Spot Testing; Ratio Testing - AEMC 1020 User Manual

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 seconds. 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
250 or less
460
There is no fixed figure for determining what is good and bad in resis-
tance readings, but a good guide would be 1 megohm for every 100
applied operating volts, as a minimum figure. This applies to motors
and transformers. When the insulation resistance gets this low, an
electrical 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 recommend-
ed minimum value (IEEE standard 43-1974: Recommended Practice
for Testing Insulation Resistance of Rotating Machinery), but this
varies with temperature and humidity.

RATIO TESTING

In time resistance testing (Dielectric Absorption Ratio), readings are
taken at 30 and 60 seconds to obtain the dielectric absorption ratio.
Insulation resistance @ 60s = Dielectric Absorption Ratio (DAR)
Insulation resistance @ 30s
This test is useful to increase the accuracy of spot testing. In gener-
al, a ratio of 1.5 or better should be required. A ratio below this indi-
cates that repair is probably needed.
10
R Minimum
2 MΩ
2 MΩ
Megohmmeter Models 1020 and 1025