Megger SG32-1500M, User Manual

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Typical Fault Locating Strategy
SG32-UG-EN-V01 Dec 2016
41
Overview of Fault Pre-location Methods
Description of TDR (Pulse Echo) Techniques
TDR also known as Pulse Echo or Radar method for cable fault location use
low-voltage pulses to locate changes in impedance along the length of the cable.
From these low-voltage pulses, a small amount of energy is reflected back to the
TDR as the result of a change in impedance of the cable. It is displayed on the
MTDR screen, as either a positive or negative reflection, depending on the
impedance characteristic (negative reflection for low impedance to shield faults
and positive reflection for high impedance faults).
In TDR mode, the time, which the pulse needs to travel from the Instrument to
the end of cable and back, is measured by means of a cursor which is positioned
at the beginning of the reflection.
Mathematical representation: L = v · t
In TDR mode, the output pulse travels twice the length of the cable (i.e. from
the output of the TDR to the change of impedance and the returning reflection
back to the TDR) so the length
to
the impedance change is shown as :
L
x
= v · t/2 = v/2 · t
Where: v = Propagation velocity; L = Measured length; t = time measured
Cursor