Baur Sys compact 2000 M pro - TDR: Time Domain Reflectometry

Syscompact 2000 M pro Cable fault pre-location
822-175-2 53 / 98
12.3 TDR: Time Domain Reflectometry
12.3.1 About the TDR method
Areas of application
 To detect the cable length and to test the velocity of propagation
 To check if all phases are equal in length and if there is a cable break
 To detect joints and other impedance changes
 To compare healthy and faulty phases
Measurement principle
A transmitting pulse is fed into the cable. When the transmitting pulse reaches a position with
impedance change (cable ends, faults or joints), a part of the pulse energy is reflected to the
time domain reflectometer. These reflections are recorded and presented in a graph.
The amplitude of a reflection is determined by the extent of the impedance change, which is
defined by the reflection factor r:
r
Reflection factor
Z
1
Cable impedance up to impedance change
Z
2
Impedance of a change in the cable route (e.g. fault or joint)
If the impedance of a change Z
2
is greater than the cable impedance Z
1
, the reflection factor is
positive. In the reflection image, the open cable end is displayed by a positive reflection.
If the impedance of a change Z
2
is less than the cable impedance Z
1
, the reflection factor is
negative. In the reflection image, a short-circuit or a low-resistive fault is displayed by a
negative reflection:
To determine the fault distance, the time gap between the transmitting pulse and the reflecting
pulse is measured. The fault distance is calculated with the following formula.
l
Fault distance
t
Time gap between the transmitting pulse and the reflecting pulse
v/2
Velocity of propagation