Introduction - Megger TDR1000/2 Guide De L'utilisateur

Introduction

Thank you for purchasing the TDR1000/2 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.
The TDR1000/2 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 pulse travels down
the cable at a velocity determined by the insulation
between the conductors and this resistance to the
flow of the pulse is characterised as impedance for
the cable. Any changes in cable impedance will
cause a proportion of the pulse to be reflected.
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 cables
characteristic impedance, such as poor joints or
discontinuities. Faults showing an impedance
higher than that of the cables normal impedance
will cause a reflection of the same polarity,
i.e.positive, whilst faults with an impedance lower
than that of the cable will cause an inverse
negative going reflection.
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 losses) 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 TDR1000/2 has an adjustable gain
setting. By increasing the gain small reflections
become more obvious.
The TDR1000/2 can be used on any cable consisting
of at least two insulated metallic elements, one
of which may be the armouring or screen of
Matched cable
5