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ISE Columnist Don McCarty, OSP Expert

The Challenges Inherent in Single-Pair Faults

Feb. 1, 2021
Taking care of copper cable has never been easy. The older the cable gets, the more companies cut copper budgets, and the harder it gets to resolve cases of trouble.  […]

Taking care of copper cable has never been easy. The older the cable gets, the more companies cut copper budgets, and the harder it gets to resolve cases of trouble. 

Today’s field technicians typically use a variety of test equipment for fault locating in an increasingly complex and adverse environment. These tools provide tremendous advantages but only if the techs are taught how to maximize use of the equipment, and if they are given the time and other support needed from managers. 

Sometimes, in spite of best efforts and good equipment, things go wrong and the job may not be completed to the satisfaction of the customer or the technician. The problem may be the equipment itself, or perhaps the maps provided haven’t been updated, or the technician hasn’t received needed training, or the tech isn’t being allowed enough time to complete the job.

Unfortunately, the outcome of an unsatisfactory result from a tech’s initial effort to fix a problem often means the tech jumps to taking a shortcut, like cut-to-clear or dumping the problem to cable maintenance. 

Food for Thought from Our 2022 ICT Visionaries

Cut-to-clear means an otherwise good pair is now out of commission when it could have been repaired. After enough pairs are deemed unusable (even though they could have been repaired), a new cable may need to be placed — and that is a very expensive and time-consuming proposition. So, although the customer may be happy that they are back in business, the long-term effect is bigger budgets needed to replace cables.

I’ve found that something many techs aren’t aware of, or not accessing, is that most RBOCs dispatch centers use the mechanized loop testing system (MLT) for testing and dispatching field repair technicians. The MLT system has a built-in resistance bridge that can be accessed by the field technician for resistance fault locating.

Resolving Single-Pair Faults 

Let’s dive deeper into solutions and procedures for resolving single-pair faults.

Single-Pair Faults: Problem #1
Pinpointing a fault isn’t a one-step process, and is not necessarily an easy process. Not all techs have advanced training that may be needed to solve the toughest cases. Let’s look at the specific problem of single-pair faults. 

In my experience, better than 70% of single-pair faults (shorts, grounds, crosses, and battery‑crosses) that are not in the CO and not in the CPE or drop, are in a ready access within 2,000 feet of the customer’s network interface, and are usually caused by another technician not doing their job correctly. Rather than identifying the cause of the fault, they cut-to-clear, leaving the problem for the next person to deal with. The rest of the cable is not affected. 

Our point here is that these resistive and open faults can be found by the first person on-site with about the same amount of time and effort as a cut‑to‑clear takes.

Procedure for Testing for Problem #1
• Any resistance bridge needs at least one good wire of the same gauge, temperature, and length for accurate fault locating.
The one-pair hook up is ideal for pre‑localization of a resistance fault, but there are subtle situations that can cause the measurement to be inaccurate.

• Many cable pairs have a certain degree of resistance unbalance between the tip and ring conductors. The circuit will function just fine until 2 events occur which upset the longitudinal balance of the circuit: 1) The resistance unbalance exceeds 5 ohms between the tip and ring; and 2) The power influence is high enough for the customer to hear line‑borne noise.

• Let’s suppose that a faulted circuit is unbalanced and has minimal power influence. Let’s say that Marvin, working on another pair in an associated terminal beyond the subscriber, closed the lid and pinched our pair, causing a ring ground. One wire of this abused pair also has a loose twist in a splice in the underground. This causes series resistance, and Marvin is dispatched to fix the new trouble.

• The trouble pair appears to meet the requirements for a MLT computer location. But we have problems here that both the computer and Marvin don’t know about: the trouble is beyond the subscriber, and the pair is unbalanced (due to the loose twist) on the clean side.

• When a technician shorts the pair and the fault is beyond the subscriber, the strap‑to‑fault (STF) measurement should read 0 feet. That would instantly indicate that the fault is either right here at the strap, or beyond this workout terminal. But this pair has lots of troubles on it and the STF doesn’t read 0. This is where the confusion starts, and techs often want to give up and move to cut-to-clear.

• The pair’s unbalance moves the electrical center of the loop (which should be at the strap) down the longer of the unbalanced wires (in this case the tip). So, a portion of the good wire is seen by the bridge to be a part of the faulted wire.

• An unbalance of 10 ohms moves the electrical center down the tip wire 5 ohms, and makes our trouble to the field look like it’s toward the CO. Let’s say we measure in 24 gauge and set in the proper temperature. The CO computer would show STF footage to be about 200 feet. Moving that distance to a suspect terminal and placing a strap, we find the STF footage is again at 200 feet. Taking another measurement at a third access point would again show 200 feet. As a matter of fact, if the pair was accessible all the way to the CO, the indicated footage would read 200 feet until the technician passed the series resistance. Then the measurement would be 0 feet, indicating an at or beyond reading.

• To circumvent this problem, if the indicated footage is within 500 feet of the workout terminal, or if the reading stays the same when moving to the first suspect terminal, call for a separate good pair and re-measure the fault. If the technician has a portable bridge, Short the pair in the CO and connect the bridge to the faulted conductor at the workout terminal; the distance‑to‑fault measurement will read 0 feet. To find the fault, move the strap to the end of the cable and remeasure the fault; STF will indicate the distance to the terminal where the wire is pinched.

Single-Pair Faults: Problem #2
A second problem with the single pair hookup is trouble on the good wire. Trouble on the good wire will electrically unbalance the circuit as current and crossed battery adds or subtracts from actual loop resistance. This will affect both distance‑to‑fault and strap‑to‑fault bridge measurements.

Procedure for Testing for Problem #2
• The good conductor (or pair) must test good above the 20-megohm range. Many of the standard field meters are incapable of testing that high and will show the pair good. But the interfering resistance it can’t see will cause the resistance bridge to miss‑measure the distance to the fault. In that case, please try the following:

• Test all other vacant cable pairs in your 25-pair compliment. Any cable pair or conductor that tests good can be used for a single pair or conductor hookup. 

• You may have to take another customer out of service for a short time and use their pair for a single or separate good pair hookup. Make sure that customer is aware of what you are doing. 

• If no potential good pair is available. then test other compliments. If you identify a good pair in another compliment, then only a separate good pair test is valid. 

• A single pair hookup will not always work. If at this point no good pair or conductor can be found, then it is time to turn the ticket over to cable maintenance.

These are examples of conditions which may discourage good technicians from making the best choice to fix the single-pair fault instead of cut-to-clear. But please don’t give up on the procedure and learning your equipment to help you be a super tech! This hookup is excellent for pre‑localization for section troubles and finding single pair faults.

The operative word here is pre‑localize. Don’t dig on it unless you’re sure of the measurements. If things are confusing or just don’t add up, use a separate good pair.

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Signing off
If you frequently use cut-to-clear rather than doing the work to keep the pair in service, I challenge you to try the procedure I’ve identified, and let me know how it works. If you are stuck, you even have my invitation to call me and I will walk you through it!
[email protected]

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