Cable Locating: Beyond The Basics

ColumnistCopper ExpertCore/LegacyCurrent IssueDSL/G.Fast/Vectoring
ISE Columnist Don McCarty

In earlier columns I have discussed how to get started with your cable locator with tips about the types of hookups, tone transmission, etc. This month I discuss some of the problems that occur when you’re actually out there, waving that locator over what you dearly hope is the cable path. 

But confusion and chaos can lurk below ground: foreign conductors, unknown laterals, slack loops, out-of-phase signals, stolen tone, and so on. It is these things — and others — that can mislead a locate. 

HERE’S THE WAY IT SHOULD WORK

STEP 1. With the transmitter properly set up, peruse the suspected path of the cable in PEAK, NULL, and DIFFERENTIAL (arrows) modes. 

STEP 2. Note any differences in the path indicated by all 3 modes. Any change in gain, volume, symmetry, or path, may indicate other conductive entities in the area. 

STEP 3.If all modes indicate the cable in the same place — and you are sure there are no more utilities in the area — mark the cable path. Pack up, and head for coffee. 

BUT ARE YOU REALLY SURE?

There is really only one way to be absolutely confident of a cable locate — and that is to be there when they buried the plant. Since that’s unlikely, let’s look at other resources, procedures, and theories, that can boost your confidence and allow you to sleep better at night. 

Cable Maps

First and foremost, access to good cable maps is a must. Sadly, by and large cable maps are not totally accurate or up-to-date. Nevertheless, engineering job orders indicate any new or pending construction, and other utilities supply information and assistance as to the placement of their plant. Smaller drops, service wires and CATV drops may not be marked on maps — and with them you’re on your own. 

But first, look around a locate site. 

  • Knowledge of construction methods by all utilities indicate to the technician possible unmarked plant. 
  • Look for CATV pedestals, water meters, handholes, and hydrant sites. 
  • Try to determine the direction of construction: if a slack loop was left in a run, 99% of the time it is left in front of the pedestal at the far-end of a section. 
  • A butt-splice can be anywhere.

Signal Radiation

Radiated signal strength decreases on both sides as the distance from the cable increases. This is a precise mathematical phenomenon called the square-law of relative radiated signal, and can best be explained using light as an example. 

Light is radiated at a very high frequency. When a photographer uses a meter to measure its intensity, he is measuring how much light is being radiated at a given distance from its source. 

  • If the photographer measures intensity at 2 feet from the source, and again 4 feet, the intensity at 4 feet is not ½ the 2-foot level, it is ¼. 
  • If the photographer measures intensity at 6 feet, it is 1/16th of the 2-foot level. 

The same law applies to radiated tone from a telephone cable in the ground. As the distance from the source increases, the intensity decreases by this mathematical square. That’s why signal decreases rapidly — and predictably — as the receiver moves to the side of the cable. Under uncomplicated, prefect conditions, we find a signal — in both NULL and PEAK — which is exactly symmetrical over and on both sides of the cable. Such conditions are rare, indeed. 

WITH THIS IN MIND, LET’S LOOK AT SOME REAL-LIFE PROBLEMS 

Because all locators use an AC tone that can (and will) induce signal to any other conductive entities in the area, any of which can mislead the locate, the technician must pay intelligent attention to the following factors:

Relative Depth (High Frequency) 

If there are other conductors which are shallower than the conductor you are attempting to locate, the electrical field may pull the locate indication off exact track. For example, a CATV drop at 2 inches below the grass will show more signal than a cable 30 inches deep — even if the cable at 30 inches has 100 times more signal. The tone induced into the CATV is shown strongest by the receiver, and you’ll be off following that sad path, causing a mis-locate. 

When you know you’re locating telephone or power cables in the vicinity of a CATV drop, beware. Most CATV drops and cables are buried 12 inches or even shallower. The Telcos and power companies are made at risk by this practice, but must understand that such shallow depths are most often the only part of an easement available to CATVs at a reasonable construction cost, as everything else reasonable is used by the power and telephone companies. Any attempt to locate
a deeper conductor in the vicinity of a CATV drop can be thwarted by the problem of relative depth.

REMEMBER:
Low frequency is for accuracy, high frequency is for convenience.   

Lower frequencies have less of a tendency to be re-radiated by other conductors. If you have a CATV cable running parallel to your locate, send tone on a conductor grounded at the far end with an independent ground rod (such as a screw driver) placed opposite the CATV cable. Make sure the transmitter ground is also opposite the CATV cable path. This minimizes tone transfer to the CATV cable. 

High frequency AC current always takes the path of least resistance, and is unencumbered by the need for a direct connection. High frequency AC current can “jump the gap” between conductors through insulation. 

A properly tuned receiver can indicate this jump by indicating relative signal strength in the area, which results in an asymmetrical signal reception (where one side of the cable path shows a different amplitude than the other). 

This problem of current transfer without a direct metallic path becomes apparent with the next 2 problems.

Relative Conductivity (High Frequency) 

Some underground conductors have the ability to conduct electricity better than others. A CATV cable with no ground for a distance conducts better than a primary power cable with a concentric neutral grounded the length of the conductor. Plastic-sheathed telephone cables conduct better than grounded lead-sheathed cables. Tone jumps from a common-grounded lead cable to just about any other cable or conductive entity in the area. Current takes the path of least resistance. And this path is enhanced by:

Proximity (High Frequency)

Any conductor in the vicinity of the toned conductor, which offers a better path for current flow, will “steal” the transmitted tone. A well-grounded lateral steals the tone from the main cable path. Gas pipes, CATV conductors, drops or power cables in the area that offer better conductivity, are threats to a good locate.

