Megger 651403, Instruction Manual

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AVTM6514J Rev. A (03/2003)
21
Section 7
Operation Notes
THEORY
All faults can be represented electrically by a gap shunted by a resistance as shown in
Figure 3. Although the electrical circuit shown is simple, variations in the conditions of
the two paths can cover an extremely wide range, with the resistance ranging from a
dead short to many megohms, and the gap breakdown voltage varying from zero to
many thousands of volts.
Figure 3: Typical Fault Diagram
When the impulse voltage wave reaches the fault, it may dissipate either through the
resistance path or by sparking over the gap. In either event, it releases energy in the
return path, thereby providing a current that can be detected. If the cable is exposed,
faults can usually be located by the sound of the arc discharge or by the flash itself. For
more difficult cases where the cable is buried or the sheath is intact, an electromagnetic
impulse detector or an acoustic impulse detector may be required. If energy passes
through only the resistance path, no spark results and an electromagnetic impulse
detector must be used. Even if the resistance of the fault is infinitely high, the fault can
be located by the impulse method, provided that the peak value and time duration of
the voltage wave are sufficient to cause sparkover. It is for this reason that the impulse
method is so generally applicable.
Basically, sparkover is determined by three factors: the nature of the fault, the
capacitance of cable, and the capacitance of the Impulse Generator. If the capacitance
of the Impulse Generator is equal to or smaller than that of the cable to which the