with a 147 type amplifier as an audio receiver. The problem here was that the maximum of 90 volts peak would not carry the tone far enough for effective cable locating.

The need to increase the tracing frequency above the range of human hearing brought about the Fisher M-Scope. This test set soon became the industry standard for cable locating. It used a direct connection hookup, and also had the capability to induce tone into the natural magnetic field of a cable simply by setting the transmitter on the ground. A large open transmitting loop was used to send the tone. This induced a high frequency broadcast tone all over the place, which was fine in rural or uncomplicated plant, but a problem in congested plant.

The Fisher M-Scope was not the end of low frequency tracing tone. There were many common grounded lead and tape armor cables that the M-Scope couldn’t locate, and telephone companies had to bite the bullet and use low frequency even though customers complained. Modern low frequency sets use a much lower voltage, which reduces range but affects fewer customers. The simple fact is no one cable locator on the market can locate all underground telephone cables in all situations in the field today.

Cable Locating Test Sets Today
Technology has had a big impact in many areas of the telephone business and CFL test sets are much improved. There are currently more than 23 major brands of cable locating test sets used to help technicians find the path and depth of underground conductors. Despite this variety, there is no one brand that will do every job completely, and, for some 10 percent of underground conductor configurations, there is no electronic device that can pinpoint them precisely. Knowing this can lead to sleepless nights, but when a technician is schooled in the plant and what might be out there, a more careful, competent, and skilled location process will greatly reduce cutoffs.

Today’s test sets can be broken down into two basic categories: 1.) low-frequency locators, and 2.) high-frequency locators. The low frequency locators are clones of the 20C and they do the best job on common grounded cables and conductors, such as direct buried lead-sheathed cables, and primary power cables with concentric neutrals. Where tone distance is a necessity, such as long fiber runs, low frequency is a natural.

High frequency locators, clones of the Fisher M scope, function best in congested areas, such as in subdivisions with common trenched power and telephone. Cables and buried service wires can be identified and separated.

NOTE: Watch out for buried cable TV. Because the conductors are usually buried quite shallow, they will pull a majority of tone and cause a miss-locate. When such a condition is suspected, use low frequency.

The purchase and use of a locator depends on many factors. The specific task of the operator needs to be discussed. Don’t be fooled by price alone. Many inexpensive (not cheap) locators can outperform the big guns in the right situation. There is no need in a non-congested area to have a locator with all the bells and whistles, because, with no other conductors present in the area, all locators work. So, a technician whose primary use of a locator is repairing buried service wires does not need the more expensive test sets with every feature known to man installed. The savings could well be spent on some form of sheath fault locator to pinpoint conductor and shield faults.

Accurate cable location requires not only a well-trained, well-informed, dedicated technician, but also an informed and cooperative manager who provides proper equipment and training, and who will carefully investigate a damaged cable before arbitrarily placing blame. In cable defense, choose your weapons well. With the right machine, a good knowledge of your plant and other underground utilities, and understanding how to locate in the first place, the number of damages can be reduced drastically.

Signing Off
I hope you are off to a great 2009. Help your fellow technicians out this year by sending me your tips and comments, and describing your tough cases of trouble. We’re all in this together, and if we can help each other out, let’s do it! Contact me at dmccarty@mccartyinc.com or call me at 831.818.3930.