Whats behind this wall? Locating your cabling

Jan. 1, 1995
Whether the cable you are looking for is copper or fiber, indoor or outside-plant, there are technologies and techniques that can ease this routine but necessary task.

Whether the cable you are looking for is copper or fiber, indoor or outside-plant, there are technologies and techniques that can ease this routine but necessary task.

Arlyn S. Powell, Jr.

What could be more frustrating than knowing the cable you are looking for is behind a wall--or in the ceiling or floor--and yet still being unable to find it? Cable locating is a routine and yet necessary task that all cabling installers must regularly perform. Sometimes the cable is copper, sometimes fiber-optic. Usually it is located indoors, but it may run outside as well in a campus installation.

If premises distribution systems were installed one cable at a time end-to-end, there would probably not be much need for cable locating techniques and technology. You could pull the cable, terminate it, connect it, test it, label it and then mark it on your as-built drawings. However, it is more efficient for most installers to perform these tasks separately, so the crew that pulls the cable may not be the one that has to identify, verify, test and label it at the end of the job. This fact of the installer`s life means that circuit-tracing equipment is used on most jobs.

Two scenarios for cable location

Indoor copper-cable locators are used most often in two situations--when an installer is verifying, labeling and testing a new installation and when a customer asks that an existing cable plant be traced. "We use them for a new installation if we want to verify that our labeling is correct," says Mike Fecteau, vice president and co-owner of Dinsmore Communications (Portsmouth, NH). "We test almost every installation that we do now, whether the customer requires it or not. We check all the cables before we leave to make sure they are properly installed and identified."

Fecteau adds that, even though technicians mark cable on the spool, and label it as it is cut from the spool, mistakes can still occur. "Labels fall off," he says, "or spools get mixed up. So if you end up with two or three cables you can`t identify, you use a tone generator to trace them."

It may also happen that another contractor--the electrician, for example--prewires a new building. "They may leave you a bundle of unlabeled wires," Fecteau says, "or they may have labeled one end but not the other. In that case, you can put the alligator clips of the transmitter on the 1,2 pair of one end of a wire and wave the receiver over the bundle at the other end. As the receiver gets closer to the transmitting pair, the signal gets stronger. When you touch the proper wire, the receiver gives a long beep, so you have a positive identification."

Rehabilitating an existing cable plant

In the second installation scenario, rehabilitating an existing cable plant, Sam Flaherty, president of Teledata Systems Corp. (Souderton, PA), recommends the tone generator and inductive amplifier as well. "The best cable locator, in my opinion, is the inductive amplifier," he says. "You put a tone generator on the cable and trace it with the amplifier. That works as well as anything for inside cable pulling." Flaherty`s company has used more sophisticated devices, but, he says, they turned out to be "not very useful."

"We recently did a project in Philadelphia," Flaherty says, "where we had to identify more than 200 coaxial cables. Nothing was labeled. It was a nightmare." Teledata technicians found the cables coming through holes in the wall and knotted together in a very small space. "It was unmanageable," he adds. "Each time the customer tried to relocate a terminal, it would take half a day just to find the right cable."

Teledata came in on a weekend and simply cut all the cables, trimming them to the proper length so that the tangled spaghetti of cables behind the controllers would not reappear. The cables were then reterminated and plugged into patch panels. The refurbished cables were identified by connecting a tone generator to each workstation jack and then locating the other end in the computer room.

"We used three different kinds of locators to help us with this," Flaherty states. "The best one was the simple old $30 tone generator and $60 inductive amplifier. We just grabbed a bunch of 30 or 40 cables, stuck the nose of the inductive amplifier into it, and saw if we got any tone. If we did, we just kept weeding out the cables until we got to the one that we thought was toned. If we got a signal on the outside of the jacket, we went to the end of the cable and touched the conductor. The tone would come out really loud if it was the right one. The procedure was very quick and effective."

Other locators were less effective because they lacked a metal probe with which to make contact with the conductor. By putting the device close to the cable, an audible signal could sometimes be detected, but it was unclear which cable in the bundle was carrying the tone, because it could bleed over into adjacent cables.

