TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
Figure 14-2. Curves showing components of measured current during insulation testing.
as to how "good" or "bad" is the insulation. By taking
Tests by this method are sometimes referred to as
readings periodically and recording them, you have
absorption tests (fig 14-3). Test voltages applied are
a better basis of judging the actual insulation con-
the same as those listed for the spot-reading test.
dition. Any persistent downward trend is usually
Note that good insulation shows a continual in-
fair warning of trouble ahead, even though the
crease in resistance over a period of time. If the
readings may be higher than the suggested mini-
insulation contains much moisture or contami-
mum safe values. Equally true, as long as your
nants, the absorption effect is masked by a high
periodic readings are consistent, they may be O.K.,
leakage current which stays at a fairly constant
even though lower than the recommended mini-
value-keeping the resistance reading low. The
mum values. You should make these periodic tests
time-resistance test is of value also because it is
in the same way each time, with the same test
independent of equipment size. The increase in re-
connections and with the same test voltage applied
sistance for clean and dry insulation occurs in the
for the same length of time. In table 14-1 are some
same manner whether a motor is large or small. You
general observations about how you can interpret
can, therefore, compare several motors and estab-
periodic insulation resistance tests, and what you
lish standards for new ones, regardless of their
should do with the result.
horsepower ratings. The ratio of two time-resistance
b. Time-resistance method. This method is fairly
readings is called a Dielectric Absorption Ratio. It is
independent of temperature and often can give you
useful in recording information about insulation. If
conclusive information without records of past tests.
the ratio is a ten minute reading divided by a one
It is based on the absorption effect of good insula-
minute reading, the value is called the Polarization
tion compared to that of moist or contaminated in-
Index. Table 14-2 gives values of the ratios and
sulation. You simply take successive readings at
corresponding relative conditions of the insulation
specific times and note the differences in readings.
that they indicate.