While the share of installed cables without a jacket generally continues to decrease, the unjacketed cables that remain continue to age. As they do so, it is increasingly likely that a fault will develop on these cables. It can be particularly troublesome to locate faults on unjacketed cables as normal Time Domain Reflectometry, or radar, based fault location techniques often do not work, due to the corrosion of the neutral wires. While there is no guaranteed action that would result in locating the fault, this paper will outline some advanced methods, based on field experience, that can help when locating these types of faults. This paper was an article presented at NETA in 2017 and will discuss the Impulse Current method, which does not rely on the neutral wires being intact, in both a theoretical and practical light.
Finding faults on shielded primary power cables is part art and part science. No two faults are exactly the same. But by developing a logical process, it is possible to minimize the amount of time spent finding the fault, minimize the customer minutes lost, and minimize stress and damage to the cable system. The current state of the art method for finding faults relies on a two-step process utilizing a TDR (Time Domain Reflectometer) to find the fault location. The first step is to identify the end of the cable using a Low Voltage TDR, typically in the 20-30 volt range, but with some models extending far above or below this value. Once the end of the cable is known, an attempt is made to find the location of the fault. This is accomplished by initiating an arc at the fault location with a high voltage surge, which is quickly followed by another Low Voltage TDR pulse. This low voltage pulse will then reflect off the temporary arc, and allow the user or the analysis software to identify the fault location. This technique is known by various trade names, but can be broadly classified as Arc Reflection.