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The Basics of (TDRs) Time Domain Reflectometers

Time Domain Reflectometers (TDRs) have been around for several decades providing the technology to quickly find and locate shorts or cuts in cable. Until recently, TDRs have been complicated, bulky and expensive to operate. This has limited their use somewhat to larger jobs. As technology has advanced, TDR instruments have finally been simplified and have become affordable for small to medium sized contractors
and even indivudual techs. With the speed and accuracy these tools provide, the TDR has become the most preferred method of cable fault location in a wide array of industries.

The TDR uses the same principle as radar but instead of using air, they use wires. With this unique simple application, technicians are able to send a pulse of energy down a cable, the transmission reflects back after it comes to the cable end or has reached a wiring fault such as an open or short circuit and finally calculates the time it took to reach one of the above converting the time into distance. This is the calculation you the technician are in search of to pin point the exact location of the fault, so if excavation is necessary, you hit it on the first try. This saves time, money and headaches.

Understanding the principle of the TDR application will help you feel more confident in the results you find using your TDR instrument. How does it work? Cable impedance is formed any time two metallic conductors are placed close together. Each connector has an insulating material that keeps the conductors separated called the cable dielectric. The TDR looks for any change in the dielectric charge meaning there is a mismatch caused from cable damage, water ingress, sheath damage, change in cable type or other fault conditions. The TDR sends energy pulses down the cable and the reflected energy from the change in impedance between conductors is displayed on the screen either as a waveform or a distance reading. The magnitude of the impedance change determines the amplitude of the reflection.

Beware of Blind Spots
Some TDRs require a certain amount of distance to jumpstart or launch. This is extremely important to remember as the fault might lie in this blind spot. To check for this area, connect another small jumper cable to the cable in question. Make sure you have the same type of cable with the same impedance to ensure you are not creating a poor connection. Also, remember to subtract the length of the jumper cable for a correct distance.

NOTE: Tecra's Cable Tool Digital TDR does not have a minimum launch distance thus it does not have a "Blind spot" problem!

Multiple Faults and Testing from Both Ends
Due to improper or faulty installation, construction, ground shift, or even structural flaws from the manufacturing process, multiple faults might be present on the cable being tested. Remember that when using your TDR it might be misinterpreted and only one fault might be caught. As the TDR pulses energy, the first fault indicated might be an open or a dead short ending the signal at that point and not beyond. The key is to always test from both ends for signs of faults on the other side of an open short or really in all fault tests being made. Not only testing from both ends for finding multiple faults but testing from both ends in all tests will ensure there are no faults being hidden by blind spots or to detect smaller faults that were overlooked due to a larger fault detected on up the cable. This also might put you closer to the fault from the other end giving you a more accurate distance.

Use Common Sense
Keep in mind that common sense can play a major role in finding the fault in a cable. Take a look at the “big picture.” If you have cables behind a wall and the fault is
shown to be 15 ft from where you are but you see a new bathroom has been installed after you ran the cable about 17 ft away, you might want to check to see if a pipe has interfered with your cable. One last thought is to always run a last check before you leave the site.

Remember you can always allow someone to sign off on your “no fault” findings for your records. This just might help you at a later date.

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