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Introduction
Thank you for purchasing this cable fault locator. Before attempting use of your new instrument please take the time to read this user guide,
ultimately this will save you time, advise you of any precautions you need to take and could prevent damage to yourself and the instrument.
This is an advanced instrument capable of identifying a wide range of cable faults. The instrument uses a technique called Pulse Echo (also
known as Time Domain Reflectometry or TDR). A pulse is launched into a cable from one end. This can be on either a pair of conductors, or
a conductor and the screen. The cable’s construction will determine its characteristic impedance and the velocity a pulse travels down the
cable.
The pulse velocity is normally described as a fraction of the speed of light and is called the Velocity Factor. By measuring the time between
the transmitted pulse and the reception of the reflected pulse, and multiplying this by the speed of light and the velocity factor, the actual
distance to the reflection point can be established.
Reflections are caused by changes in the cable’s characteristic impedance, such as poor joints or discontinuities. Faults showing impedance
higher than that of the cables normal impedance will cause a reflection of the same polarity, i.e. positive, while faults with impedance lower
than that of the cable will cause a negative going reflection. Matched cable terminations absorb all the pulse hence no “end of cable”
reflection will occur, the cable appearing endless. Open or short circuits will reflect all the pulse and a large reflection will be displayed. At an
open or short circuit all the transmitted energy is reflected and the TDR will not ‘see’ the cable beyond that fault.
As a pulse travels down a cable, the size and shape of that pulse is gradually attenuated by the cable. The pulse reduces in amplitude and
becomes more elongated or stretched. The level of attenuation (or loss) is determined by the cable type, the condition of the cable and
any connections along its length. The limit of how far you can see is determined by the point beyond which you will not be able to see or
distinguish a reflection. To help identify small reflections, especially at greater distance the instrument has an adjustable gain setting. By
increasing the gain small reflections become visible.
The velocity factor of the TDR must be adjusted to match that of the cable under test, allowing an accurate distance measurement to be
read directly from the instrument. Where the VF of a cable is not known, but the length is, the cursor can be set to the end of the cable and
the VF on the TDR adjusted until the correct cable length is displayed.
