Resistance Bridge Fault Locating

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or the fallacy of single-pair hookups —

Over my many years in the business, I have written and taught about resistance fault locating (RFL) using the single-pair hookup to find a ground or crossed battery on one side of a cable pair when the other side tests clear. Regular column readers and my past students know that I’m adamant about this topic because without a good process, in-depth knowledge, quality tools, and management support allowing the needed time to implement the correct repair, technicians will cut-to-clear rather than locating and repairing the problem. This approach, while easy and fast to implement, is a very expensive solution in the long run, eventually requiring that cables be replaced.

Since my last column on this topic, several quality field techs have queried me on their failure to get accurate measurements to the fault. Given that there have been changes to this process since my last column on this topic several years ago, we’re going to take a look today at creating a flow that will help you.

Understanding single-pair RFL hookups
The most efficient method of fault locating using RFL on your multi-function is the single-pair hookup when chasing terminating grounds or crossed-battery on a cable pair. However, I get a lot of moans and groans when I make that statement. The reason for the complaining? The technicians tell me that single-pair hookups often don’t work, and then they feel they’ve wasted valuable time in addition to creating a lot of frustration.

I’m hoping to change your mind by explaining why it doesn’t work all the time, and how to remedy the situation. If you practice single-pair hookups when trouble-shooting single-pair terminating faults you will be more efficient, and your boss will be much happier with the end product because his budgets will decrease dramatically, and long-term engineering and maintenance will improve with less cut-to-clear by moving the customer’s circuit to another cable pair.

Why some single-pair RFL hookups don’t work
It’s ultimately easier to measure the distance to a resistance fault or an open, and fix that pair to restore service. Otherwise, you’ll be coming back to the same cable again and again. You should be able to run it down the first time, but if a technician has tried this approach a few times and not gotten good results, he gets gun shy and wants to cut-to-clear.

Before you do that, please consider that most of the time, the poor results are not an equipment failure which is what many techs believe. While older equipment can be troublesome, today’s equipment is quite reliable. If you keep coming up with a “mis-location” reading, the problem is more likely due to 1 of 2 reasons:

Reason #1. The cable pairs you are testing are not resistively balanced.

Reason #2. There is undiscovered trouble on what you believe is a good pair.

Let’s look further at the issue of mis-locations.

Cable pairs not resistively balanced
Any resistance bridge needs at least 1 good wire of the same gauge, temperature, and length, for accurate fault locating. The one-pair hookup is ideal for pre‑localization of a resistance fault, but there are subtle situations that can cause the measurement to be inaccurate. This can discourage the average user.

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:

Event #1. The resistance unbalance exceeds 5 ohms between the tip and ring.

Event #2. There is trouble on the good conductor.

For example, suppose that a faulted circuit is unbalanced and has tested and identified a ring ground on the pair and the tip side tests clear. But this damaged pair also has a series resistance on the tip side in a splice in the underground.

The trouble pair appears to meet the requirements for computer location. But the technician doesn’t know about the series resistance on the clean side.

When the tech shorts the pair at one end, let’s say at the customer’s terminal, and runs a single-pair RFL from the cross-box is where the tech gets bushwhacked.

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 or remote. Let’s say we measure in 24 gauge and set in the proper temperature. When Measuring from the cross-box, the test set RFL would show the STF footage 200 feet farther than it really is. If the ring ground was actually 500 feet from the strap, then the test set would show STF 700 feet.

To circumvent this problem, strap the pair at the cross box and measure again from the end user’s terminal. If the distance to fault (DTF) decreases to 500 feet, as in our example, then that DTF of 500 feet is accurate.

The “good” conductor isn’t good
A second problem with the single-pair hookup is trouble on the good wire. Trouble on the good wire electrically unbalances the circuit as current and crossed battery adds or subtracts from actual loop resistance. This affects both distance‑to‑fault and strap‑to‑fault bridge measurements.

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 causes the resistance bridge to miss‑measure the distance to the fault. The only solution is to strap the faulted conductor to a tested good pair, and use a separate good pair hookup.

In summary
I’ve provided you with examples of conditions that may prevent you from being successful with single-pair fault locating and may discourage good technicians from using fine test equipment. But don’t give up on the single-pair attachment altogether. Over 90% of single-pair faults (shorts, grounds, crosses, and battery‑crosses) are most often in a ready-access terminal or pedestal within 2,000 feet of the end user’s terminal. The rest of the cable is not affected.

Our point here is that these resistive and open faults can be found by the first man on-site with about the same amount of time and effort as a cut‑to‑clear takes. But cut-to-clear is a bad long-term decision in most cases and can be avoided.

This single-pair hookup is excellent for pre‑localization for section troubles and finding single-pair faults in pedestals and terminals. The operative word here is pre‑localize. Don’t dig on section troubles such as sheath damage or water in a splice in a buried section of cable unless you’re sure of the measurements. If things are confused or just don’t add up, use a separate good pair.

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Signing Off
About 20+ years ago, while the hope and promise of fiber was seen as a great solution that would happen soon, I remember feeling a bit sad that my expertise and many years of training would one day no longer be needed. Now, all those years later, while there is a great deal of fiber laid, copper is still in play, and it is now evident that it will be around for a long time to come. I hope your company supports your role as a copper technician providing quality training and tools and, most importantly, allowing you the time to implement the best process, even if it takes more time initially. Smart companies realize the long-term health of copper is critical. If you have other topics you’d like me to address, please text, call 831.818.3930, or email dmccarty@mccartyinc.com.

 

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About Author

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.

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