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ISE Columnist Don McCarty, OSP Expert

The unknowns that make fault locating in sections of buried telephone plant difficult.

March 1, 2019
Most bad sections of buried copper plant are a slam dunk to fix, especially when the well-equipped and well-trained technicians are given time to find and repair the root cause […]

Most bad sections of buried copper plant are a slam dunk to fix, especially when the well-equipped and well-trained technicians are given time to find and repair the root cause of section failure. These buried sections are usually in the distribution plant, and they are usually 25-pair or 50-pair PIC cables. They have failed because of sheath damage, splice or encapsulation failure. It is usually fresh trouble if there are no prior complaints from the customer.

First, when trouble-shooting a bad cable pair, the skilled technician using a multifunction test set that includes a resistance fault locator (RFL), an open meter, and a time domain reflectometer (TDR) will take measurements from the CO, cross-connect box, and/or the customer’s terminal or pedestal. These measurements provide an approximate distance to the fault. If the fault is found in a terminal or a pedestal, it is repaired, service is restored, and the technician moves on to the next case of trouble.

If the problem is proven into a buried section, using the same measurements to find the approximate distance to the fault into the section, the technician locates the cable path between access points, and uses the earth frame to pinpoint the fault location. Given time, the field technician digs up the fault, makes a quality repair, and heads on to the next case of trouble. If that sounds easy, it is. We refer to this as the one-hole case of section trouble. There are extenuating circumstances that make some sections of cable more difficult to analyze.

If things are not coming out right, take a step back and do more analysis. First, you will need the above-named features of your multifunction test set, but you will also be using the digital multimeter (DMM) function of your test set. Additionally, you will need a cable locator to find the path and depth of the buried section, an earth frame that will pinpoint the location of the fault, a #2 shovel, and a guest shovel if your company SME wants to run the show. Yes, Greg and Red, I am referring to you.

However, even if all the readings from your test set agree, you must know exactly how much cable is in the section. Using a cable locator, measure the section distance with an engineering wheel. Next, measure the length of the section with the RFL, open meter, and TDR functions of your test set. The test set measurements should agree within about 3% of the actual section distance.

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Following are some examples of faulted cable sections when the test set measurements didn’t agree within 3% of the actual distance:

Example #1
Taking measurements on a good pair in a 1,000-foot section of cable with a multi-function test set, the RFL measurement set to the proper gauge and temperature measured 970 feet. The TDR set to the proper velocity of propagation (VP) measured 975 feet, and the open meter measured 1,205 feet. The extra capacitance was indicative of an unknown lateral (bridged tap). The TDR showed where the lateral was spliced into the cable and the lateral was 200 feet long. The RFL measurement showed that the fault was in the lateral splice. Keep in mind that any fault anywhere on the lateral would have measured to the lateral splice. The lateral splice was repaired, restoring service and eliminating any future unnecessary truck rolls.

Example #2
In a 300-foot section of a 50-pair cable the RFL, TDR, and open meter on a good pair between 2 pedestals each should measure around 300 feet, but in this case the TDR and open meter both measure about 300 feet and the RFL measured 280 feet. This indicates that there is a gauge change in the section. If there is 24-gauge cable in each pedestal, more than likely the cable was cut, and the cut section was replaced with 22-gauge cable. The TDR showed a splice 100 feet from the test set and a splice 150 feet from the test set. RFL measurements on several faulted pairs showed trouble in both splices. Both splices were repaired, and service was restored.

Example #3
In a 700-foot section of 100-pair 26-gauge air-core cable, the RFL set to the proper gauge and temperature on a good pair measured 710 feet, the TDR set to the proper VP measured 690 feet, and the open meter measured 1,400 feet tip- and ring-to-ground. The tip-to-ring measurement showed 1,350 feet. The increase of open meter footage was due to water in the section of cable. Normally when measuring a clean capacitively balanced vacant cable pair with an open meter, the tip-to-ring measurement is longer; however, when water is present in the section tip and ring-to-ground it will measure longer than tip-to-ring. When the bonds were removed on both ends of the section, -18VDC was measured shield-to-ground. This confirmed that there was water in the section. The TDR showed that the water started 200 feet from one end and 300 feet from the other end, indicating 200 feet of water in the section. This section of cable needed to be replaced.

Working with the field technician, we put together a business case to get the cable replaced. The RFL, open meter, and TDR measurements, were recorded on more than 30 faulted cable pairs in the section. Distance-to-fault measurements were anywhere from 200 feet to 400 feet from the test set. The measurements were recorded including a TDR picture. When engineering looked at the business case for replacement they rejected it. They stated that the cable was up for replacement with fiber in 1 year.

I do not have an answer to this. I do know, through experience, that over the years thousands of buried sections of telephone cables turned in for replacement could have been repaired within a short period of time by digging 1 or 2 holes and fixing the root cause. Instead, thousands of cables that should have been replaced were not. Many, many customers forced to wait too long for quality service they paid for each month gave up and moved to alternate vendors. Additionally, millions of dollars have been lost on expensive unnecessary truck rolls working on the effects of section troubles rather than working on the root cause.

A Reader Writes
Hi Don,
Your columns are always my favorite in ISE magazine.

One thought you expressed in your January 2019 article (A Hitch in the IoT) especially rang true for me.

Re: The ever-dwindling number of air pressure technicians being pulled off their job and put into the load. So true, and so short-sighted of the decision makers. Once a cable gets wet, the amount of time, effort, and expense, to restore service(s) is inexcusable. Not to mention downtime for the customer.

Keep ‘em coming. You’re the voice of reason out there.


Signing off
Sadly, as a technician, you often have your hands tied when trying to do the right thing and instead end up doing a less than optimal job. I do know from working within all the states and many other countries that many of you do work for companies that truly care and have a long-term vision of business and creating the best customer relations. Please let me know what prevents you from doing your job as you want to do it. I won’t use your names or companies’ names, but it is helpful for other technicians to learn from you! Reach out to me at [email protected] or 831.818.3930.

About the Author

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 [email protected] or visit