“A Stitch in Time...”

There are also cases, such as found in two winding transformers or multiconductor
cables, where a three-terminal network is formed as shown in
Fig. 25b. Figures 30 and 33 and others show practical applications of this
form of three-terminal network.
In making a three-terminal test involving only one measurement, the Line
terminal of the Megger instrument is connected to Terminal 1, Fig. 25a, the
Guard terminal to Terminal 3, and the Earth terminal to Terminal 2. This will
give the true value of r 12
, provided r 23
and r 13
are not too low in value. The
leg r 23
, which is connected across the Megger instrument generator, should
be about 1 megohm or higher to prevent excessive load on the generator,
and maintain satisfactory generator voltage.
In using the Guard terminal, particularly in the case of motor-driven or
rectifier-operated Megger instruments, also make certain that there is no
chance of an arc-over between the guarded terminal of the sample and
ground. Such an arc-over may cause undesirable arcing at the commutator of
the instrument generator.
The leg r 13
, which shunts the Megger deflecting coil, should be at least 100
megohms for a measuring accuracy of approximately 1%. The 1% accuracy
figure is based on the R' ballast resistor being 1 megohm, which is typical.
For more precise determinations of accuracy, obtain the exact value of R' by
writing to Megger and giving the serial number of the instrument in use.
Figure 27-Showing how to use the Guard connection to eliminate the effects of surface leakage
across exposed insulation at both ends of a cable when a spare conductor in the cable is available for
completing the Guard connection.
52
A STITCH IN TIME