Types Of Tests; Spot Reading Test - AEMC 5100 User Manual

Megohmmeter Models 5000N/5100/5110

TYPES OF TESTS

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
1
above the dewpoint . The operator should note the winding temperature
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.
1
Dewpoint temperature is the temperature at which the moisture vapor in the air condenses as a liquid.
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