“A Stitch in Time...”

of the ohmmeter can be explained by referring to Fig. 21. In this figure, the
change which may occur in leakage current after the absorption current has
disappeared is shown plotted in terms of insulation resistance against three
different voltages. Note that there is no change in resistance shown in the
figure between 500 and 1000 volts, indicating no change in the insulation
as a result of applying these two voltages. This is an assumption, but is a
condition which is not uncommon in practice. If the insulation continues to
be stable at 2500 volts, there will be no change in the insulation resistance
value obtained, which is shown by the dotted extension of the horizontal
line in the figure. When non-linear conditions appear at a higher voltage, the
voltage resistance curve reveals this very clearly by a lower resistance value,
indicated by the downward curve in the figure. The figure, therefore, reveals
the simplicity of determining the change in insulation stability by using three
fixed voltages which are easily reproducible when making three-voltage tests
on a routine basis.
Figure 21
We wish to emphasize that the curve in Fig. 21 indicates the resistance
change due to leakage current only, and not the absorption current which
may appear for a period of time with each change in voltage. It may be
necessary to wait an appreciable amount of time after each voltage change
for the absorption current to disappear before taking a reading.
A STITCH IN TIME 43