Cable-tracing devices can help find that missing link

Although various types of cable-tracing devices are available, this product update focuses on the basic devices used for locating copper cables inside a building: the tone generator and probe (see Product Update table, page 26). Installers routinely use copper-cable-tracing devices to verify, label and test a new installation or to trace existing cabling during renovation or maintenance projects. For example, when you are at an installation where someone has left a bundle of unlabeled wires, or

Aug 1st, 1996
Th Cim62644 28

Barbara E. Thompson

Although various types of cable-tracing devices are available, this product update focuses on the basic devices used for locating copper cables inside a building: the tone generator and probe (see Product Update table, page 26). Installers routinely use copper-cable-tracing devices to verify, label and test a new installation or to trace existing cabling during renovation or maintenance projects. For example, when you are at an installation where someone has left a bundle of unlabeled wires, or has labeled only one end of a cable, you can use these products to identify one wire pair within a bundle of wires.

What`s in a name?

In the telecommunications industry, manufacturers and end-users use different--and sometimes confusing--names to identify products. According to Bruce Bond, marketing director at Chesilvale Electronics Ltd. Corp., "In the vernacular, these devices are known as tones and probes."

However, Dave Long, data communications market manager at Ideal Industries Inc., suggests the term "cable-tracing devices" to describe the product. "Some products known as cable locators are used by plumbers, electricians and data communications installers to find conduit and pipe as well as cable," he explains.

The basic cable-tracing device consists of two parts: the tone generator and the probe. The tone generator (transmitter) sends a signal down the cable, and the probe (receiver) picks up and amplifies the signal at the other end of the cable.

Underground cable locators for outside-plant projects use somewhat different technologies, but the technique is similar (see "Underground Cable Locators," page 22).]

Transmitting the signal

The tone generator emits either square-wave or sine-wave signals. "Sine-wave [tone generators] are much better, because they are more accurate and don`t generate or pick up extraneous signals," explains Bond. "However, they are also more expensive."

Other features to look for in a tone generator are the number and type of signals it produces. In older technology, only one constant tone was used, making it difficult to distinguish from other noises. Most devices today generate at least two tones.

"The Bell Standard requires two tones," says Bob Gill, marketing manager at Progressive Electronics Inc. "One is an alternating tone, known as a warble, which is a high 900- and 1100-Hz signal. The other is the constant tone, which is used infrequently because it`s difficult to locate."

A warble tone is good to use if the cable is in a manufacturing area with considerable machinery noise, because you can hear it above the hum of other electrical noises. Some devices provide more than two different tones that can be used simultaneously to trace more than one cable (signal) at a time.

Probes receive the signal

Once the transmitter is attached to one end of the cable you are tracing, you can use a probe to detect the audio or visual signal at the other end. As you wave the probe over a bundle of cables, the closer you get to the signal you want to find, the stronger the signal becomes.

"Most probes use an inductive pickup," says Bond. "And, because the probe picks up and amplifies an audio signal, it`s sometimes known as an inductive amplifier-- but it`s still a probe."

Probes are manufactured with metal or plastic-coated tips and do not necessarily have to be in metallic contact with the conductor to pick up the tone signal.

"Traditional metal probes make it easier to pick up the signal," says Gill. "However, a technician looking for a tone may run the tip down a block in the wiring closet and create a short or disrupt the circuits. We have developed a fiber-carbon-powder plastic tip that is conductive enough for the tone signal to be picked up well, and resistive enough that it doesn`t short 66 blocks."

Linda Hathorn, product manager at Harris Corp./Dracon Division, agrees that "if you`re around data lines, you may want to choose a more expensive, nonmetallic tip."

In some units, a light-emitting diode display provides the visual signal, which can also be used to indicate continuity, polarity and voltage.

Sensitivity (or volume) control is another helpful feature on some probes. "For example, when you have bleed-over noise from your tone signal onto another wire-pair, you can turn down the volume," says Gill. "This reduces the sensitivity of the probe and makes it easier to find the conductor."

Battery type and operating life, warranty, and price are other issues that you may wish to consider when purchasing these products. These devices typically use a 9V battery with an operating life of 8 to 500 hours, and warranties are for one year--although one manufacturer offers a lifetime warranty. Prices vary from $49 to $900, depending on features.

You may want to consider other features as well, depending on your application; for example, terminals for headsets or butt-in sets, and filtration for AC noise.

In addition to powering the cable-tracing devices, battery power can be used to establish communications between two technicians in different telecommunications closets. Some probe instruments have terminals to clip on a butt-in test set, which are practical if you are installing cable in an office building and do not want to disturb those around you with the constant "warbling." You can attach the butt-set to the terminal clips and listen for the tone through the receiver.

AC power "noise"

Another important issue in the telecommunications industry is the presence of AC power "noise," which has a tendency to jump over onto communications pairs. When this happens, the inductive amplifier will pick up the noise and make it difficult to find the tone on your cable. "To help eliminate this AC noise problem, we have added filtration to one of our probes," says Gill.

Durability--such as sealed electronics and water-resistance, and whether the product is dust-proof or shock-proof--may also be an important feature to consider. "The worst thing you can do is drop the product and break it. One of our tone probes can take a drop from approximately six feet," says Hathorn.

After so many choices, followed by an exhaustive search for that elusive cable, you will probably be glad that you use cable labeling and installation practices according to the EIA/TIA-606 administration standard. As a wise installer once told me, "Careful planning up front saves time at the end of the day."q

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Installer uses a Harris Corp. probe with an insulated tip for safety around data lines.

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