Megger TDR1000/2 Guide De L'utilisateur page 4

Introduction
Thank you for purchasing the TDR1000/2 cable fault
l o c a t o r. 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 re f l e c t i o n .
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
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.
increasing the gain small reflections become more
obvious.
The TDR1000/2 can be used on any cable consisting
By
5