Types Of Tests; Spot Reading Test; Ratio Testing - AEMC 1000N User Manual

Megohmmeter Model 1000N/1100N
7\SHVRI7HVWV

SPOT READING TEST

Method
For this test, the megohmmeter is connected across the insulation of the
windings of the machine being tested. A test voltage is applied for a fixed
period of time, usually 60 seconds, and a reading is taken. The spot
reading test should only be carried out when the winding temperature is
above the dewpoint
so that the reading may be corrected to a base temperature of 20°C
(68°F).
Test Duration
To obtain comparable results, tests must be of the same duration.
Usually the reading is taken after 60 seconds.
Interpreting the Results
Proper interpretation of spot reading tests requires access to records of
results from previous spot reading tests. For conclusive results, only use
results from tests performed at the same test voltage for the same
amount of time under similar temperature and humidity conditions. These
readings are used to plot a curve of the history of insulation resistance. A
curve showing a downward trend usually indicates a loss of insulation
resistance due to unfavorable conditions such as humidity, dust
accumulation, etc. A very sharp drop indicates an insulation failure.

RATIO TESTING

In time resistance reading (dielectric absorption ratio), readings are taken
at 30 seconds and 60 seconds to obtain the dielectric absorption ratio.
Insulation resistance @ 60s
Insulation resistance @ 30s
This test is useful for increasing the accuracy of spot testing. In general,
a ratio of 1.25 to 2 or better should be required. (Below 1.1 is dangerous;
1.1 to 1.25 is questionable; 1.25 to 1.4 is fair; and 1.4 to 2 and above is
good.) A ratio below this indicates that repair is probably needed.
1
Dewpoint temperature is the temperature at which the moisture vapor in the air condenses as a liquid.
1
. The operator should note the winding temperature
= Dielectric Absorption Ratio (DAR)
- 22 -