Time domain reflectometry (TDR)

Cable fault pre-location method

Cable fault location process: Time domain reflectometry | BAUR GmbH

This method is one of the most frequently used methods for fault location, because as well as providing information about the cable length and joint locations, it also helps to locate low-resistive faults (e.g. short-circuits).

How does that work?

With time domain reflectometry, a LV pulse is fed in at the cable end, which then propagates along the cable. Wherever there is a deviation in impedance, the pulse is reflected and recorded by the measuring device. The distance to the point where the reflection took place can be determined by means of the duration (the time difference between the pulse transmission and the reflection registration). The software in any modern measuring device will calculate the distance
for you.

Time domain reflectometry can be used when the fault lies between two parallel phases or between a phase and the cable screen. The pulse reflection is visible at those points where the impedance between the two phases changes.

Detecting cable end or cable break
At the end of a cable or at a break, there is a positive reflection of the pulse and the reflection factor will be around 1. If the reflection takes place at a point before the cable end, there is a break.

Tracing short-circuits

Similarly, short-circuits are shown clearly in the reflection image.

It is likewise easy to detect an impedance change, as it takes place at the transition from one cable section to the next – i.e. at a joint. In the evaluation logic, the reflection at the joint appears in the form of a horizontal S.

TDR measurement in mixed cable routes

Cable routes with unknown length or consisting of varying partial sections are an exception. Here, the velocity of propagation can vary in the partial sections. Therefore, it is not possible to calculate the fault location exactly. However, to determine the fault location with high reliability, you can carry out the measurement from both cable ends. Both measurements produce two different fault locations in mixed cable routes. The actual fault is located between these two points, generally approximately in the centre. In any case, careful pin-pointing of the fault is recommended for mixed cable routes.

Overview of cable fault location

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