LG RCWW Operation & Maintenance Manual page 83

7) Other factors affecting the insulation resistance
<Insulator surface contamination> If absorbable or deliquescent substances such as acid or salt are attached to
the insulator surface, they will affect insulation resistance. Remove these substances before test.
<Condensed water> If the temperature of insulator is below dew point temperature, water will condense on the
surface of insulator (particularly on the cracks or dents) affecting the insulation resistance greatly. The inspection
should be done when the insulator temperature is higher than the dew point temperature of ambient air. The tem-
peratures of a dry bulb and a wet bulb of the humidity meter should be recorded.
<Absolute humidity> Although the temperature of the insulator is above dew point temperature, the water vapor
in the atmosphere still affects the insulation resistance. Avoid conducting the test where the absolute humidity is
high.
8) Apply electric current for one minute to the spot where insulation is to be measured and read the scale and record.
Apply the inspection standard for the temperature of insulator at 20°C.
(If measured at a different temperature, convert the indicated value after one minute using the temperature coeffi-
cient.)
9) Take following actions based on the insulation condition.
Condition
Dangerous
In poor state
Re-inspection required
Fair
Good
Excellent
h Motors falling in "In poor state" or "Re-inspection required" in the megger insulation test are required to take PO-
LARIZATION INDEX Test.
10) POLARIZATION INDEX Test
Write down the indicator values after one minute and after ten minutes in the megger test.
Condition
Dangerous
In poor state
Uncertain
Excellent
Motors falling in "Dangerous" range in the insulation absorption rate should be returned or replaced.
Motors falling within "In poor state" range in the insulation absorption rate should be run carefully for four hours, and
an additional test should be conducted.
11) Record the followings in megger testing
- Type and voltage of megger tester
- Connecting parts of megger tester
- Ambient temperature and humidity of testing place. For inclosed type, pressure of the container inside.
- Residence time of the machine before test
Indicated value after 1 minute
Less than 50 MΩ
500~1000 MΩ
1000 MΩ or bigger
Table 13 Insulation conditions
Insulation absorption rate =
Fair
Good
Table 14 Insulation absorption rate status
2 MΩ or less
50~ 100 MΩ
100~500 MΩ
indicated value after ten minutes
indicated value after one minutes
Insulation absorption rate
1.5 or higher ~ lower than 2
2 or higher ~ lower than 3
3 or higher ~ lower than 4
6. Commissioning
Action
Repair or replace
Troubleshooting
Troubleshooting
1 or lower
Lower than1.5
4 or higher
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