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Using TDR to Identify Neutral Corrosion and Splices

October 1, 2011

Using TDR to Identify Neutral Corrosion and Splices

TDRQ: The blog post titled Neutral Corrosion — How Much Is Too Much? includes a waveform from a TDR (time domain reflectometer, often called a radar) that is used to pinpoint bad sections of cable neutral. The TDR is also used to pinpoint splice locations on the cable. How does the TDR determine the neutral corrosion and splices on the cable? And how is the waveform read to tell them apart and to pinpoint their locations?

A: Step-by-step instructions for how to identify and pinpoint neutral corrosion and splices on concentric, medium-voltage power cables are provided in Novinium Rejuvenation Instruction 260, TDR Diagnosis. The TDR sends a low-voltage (10–20 volts), short-wavelength (1–20 nanoseconds) pulse down the cable. A portion of the wave is reflected when it encounters a change in impedance. The four main types of impedance changes encountered along the length of a test cable are discussed below. (Remember that impedance includes three elements: resistance, capacitance, and inductance.)

1. Instrument-Cable Interface

The first impedance change that is encountered results from the mating of the test instrument lead, an RG59 coaxial cable, which has a characteristic impedance of 75 ohms, with the power cable, which has a characteristic impedance of 8 to 38 ohms, depending upon its geometry and polymer system.

Some reflection is unavoidable, but to minimize the reflection from this impedance change, Novinium uses a proprietary impedance streamliner. The impedance streamliner is like the smooth curves of an aerodynamic sports car, versus the squarish shape of a pickup truck. Figure 1 shows the signature of Novinium’s impedance streamliner in red, superimposed upon the green signature of an older impedance technology device.

Because the impedance streamliner reflects less of the input pulse, it minimizes signal attenuation and dispersion. Attenuation is the reduction of signal amplitude, and dispersion is the smearing of the narrow pulse into a broader, less discrete pulse. Both are undesirable. Because untoward noise and reflections are avoided, the usability and hence the sensitivity and accuracy of the TDR are improved.

Figure 1
Figure 1

2. Splice

Figure 2 shows a very typical splice. The neutrals are all dirty as they are prone to be in a pit, but if you look carefully along the orange annotation, you can see how the neutrals are close to the conductor on the cable, then are pigtailed together and lay farther from the conductor as they jump across the molded splice body. On the far end of the splice, the neutrals again come back to intimate proximity.

Figure 2
Figure 2

This change in the separation of the two signal conductors (the conductor and the neutral) changes the circuit impedance. Splices are characterized by an increase and then a decrease in impedance. The resistance is not significantly changed—the already low capacitance decreases with increasing distance—but that capacitance change is trivial compared to the change in inductance. The inductance and hence the impedance skyrocket as the neutrals leave the insulation shield and then plummet when the neutrals return to the cable.

The actual TDR image of a splice, a characteristic sine wave, is superimposed in the lower-right corner of figure 2.

3. Neutral Corrosion

The physics are even simpler for neutral corrosion. The capacitance and inductance components are insignificant. Neutral corrosion is a simple increase in impedance, driven by resistance. A good old-fashioned resistance increase is displayed as an impedance increase. Figure 3 shows how this looks on the TDR.

Figure 3
Figure 3

4. End of Cable

Simpler still, the end of the cable is characterized by either an infinite impedance increase (if the circuit is open) or an infinite impedance decrease (if the conductor is grounded to the neutral). When used, grounding devices add some more color to the wave shape, but the basic idea remains the same.

90-20110930_Reflections on TDR4
Figure 4

The TDR signal is reflected by each of the above impedance changes, and the time that the signal takes to travel to and then from the impedance change can be used to estimate the distance to that change.

Note that the TDR is not a pinpointing technology; rather, it provides a location estimate. To pinpoint splices and corrosion, radio-frequency (RF) locating is utilized. RF Locating provides step-by-step instructions for RF pinpointing.