Megger PFL22M1500 Series, User Manual

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AVTMPFL22M Rev 1 April 2011
63
Overview of Fault Prelocation Methods
Description of TDR or Pulse Echo techniques
TDR also know as Pulse Echo or radar methods of 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 from a change of impedance and is displayed on the MTDR screen, as
either a positive going or negative going pulse, depending on the impedance
characteristic (negative pulse for low impedance to shield faults and positive
pulse for high resistance faults).
With Pulse Echo. 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
With Pulse Echo, the output pulse travels twice the distance 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