User Manual Megger Measuring Instruments Tdr500-3 [5/29 Pages]

Megger measuring instruments TDR500-3

Page 5 of 5

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.