Phase Shift (High Frequency) 

Canceled signal occurs when signal from a toned conductor returns on another — and parallel — conductor from the opposite direction. The current flowing in a clockwise direction down a conductor is canceled by the returning counterclockwise current. Current going positive in one direction is canceled by current going negative in the opposite direction. Therefore, no current flows, and no locator tone can appear on the section.

Let’s look at a situation where a buried cable is trenched laterally to a pedestal 15 to 20 feet off the cable path, and the cable is returned to the main path in that same lateral trench. Tone enters the path to the pedestal in one phase on the entering cable and returns from the pedestal on the parallel cable in the opposite phase. Therefore, the locate is impossible because they self-cancel. 

Because of phase shift, never use a coupler to put tone on a conduit run. The coupler sends tone on one cable to the bonding ribbon in the next manhole and returns the tone in the opposite phase on all other cables. This creates a canceling effect that can throw the locate off by many feet. Symptoms of this on the receiver are a one-sided NULL where the tone drops off and never comes up again. This one-shouldered NULL indicates a good possibility of mis-marking the conduit run. 

A second symptom shows up in peak. A rise in tone occurs on one side of the run, but the tone never properly drops off; tone just fades away. Don’t use the coupler in conduit. I am aware that some cable locate vendors have even made an extension handle to reach a coupler into a manhole without entering. But in this practice lies the potential for major disaster.

With a high-frequency locator, the best method for transmitting tone into a conduit run is to set the box on top of the cables in the manhole. This assures that tracing tone goes in one direction down the run, and keeps the receiver on the conduit path. 

When using a low frequency locator, use a direct connection to the bonding ribbon. Do not disconnect the bonding ribbon. If the ribbon is disconnected, the same misleading phase shift occurs as with the coupler. 

Using The Receiver

To begin, follow the suspected path of the conductor in NULL mode. Adjust the receiver for a 12- to 18-inch quiet zone in the center of the locate. Carefully listen to monitor the amplitude of the null lobes, and use the receiver meter needle (if one is available). Any shift — or difference — in amplitude on one side or the other indicates a change in current flow and the presence of some anomaly that should be resolved. 

Here Are Examples Of Conditions Which Affect NULL.

  • The cable changes direction. When the cable turns, the amplitude of signal opposite the turn is less than in the direction of the turn. When the turn is passed, the tone drops off completely. 
  • A lateral leaves the cable path. When approaching the point where the lateral breaks off, tone rises in amplitude on one side and decreases on the opposite side within 10 to 15 feet of the lateral. 
  • Another conductor is in the vicinity of the toned conductor. This conductor is sympathetic to the induced tone from the toned conductor. NULL shifts as the toned conductor induces signal into the dead conductor. The shoulders change in amplitude. 

These signal shifts are subtle, and a good ear is necessary as well as a good receiver. This non-symmetrical signal is a warning. Be cautious. You may miss-mark the conductor. 

MORE INSTRUCTIONS TO HELP YOU WHEN THIS HAPPENS

  • Use PEAK to recheck the locate. PEAK indicates maximum signal to the operator. DIFFERENTIAL always falls somewhere between NULL and PEAK on a non-symmetrical signal.
  • When the cable changes direction, back up to symmetrical signal and set a reference. Mark the tone change and circle the area of change in a 15-foot radius. Bump the gain a bit when you step off the cable path. Reduce the gain as tone is encountered (don’t overdrive the receiver). Maximum tone in PEAK finds the cable and indicates the new direction. 
  • Where a lateral leaves the cable path, mark the spot where the tone shift is noted in NULL OR DIFFERENTIAL. Back up to symmetrical signal and set a reference in PEAK. Circle the area of change in a 15-foot radius. Bump the gain a bit when you step off the cable path. Reduce the gain as tone is encountered. Maximum tone in PEAK indicates the direction of the lateral. Circle the area a complete 360 degrees. A second time around indicates relative signal strength on all cables radiating from the splice.
  • Again switch to PEAK if another conductor in the area is suspected by a tone shift in NULL or DIFFERENTIAL. If PEAK indicates the conductor path different from NULL, PEAK is the more accurate — most times. 

A constant adjustment of the receiver gain control is necessary when using PEAK. Tone changes when depth changes, soil consistency and conductivity changes, or when there is a phase shift. Tone decreases when other conductors steal tone. These noted changes can give the operator a fairly accurate picture of the underground conductors. 

REMEMBER: when in doubt, expose the conductor carefully by hand-digging. as the conductor is excavated, move the receiver closer, and the inaccuracy decreases. 

SIGNING OFF
Cable location is an art, not an exact science. In very congested areas, accurate location is difficult, if not impossible. Knowing when your locator is being pulled awry — giving an inaccurate locate — is one of the best means of avoiding cable damage. This decreases what we call the pucker factor considerably, and just may allow you to sleep tonight.

I hope my input is helpful. Let me know! Email dmccarty@mccartyinc.com or text or call me at 831.818.3930.

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About Don McCarty

Don McCarty is the OSP EXPERT columnist for ISE magazine, discussing the issues around provisioning, testing, and maintaining copper for all services from POTs to IPTV. Don is also president of and the lead trainer for McCarty Products, a technical training and products company training field technicians, cable maintenance, installation repair, and Central Office technicians and managers. For more information, email dmccarty@mccartyinc.com or visit www.mccartyinc.com.