GE GEH-5304A, Instructions Manual, Page: 27 / 45

GEH-5304A
27
For maximum insulation life, these three causes of insulation failures should be prevented:
1. Contamination.
2. Mechanical factors.
3. High temperatures.
L. Testing Methods
1. Visual Inspection
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-
spections with insulation resistance measurement. GE Service Shops have personnel who can inspect
equipment and point out potential trouble areas. Their services can help build experience and judge-
ment for future visual inspections.
2. Insulation Resistance Measurement
A method of measuring the insulation resistance is described in Report 43, “Recommended Practice
for Testing Insulation Resistance of Rotating Machinery”, published by IEEE, 345 E. 47
th
Street,
New York, NY 10017. The resistance measurements should be taken with a 500 or 1000 volt megger
and corrected to 104
°
F (40
°
C).
The insulation resistance measurements are affected by the following:
a. Magnitude of test voltage.
b. Time the test voltage is applied.
c. Temperature.
d. Surface condition (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 resistance increases in accordance with Table 13.