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ISE Columnist Don McCarty, OSP Expert

Cable Fault Locating and Repair

June 1, 2020
Part 1. Short-Term and Long-Term Approaches — Are your technicians efficient in quickly finding and repairing copper cable problems? Or are they doing what’s been done before and it seems […]

Part 1. Short-Term and Long-Term Approaches —

Are your technicians efficient in quickly finding and repairing copper cable problems? Or are they doing what’s been done before and it seems to work well enough? Yet, under close examination, if you’ve adopted the "quick fix" approach, you are costing your company more in the long term.

2 Major Problems: Cut-to-Clear, and Divide-and-Conquer

When the root cause of a service interruption is a single pair fault in an aerial terminal or in a buried pedestal, many field techs under management direction, or because they haven’t been trained, move the customer’s service to a good cable pair to restore service: AKA Cut-to-Clear.

Eventually, after all the good pairs have been used because of multiple pairs in trouble, the tech is pressured to fix the trouble rather than cut-to-clear. Yet, even when attempting to fix a problem, technicians often take another bad turn. Unable to depend upon a good test set to quickly locate the problem either because they don’t have one or they haven’t been appropriately trained, the tech adopts the Divide-and-Conquer Method.

Food for Thought from Our 2022 ICT Visionaries

The Divide-and-Conquer Method is time-consuming and expensive. The tech goes halfway along the cable route, opens a terminal, finds the faulted cable pair, cuts it in half, and then tests the cable pair to see which direction the trouble is. Or, even worse, they dig multiple curiosity holes to find the problem in a cable section, and if they cannot find the buried section fault, then they report that a replacement cable is needed. Management is never happy to hear this because of the cost of cable, trucks, and personnel.

Or, You Can Adopt a Proactive Approach

Alternatively, there is a proactive approach: invest in good test sets and excellent training for technicians so that they can quickly find the fault with a good test set, and then they can spend the needed time to fix the fault rather than cut-to-clear, which is faster but a very bad long-term approach. Arming the techs with knowledge and good equipment allows them to save pairs, avoid digging expensive curiosity holes, and save the unnecessary costs of replacing a cable.

Why don’t companies do proactive maintenance? First of all, uninformed managers think that copper is dead not knowing that there are millions of paid for good copper cable pairs earning revenue daily. While nearly everyone involved wants to see fiber everywhere, we are a long way from that happening. However, given the desire to expand fiber installs, budgets for training and equipment are often cut.

Additionally, management often adopts an approach to solving as many cases of trouble in a day as possible — that means cut-to-clear because it does take more time to be proactive. However, in the long term, a proactive approach is much less expensive because you save money on cable replacement.

Also, digging curiosity holes to find a case of trouble is more expensive in time than using good multi-function test sets to find and fix cable pair faults.

The Multi-Function Test Set

Multi-function test sets have many functions for provisioning both narrowband and wideband service over copper cable pairs.

Following is a process for qualifying a cable pair as "good."

When testing a good vacant cable pair, the longitudinal balance should test greater than 60dB.

AC volts should test less than 50VAC.

If you test more than 50VAC, hazardous voltage may be present — use your voltage detector and follow all safety practices.

When testing for DC volts, nothing more than 0VAC is acceptable.

If you restore service on a cable pair with as little as -3VAC tip-to-ground or ring-to-ground, then that pair will go bad again, disrupting service, and most likely bringing in a repeat report.

With no DC voltage present, test for unwanted resistance tip-to-ground, ring-to-ground, and tip-to-ring.

Any resistance more solid than 20 megohms (20 million ohms) should be found and fixed.

When testing with the open meter, tip-to-ground should measure the same distance as ring-to-ground, and the tip-to-ring measurement should be subtly longer than tip- or ring-to-ground, indicating that the cable pair is not split with another cable pair.

The TDR set to the proper velocity of propagation (VOP) should show a flat line with the exception of splices, and show approximately the same distance as the open meter.

There are countless cable pairs that meet the above-listed requirements that are providing quality service for your customers and generating revenue for your company. The root causes of faulted cable pairs that don’t meet these requirements can be proactively found and repaired.

In subsequent columns, I will describe how test set functions work:
The Longitudinal Balance Test
The Digital Multimeter (DMM)
The Resistance Bridge
The Open Meter
The Time Domain Reflectometer (TDR)

Additional field test sets will be covered, such as the cable locater and the earth frame.

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Signing off
Email ([email protected]) or call me (831.818.3930) with questions or comments on the test set or procedures that will help you in fault locating the copper infrastructure. Also, if you have ideas on other topics I can address that will help you do your job, please let me know. Thank you!

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