GEH-5304A
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For maximum insulation life, these three causes of insulation failures should be prevented:
1. Contamination.
2. Mechan ical factors.
3. High temperatures.
L. Testing Methods
1. V isual I nspect ion
In addition to collecting contaminants, insulation shrinks, cracks and becomes brittle with heat and
age. These changes allow movement of coils, loose filler strips, loose ties, chafing and abrasion, all of
which can be picked up by visual inspection.
Experience and judgement can be gained by careful observation and comparing results of visual in-
spection s with in sulation resistance measurement. GE Service Shops have personn el wh o can inspect
equipmen t and point out potential tr ouble ar eas. Their services can h elp build experience an d judge-
ment for future visual inspections.
2. Insulation Resistance M easurement
A method of measuring the insulation resistance is described in Re port 43, “Re commende d Pract ic e
for Testing Insulation Resistance of Rotating Machinery”, published by IEEE, 345 E. 47th Street,
New York, NY 10017. Th e r esistan ce measur ement s should be taken with a 500 or 1000 volt m egger
and cor rected t o 1 0 4 °F (40°C).
The insulation r esistance measuremen ts are affected by the following:
a. Magnitude of test voltage.
b. Time the test voltage is applied.
c. Temperature.
d. Sur face con dition (contaminants).
e. Moisture.
When a 1000 volt megger is used, taking readings of one minute and converting the data to 40°C
(104°F), the data will evaluate the other two factors, i.e., the contaminants and the moisture present.
The insulation resistance varies inversely with the winding temperature. That is, as the temperature
decreases, the insulation r esistance increases in accordance with Table 13.