“A Stitch in Time...”
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The highest accuracy is desired <strong>in</strong> cases as shown <strong>in</strong> Fig. 25a, or where<br />
the true resistance of each leg is wanted as <strong>in</strong> the case of Fig. 25b, three<br />
measurements are required and the follow<strong>in</strong>g equations are used:<br />
r 12<br />
= R 12<br />
R 13<br />
– (R') 2<br />
R 13<br />
+ R'<br />
r 23<br />
= R 12<br />
R 23<br />
– (R') 2<br />
R 12<br />
+ R'<br />
r 13<br />
= R 12<br />
R 13<br />
– (R') 2<br />
R 12<br />
+ R'<br />
where R 12<br />
, R 23<br />
and R 13<br />
are the actual read<strong>in</strong>gs <strong>in</strong> megohms measured across<br />
the term<strong>in</strong>als of the network which are connected to the L<strong>in</strong>e and Earth<br />
term<strong>in</strong>als of the Megger <strong>in</strong>strument with Term<strong>in</strong>als 3, 1 and 2 respectively<br />
connected to the <strong>in</strong>strument Guard term<strong>in</strong>al. R' is the value of the ballast<br />
resistance <strong>in</strong> megohms of the <strong>in</strong>strument <strong>in</strong> use. In mak<strong>in</strong>g these three<br />
measurements, do no connect the L<strong>in</strong>e term<strong>in</strong>al of the <strong>in</strong>strument to the<br />
grounded term<strong>in</strong>al of the network, as any leakage over the <strong>in</strong>strument case<br />
between the Earth term<strong>in</strong>al and ground will shunt the resistance be<strong>in</strong>g<br />
measured.<br />
Bush<strong>in</strong>gs, Potheads and Insulators<br />
Figure 30-Connections for test<strong>in</strong>g <strong>in</strong>sulation resistance between one wire and ground, without be<strong>in</strong>g<br />
affected by leakage to other wires. Note use of Guard connection.<br />
54<br />
A STITCH IN TIME
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