MM-13- rev 01-EN
29
MEASUREMENT OF INSULATION RESISTANCE
The insulation resistance test is carried out to check if the resistance of the insulations and
impregnation varnishes used is sufficient to allow a good operation of the generator.
The test is carried out using a Megger, which is able to supply a voltage of 500V. The Megger is
connected to the windings and the earth by means of appropriate push rods.
The instrument scale is calibrated directly in mega ohm and indicates the insulation resistance value.
The test is then repeated by connecting a winding phase to a push rod of the instrument and the
earth to the other push rod (of course after having opened the winding connections).
Measurement of the insulation resistance does not supply the quantitative measurement but only
qualitative.
Generally, for new generators in normal climatic conditions, the insulation resistance value easily
exceeds the value of 100 Mohm.
In the case of rewound generators or generators exposed to wet operating conditions, the minimum
insulation resistance value can be estimated around 20 Mohm at 15 ÷30°C.
If the measurement is carried out at temperatures higher than 40°C, the values detected must be
brought to 40° C using the following formula:
R
K
R
t
t
40
where:
R
40
= insulation resistance in Mohm corrected to 40°C
R
t
= insulation resistance at temperature t
K
t
t
10
0 0301
1 2041
(( .
)
.
))
The Kt coefficient can also be obtained from the undermentioned diagram.
CAUTION
BEFORE MEASURING THE INSULATION RESISTANCE, THE MACHINE MUST BE AT A
STANDSTILL FOR A SUFFICIENTLY LONG PERIOD OF TIME TO ALLOW TOTAL DISCHARGE
OF RESIDUAL VOLTAGE.
TEMPORARILY EARTH THE WINDINGS THROUGH THE FRAME TO CHECK THAT
DISCHARGE HAS TAKEN PLACE.
