DMEG2-MAN P. 5
Typically, a ratio of 1.25 represents the borderline between an insulation
resistance of questionable integrity and of fair integrity. A ratio of 1.6
and above is indicative of insulation of very good integrity. The
Dielectric Absorption Ratio method of testing insulation resistance is
generally not affected by the temperature at which the measurements
are taken. This is one of the advantages of this method. The Dielectric
Absorption Ratio of the example shown in (Fig 6) is 1.5.
Interpretation and Recording of Data
Periodic measurements of insulation resistance of a device, taken
under the same conditions, generally result in much more meaningful
results than measurements taken just once, or at random intervals.
The measurement interval may be weekly, monthly, quarterly or
yearly. This interval chosen will depend in general upon the conditions
under which the device is operated and the cost of suffering an
unexpected breakdown.
The device which operates under conditions of high temperatures,
humidity, load and vibration should be tested more frequently than the
same device which operates under less stressful conditions.
The following “rules” are guidelines which may be used to help
determine whether a piece of equipment is operating normally or
whether it should be pulled out of service and repaired or replaced.
However, the equipment manufacturer’s data on insulation resistance
values and test procedures should be consulted when available.
One Kilovolt/One Megohm Rule
This is an old, generalized rule that states that electrical equipment
rated up to 1000 volts should have minimum insulation resistance
of one-Megohm. Above 1000 volts rating the minimum insulation
resistance should be one Megohm for each 1000 volts of rating.
NOTE: This rule of thumb does not apply to the testing of
hermetic compressors.
Trend Rule
Hermetic Compressor
Dielectric Absorption Ratio Rule
Recording Data
A package of Data Log Cards is supplied with the DMEG2. The use
of these log cards will facilitate the recording and plotting of data
necessary to monitor and evaluate the insulation resistance history and
integrity of an individual piece of equipment. Space is provided on the
front of the card to record the equipment identity and to plot a graph
of insulation resistance values from 0.1 to 1000 Megohms. Space is
provided on the back of the card to record the supporting tabular data.
It is not necessary to make an entry in every column on the back of the
Data Log Card each time a measurement is taken. However, the more
data that is recorded the easier it will be to determine the reason for
changes in the measured insulation resistance.
Determining Moisture Content
The amount of insulation resistance, when drying out or baking
transformers, motors, and generators is an excellent indicator of the
amount of moisture remaining in the device. The insulation resistance
reading will increase as the moisture is driven off. In this way optimum
curing times can be determined.
Insulation Resistance Values Interpretation
High and holding steady Condition good
High but tapering off Breakdown may be starting
Decrease test interval or
repair equipment
Moderately low and holding May be all right. Depends on
steady history of device. Should try to
identify cause of low reading.
Low and declining Failure probable in rear future.
Repair or replace.
Insulation Resistance Values Interpretation
100 Megohms & above Condition good
50 to 100 Megohms Evidence of moisture in
refrigerant. Check drier.
20 to 50 Megohms Excessive moisture in refrigera n t .
Examine system.
Below 20 Megohms Failure of system likely.
Purge system
Ratio Interpretation
Above 1.6 Condition good
1.25 to 1.6 Condition moderately good
1.1 to 1.25 Condition questionable to
unstable