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What Does It Take to Get a Cable Replaced?

What Does It Take to Get a Cable Replaced?

by: Don McCarty, OSP Magazine

In all of the years that I have been writing for OSP and training field technicians, this is the most-often-asked question from our maintenance forces. This one issue alone costs telephone companies more money than Mother Nature ever could.

When cables are not replaced in a timely manner, two things happen: truck rolls increase because field technicians are working on the effect of the problem and not working on the root cause. And secondly, customer frustration increases because of repeat trouble.

When sections of telephone cable fail, the problem is initially masked by flawed repair procedures. The goal is to get the customer back in service quickly. The most frequently adopted solution to this problem is to transfer the customer to another pair in the cable. I refer to this process as cut-to-clear.

Cut-to-clear is a reactive management decision and it is generally a bad decision. I would agree that it can be a temporary solution if the customer is moved to another circuit while the root cause is pursued, identified, and repaired, and the customer is moved back to the original circuit. The above process could save telcos millions of dollars.

When cutting-to-clear is the typical solution, you eventually run out of pairs and then you have to identify the root cause. Field technicians prove the trouble into a section of aerial or buried cable, and then the fun begins.

A Business Case for Cable Replacement
After identifying the bad section, the field technician approaches his supervisor with the good news: “I have a section of cable that needs to be replaced.” The supervisor says, “What’s wrong with the section?” The technician says, “It’s got water in it.” The supervisor says, “How much?” The technician says, “A lot.”

While it may be true, it is not a business case for cable replacement. The engineer sits in a room with a closed door and shades pulled. Most engineers are not able to make a field visit so they need accurate, quantitative data from the technician if you expect them to authorize cable replacement.

A business case can be built using a resistance bridge, an open meter, a Time Domain Reflectometer (TDR), a cable locator (path and depth), and an earth frame.

To build a business case the first requirement is the actual length of cable in the section. Without this information the test set information can be misinterpreted. If necessary, locate the path of the cable using low frequency and measure the distance with an engineering wheel. Remember to allow for the amount of cable in the pedestals if working a buried section.

This Is a Test
Section faults are caused by:
A. Water in the section
B. Splice or encapsulation failure
C. Sheath damage
D. Phase power faults
E. Lightning faults
F. Any of the above

Engineers want to know the root cause, and will replace the cable under certain conditions. First, they need proof that water is present in the section. Keep in mind that only about 20% of air core cables have water in the section. This information can be provided with the open meter and the TDR.

For example, a 700-foot section of cable with 200 feet of water present would show 1,100 feet of cable. The TDR would show 800 feet of cable because the TDR pulse slows down through the water indicating more cable. The TDR will show where the water starts from each end indicating dry cable. When these two measurements are added together and the total amount of dry cable is then subtracted from the actual section length, the difference is the amount of cable with water present.

Most section faults, around 70%, are splice or encapsulation failures or sheath damage in one spot. Distance to fault or strap to fault resistance measurements will show a cluster of faults in one place in the section, and the exact spot can be pinpointed with a cable locator (path and depth) and an earth frame.

Phase power and lightning faults from 5% to 10% will usually destroy the section. These solid faults, shorts, and crossed cable pairs will occur throughout the section measure to several places in the cable section indicating that the section needs to be replaced.

So, the Answer to the test is:  F. Any of the above.

Anomalies That Skew Measurements
Most failed cable sections are of one gauge but there are circumstances that will affect the test sets and skew the results. These circumstances are: unknown gauge changes, unknown load coils, unknown slack loops, unknown laterals (bridged tap), and water.

These anomalies will show different measurements to the trouble when using the resistance bridge, the open meter, and the TDR, even when the trouble is all in one spot.

Following is an example of a business case for cable replacement:


The water and the load coil skewed the measurements on the test sets. Any distance to fault measurements with the resistance bridge that show from 600 feet to 845 feet are in the load splice. The open meter and TDR are measuring long because of the water and the capacitance of the load coil.

As you can see, the business case report shows the pedestal numbers, the gauge of the cable, the actual section length, the resistance bridge, open meter, and TDR measurements, the amount of water in the section (where water starts from both pedestals), and the location of the load coil.

A quality business case puts the responsibility directly on engineering. Cables that need replaced can be replaced in a timely manner and those sections with wet splices or encapsulations and sheath damage can be repaired.

Signing Off
Building a business case for whatever you need, be it new equipment or cable replacement, is critical, particularly with such tight budgets nowadays. But if you do a good job building the business case, you can demonstrate that you can save money in the long term. I’d like to hear more about your experience in getting what you need to do your job. Email me: