Tec-Alert Newsletter

TDR 101 or Using Cool Technology to Find Cut Cable Behind Drywall

Someone cut my cable and now the drywall is up! How in the world am I gonna find it?

Sound familiar? If you have installed cable for more than a week, it should. Cut or damaged cables on worksites are not the exception, they're the norm. In Arizona, a recent article pointed out that the workforce in homes has never been more unqualified due to red-hot housing demand. At the same time, there have never been more homeowner complaints. Why? Cause the guy who followed you in didn't know what he was doing and decided his pipe was more important than your wire.

Ok, so we aren't going to change the situation overnight. But what do you do when the cable is cut or a nail has shorted conductors (and the answer isn't shoot the other guy)? Pull out your TDR and measure the distance in feet to the fault, wheel out the distance, then fix it. That wasn't your answer? What is a TDR? Keep reading.

TDR's have been around for about 40 years, mostly with CATV and telcos. Unfortunately, they cost 2 arms and a leg, anywhere from $1000 to $5000. Not your typical contractor tool. Ah, but isn't technology wonderful, as the recent cost to own a simple handheld TDR has dropped to less than $300 in the last couple of years. Suddenly you and your brother can have one.

TDR's work on a simple principal; send out a pulse of energy, wait for an impedence mismatch to reflect it back to the tester, then measure the time it took to do so. Then convert the time to a distance measurement. Pretty simple and the tester does all the calculating for you. You just have to be able to read a number.

The actual practical application would go something like this. You go to a job site and discover you have a shorted cable somewhere. You pull out your trusty TDR and hook an alligator clip to each of the shorted conductors. In this case, the wiring is the familiar 24 gauge security wire you constantly use. You turn on the tester and go to the set up screen. You select the 24 gauge wire from the menu, push test, and voila, the number 25 comes up, indicating that 25 feet plus or minus the error percentage of the tester in use is where your short is.

Ah, but you say, I still own several doo-hickies that the last guy told me would cure all my ills and was easier to use than Kleenex. Well, simple TDRs are easy to use as long as you understand the principles and the limitations. And (drum roll please) here they are.

1. First and foremost, the TDR has to be calibrated for the cable to be tested.
Every cable type acts differently and is rated for NVP (nominal velocity of propagation). This is a comparison of the cables speed of reflection versus the speed of light. So an NVP of 65 means that this cable transmit's the signal of the TDR at 65% the speed of light. It is important to know this and calibrate the tester accordingly or the time measurement for the reflection will not relate correctly to the distance (uh, it won't work right). So always be sure to set the NVP up for the cable to be tested.

Several manufactures provide internal cable NVP libraries in their tester. In addition, Psiber's Cabletool stores custom cable NVPs by the user that may not be included in this library. Nice feature!

2. Get a good, solid connection.
Weak or wimpy connections cause misreads.

3. Understand that, while TDRs see both opens (cut) and shorted cables, not all shorts are solid enough to be seen.
Remember, we are looking for impedence mismatches. High resistance shorts may not provide enough mismatch to be seen by a tester. My personal rule of thumb is 100k ohms or more, forget it, even for the $5000 testers.

4. Beware the TDR with a dead zone at the near end.
Some TDRs don't see a mismatch until after several feet. Heaven help you if the fault lies within this range. Look for a TDR without a minimum range and you won't have this problem.

5. NVP variance can cause errors.
The very best and most accurate way to get a good calibration is to actually calibrate the TDR with a known length of the cable you are testing. Then you aren't relying on a library of values that have to relate to cables that differ in NVP from manufacturer to manufacturer. Then store that reading into the tester. The next time you use it with that cable type, you won't have to manually calibrate it.

Last thing. You can also use TDRs to measure how much cable you have left on a spool. In this situation, it acts like a cut cable (open). It's kind of a neat feature.

And of course, the CableTool TDR from Tecra Tools has all the features we recommend above (although it won't slice bread or wash your windows).

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