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TM 5-685/NAVFAC MO-912
be avoided if at all possible. If however testing must
f. The ratio of two time resistance readings is
be conducted in an explosive atmosphere, then it is
called a Dielectric Absorption Ratio. It is useful
suggested that test leads not be disconnected for at
in recording information about insulation. If the
least 30 to 60 seconds following a test, so as to allow
ratio is a lo-minute reading divided by a l-minute
reading, the value is called the Polarization Index.
time for capacitance discharge.
Table 4-4 gives values of the ratio and correspond-
(6) Do not use a megohmmeter whose terminal
operating voltage exceeds that which is safe to ap-
ing relative conditions of the insulation that they
ply to the equipment under test.
d. To take a spot insulation reading, connect the
megohmmeter across the insulation to be tested and
Table 4-3. Interpreting insulation resistance test results.
operate it for a short, specific timed period (60 sec-
onds usually is recommended). Bear in mind also
that temperature and humidity, as well as the con-
What to Do
dition of your insulation, affect your reading. Your
1. Fair to high values and
No cause for concern
very first spot reading on equipment, with no prior
test, can be only a rough guide as to how "good" or
"bad" the insulation is. By taking readings periodi-
2. Fair to high values, but showing Locate and remedy the cause and
a constant tendency towards
check the downward trend
cally and recording them, you have a better basis of
lower values
judging the actual insulation condition. Any persis-
tent downward trend is usually fair warning of
3. Low but well-maintained
Condition is probably all right, but
cause of low values should be
trouble ahead, even though the readings may be
higher than the suggested minimum safe values.
Equally true, as long as your periodic readings are
4. So low as to be unsafe
Clean, dry out or otherwise raise
consistent, they may be OK, even though lower than
the values before placing
equipment in service (Test wet
the recommended minimum values. You should
equipment while drying out)
make these periodic tests in the same way each
time, with the same test. connections and with the
Make tests at frequent intervals
5. Fair or high values, previously
well-maintained but showing
until the cause of low values is
same test voltage applied for the same length of
sudden lowering
located and remedied; or until the
time. Table 4-3 includes some general observations
values have become steady at a
about how you can interpret periodic insulation re-
lower level but safe for operation;
sistance tests and what you should do with the
or until values become so low that
it is unsafe to keep the equipment
in operation
e. Another insulation test method is the time re-
sistance method. It is fairly independent of tem-
perature and often can give you conclusive informa-
Table 4-4. Condition of insulation indicated bY dielectric
tion without records of past tests. You simply take
absorption ratios. *
successive readings at specific times and note the
-Minute Ratio
differences in readings. Tests by this method are
sometimes referred to as absorption tests. Test volt-
ages applied are the same as those for the spot
Less than 1
reading test. Note that good insulation shows a con-
1.0 to 1.25
1.0 to 2
tinual increase in resistance over a period of time. If
the insulation contains much moisture or contami-
2 to 4
1.4 to 1.6
nants' the absorption effect is masked by a high
Above 1.6**
Above 4**
leakage current which stays at a fairly constant
value-keeping the resistance reading low. The
* These values must be considered tentative and relative; sub-
time resistance test is of value also because it is
ject to experience with the time resistance method over a period
of time.
independent of equipment size. The increase in re-
sistance for clean and dry insulation occurs in the
** In some cases with motors, values approximately 20 percent
same manner whether a generator is large or small.
higher than shown here indicate a dry brittle winding which will
fail under shock conditions or during starts. For preventive
You can therefore compare several generators and
maintenance, the motor winding should be cleared, treated and
establish standards for new ones, regardless of their
dried to restore winding flexibility.
kW ratings.

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