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NOTE: Windings should be earthed briefly immediately after measurement in
order to avoid risk of electric shock.
Conversion of measured insulation resistance values
In order to be able to compare measured insulation resistance values, the values
are stated at 40 °C. The actual measured value is therefore converted to a
corresponding 40 °C value with the help of the following diagram (see
Figure 7-2.
Correlation between the insulation resistance and the temperature
). The use of
this diagram should be limited to temperatures fairly near to the standard value of
40 °C, since large deviations from it could result in errors.
0,01
0,1
1
10
100
0
10
20
30
40
50
60
70
80
90
100
Winding temperature/[deg. of C]
C
oe
ff
ic
en
t f
or
in
su
la
tio
n
re
si
st
an
ce
Figure 7-2. Correlation between the insulation resistance and the temperature.
•
R
=Insulation resistance value at a specific temperature.
•
R40
=Equivalent insulation resistance at 40°C.
•
R40
=k x R.
Example:
R
=30 M measured at 20 °C.
k
=0.25.
R40
=0.25 x 30=7.5 M .
Table 7-10. Temperature values in degrees Celsius [°C] and degrees Fahrenheit [°F]
°C 0
10 20 30 40
50
60
70
80
90
100 110
°F 32 50 68 86 104 122 140 158 176 194 212 230
General considerations
The following consideration should be noted, before deciding any actions based
upon the insulation resistance tests:
•
If the measured value is considered too low, the winding must be cleaned
and/or dried, see
Chapter 7.6.8. Drying
for details. If these measures are not
sufficient, expert help should be acquired.
•
Machines, that are suspected to have a moisture problem, should be dried
carefully independent of the measured insulation resistance value.
•
The insulation resistance value will decrease when the winding temperature
rises.
•
The resistance is halved for every 10-15 ºC temperature rise.
