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How to find a cable fault with a cable tester

Imagine that your job is to find cable faults which occur in aircraft during flight. Modern wiring can run over 100 km in commercial aircraft, and in-service testing is a challenge in any situation. We’ve come a long way from mobile testing units, high voltage test spikes and other rough and ready testing methods — testing is more elegant now for many types of wiring.

TDR Magic

Things have gotten a lot easier for techs working on complex and hidden power and data wiring. Time Domain Reflectometry (TDR) uses some simple electrical principles to check cabling for faults and also locate those faults even on a thousand feet of cable or more. Techs can calculate the length of cable segments and observe any shorts or opens by watching reflected signals on a waveform monitor. But there have even been a few key improvements to this basic technique.

For the in-flight example above, a variation on TDR called SSTDR using spread spectrum technology lets techs check cables without interfering with data and other signals, and without being affected by activity on the wire. Spread spectrum, used in cellular telephones, increases the reliability of signals and avoids interference from them by keeping them jumping to different frequencies in predictable patterns.

From Waveforms to a Simple Digital Display

SSTDR testing of copper cable of almost any kind, from Romex to coax, is now available in a simple, hand-held device that looks somewhat like a digital voltmeter, and costs only a bit more than a quality voltmeter. The Platinum SnapShot does the waveform calculations and presents simple open/short indications with cable length calculations. It requires an NVP (nominal velocity of propagation, also known as VOP) value which describes the cable transmission velocity (as a fraction of the speed of light). Typical values are listed on the device for types of cable such as CAT6 or RG58, but it can also calculate NVP from a known length of sample cable over 25 feet.

Simple Cable Tester Operation

The SnapShot has three basic modes: calibration mode, where it calculates NVP if you don’t know the NVP; test mode, where it measures cable lengths and determines distances to faults; and tone mode, a convenience feature to help locate cables under test or locate opens/breaks in the cable. With typically better than 1% accuracy, it can locate hidden faults in seconds. It can also tell you how long those unlabeled cable runs are, check or verify spooled cable lengths, and tone out your cables before testing. Test mode also offers continuous testing, where you just connect each cable and watch the display without pushing any buttons.
Interpreting Results

TDR-type testing relies on cables that don’t have proper termination impedance on the other end. If the cable’s characteristic impedance, such as 75 ohms, is present on the other end (equipment connection) you’ll either get an error, or a “short” indication. That’s because a proper termination doesn’t reflect the test signal. When there are cable faults, the type of reflected signal tells the tester whether it’s coming from an open connection or a short, and the tester will show the cable length to that problem. Testing spooled cable, you’re just finding the distance to the open connection at the other end.

With the precision of SSTDR testing using the SnapShot, you can save time, test live (with limited voltage on the line, less than 60V DC or peak AC). Measuring to the fault allows you to dig, open walls, even send down divers pretty close to the fault. If you happen to work on aircraft wiring or something similarly complex, this unit can make you really popular!

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