Enhancements are available

Manufacturers offer many enhancements to the basic (and inexpensive) tone generator and inductive amplifier. With voice and audio circuits, for example, a synthesized voice-based audio signal generator can substitute for a technician talking at the other end of a cable during circuit identification. Butt-in test sets are now available that let technicians talk on voice lines without disturbing data traffic. Another location device is the digital carrier detector, which can identify different voice/data circuits, such as T1 and integrated services digital network.

Cable tracing for optical fibers involves a fiber identifier, a device that locates a light signal without breaking the fiber or interrupting traffic. This is accomplished by introducing a bend (less than the fiber`s minimum bend radius) and identifying the light signal that bleeds through the cladding at that point. End-to-end fiber identification can be accomplished using another fiber-optic testing tool, the visual fault locator.

Outside-plant cabling is a separate field (see "Before You Dig--New Technologies Simplify Campus Cabling," June 1994, page 19), with its own techniques and technologies. Suffice it to say here that cable-tracing principles are similar. The most common approach is to inject an electrical signal into a cable, conduit or pipe and then trace the route of the cable on the surface using an inductive amplifier. Most signal generators must be connected to the transmission medium, but some can actually induce a signal from aboveground. These outside-plant devices, some of which can be used as indoor cable locators as well, are distinguished by operating range and safety, durability and ease-of-use features.

Perhaps the best method of cable location, though, is careful up-front planning and thorough documentation. This can lessen the need for toning or some other location technique later in the job. For example, project manager Sam Parthemer of Datalink Solutions (San Diego, CA), says, "We try to minimize the toning. Some vendors will pull wire, punch down and then tone back to the workstation, labeling the cables in whatever sequence they happen to fall in. This leads to a scrambled numbering sequence. Line number 1 may be in this office, number 54 next door, and number 13 next door to that.

"We don`t do it that way," Parthemer continues. "We prenumber our blueprint, so the cable number can be the same as the office number on the architect`s plan. Then we label every cable that we pull, at both ends, while we`re pulling it. If something unforeseen happens--let`s say we lose a label--we know that location 75 is going to be next door to location 74. It`s going to be easy to find. That helps a lot."

There are many labeling schemes that can assist the installer in making sure a cable is routed correctly (see, "Marking Cables to Save Time and Money," December 1994, page 42). Voice lines can be labeled with a `V` suffix or prefix, and data lines can be labeled `D.` Cables are often color-coded as well; this coding can substitute for some labeling--for instance, if Category 3 cables are one color and Category 5 another. When planning a color-coding scheme, make sure to adhere to the guidelines in the telecommunications administration standard, ANSI/EIA/TIA-606.

By using a rational labeling and numbering system, Datalink Solutions saves time and money by toning only where there is confusing or missing information. "Some other companies try to get the numbers straightened out when they are punching down," Parthemer adds. "When you tone at this point, there`s a very good chance you`ll get ring-over from another cable. If you have four cables with equal tone, what do you do? To figure out which of the four is the correct cable, you can short at one end or run a continuity check, or you can just go for it and say, `This one must be it.` Eventually you`ll make a mistake and pick the wrong one."

To avoid problems later with the customer, Datalink Solutions insists on getting it right before the crews leave the job site. "If we miswire," Parthemer says, "if we pull to the wrong location--if, for instance, 78 voice and 79 data go to one office, and 79 voice and 78 data to the next--we switch the cables, so it`s done correctly. Even if we have to pull the cables again at our own expense, we do that just to make sure the customer gets the package we offered."

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The basic device for copper-wire cable location is the tone generator and inductive amplifier. TeleTronics (Sheboygan, WI) recently introduced its TG1 tone generator and AM1 inductive amplifier.

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The CT-1 Cabletalker from Bicom (Clearwater, FL) provides a synthesized voice signal at one end of a voice or audio line, so that a second technician is not necessary for cable location.

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Portable cable locators, such as these units from Rycom Instruments (Raytown, MO), are used in outside-plant work to find cables, conduits and pipes. Some may also be used indoors.

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