“A Stitch in Time...”

You only need to keep the test voltage constant between steps for about
60 seconds. This short period will not affect the trend in resistance change.
The timed period, however, should always be the same for a given piece
of equipment. All the absorption current may not have disappeared in this
time, but your resistance measurements will be made on the same basis and
therefore will be meaningful. Your results are independent of the insulation
material and its temperature because you’re looking at the change in
resistance – not the absolute resistance values.
As is true with the spot-reading and time-resistance measurements, the
step-voltage method is more valuable to you when repeated on a periodic,
scheduled basis.
The step-voltage method is particularly useful in determining the presence
of excessive moisture or other contaminants in the insulation of equipment
that is rated at voltages equivalent to or greater than the highest voltage
available in your multi-voltage Megger instrument. In other words, even
though your highest voltage does not stress the insulation beyond its rating,
a two-voltage test can, nevertheless, often reveal the presence of such
contaminants.
For example, assume that you first test insulation resistance on a short-time
reading basis – at a voltage of 500 VDC, and then at a higher potential of,
say, 2500 VDC. Even though the latter voltage may be nominal in relation to
the voltage rating of your equipment, a lower value of insulation resistance
at the higher test voltage usually indicates the presence of contaminated
fractures or other leakage paths through the insulation to ground.
The applied voltages should preferably be in the ratio of 1 to 5 or greater
(500 and 2500, for example). Results to date show that a change of 25%
in the insulation resistance value, with a 1 to 5 ratio in test voltages, is
usually due to the presence of an excessive amount of moisture or other
contaminant.
Use of a Guard Terminal
All Megger insulation testers having ranges of 1000 megohms and higher
are equipped with Guard terminals. The purpose of this terminal is to
provide facilities for making a three-terminal network measurement, so that
the resistance of one of two possible paths can be determined directly. It
has the further or secondary purpose of providing a source of DC voltage of
good regulation and of limited current capacity.
50
A STITCH IN TIME