“A Stitch in Time...”

Fig. 9 shows how a 60-second test would appear for good and perhaps bad
insulation. When the insulation is in good shape, the 60-second reading is
higher than the 30-second reading.
Figure 9–Typical card plot of a time-resistance or double-reading test.
A further advantage of this double-reading test, as it is sometimes called,
is that it gives you a clearer picture, even when a spot reading says the
insulation looks fine.
For example, let’s say the spot reading on a synchronous motor was 10
megohms. Now, let’s assume that the double-reading check shows that the
insulation resistance holds steady at 10 megohms while you hold voltage up
to 60 seconds. This means there may be dirt or moisture in the windings that
bears watching. On the other hand, if the pointer shows a gradual increase
between the 30-second and the 60-second checks, then you’re reasonably
sure the windings are in good shape.
Time-resistance tests on large rotating electrical machinery – especially with
high operating voltage – require high insulation resistance ranges and a very
constant test voltage. A heavy-duty Megger test set, line-operated, serves
this need. Similarly, such an instrument is better adapted for large cables,
bushings, transformers and switchgear.
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A STITCH IN TIME