Megohmmeter Model 1015
13
4.2.2 Spot Testing
As a general rule in spot testing, test voltage should be applied until no
variation in reading is noted for 15 seconds, or applied for a fixed 60 sec
-
onds.
What minimum value should be measured?
The IEEE standard No. 43-1974 states that it is impossible to specify the
value of insulation resistance at which a winding will fail electrically, but on
motors, minimum readings generally stated are:
Rated Voltage
R Minimum
250 or less
2M
Ω
460
2M
Ω
There is no fixed figure for determining what is good and bad in resistance
readings, but a good guide would be 1 megohm for every one hundred
applied operating volts, as a minimum figure. This applies to motors and
transformers. When the insulation resistance gets this low, an electri-
cal breakdown can be expected at any time, and rewinding or replacing
should be considered.
It is not unusual for a winding to be 10 to 100 times the recommended
minimum value (IEEE Std. #43-1974:
Recommended Practice for Testing
Insulation Resistance of Rotating Machinery
), but this varies with tem-
perature and humidity.
4.2.3 Ratio Testing
In time resistance reading (Dielectric Absorption Ratio), readings are taken
at 30 and 60 seconds to obtain the dielectric absorption ratio.
Insulation resistance @ 60 s
= Dielectric Absorption Ratio (DAR)
Insulation resistance @ 30 s
This test is useful to increase the accuracy of spot testing. In general, a
ratio of 1.25:2 or better should be required. A ratio below this indicates that
repair is probably needed.
Remember, a DC insulation test may be used for acceptance testing, but
is more commonly used to check the gradual deterioration of equipment
over its life. Consult your equipment manufacturer for specific test or test
voltage if not known.
Insulation resistance decreases with moisture, temperature and age and
should be recorded over time at a given temperature and corrected.
