12. Polarization Index.
- The steepness
of the dielectric absorption curve taken at a
given
temperature indicates the relative dryness of
the insulation. This steepness may be ex-
pressed as "polarization index" and is defined
as follows:
given
temperature indicates the relative dryness of
the insulation. This steepness may be ex-
pressed as "polarization index" and is defined
as follows:
Polarization index = R
10
/R
1
= I
1
/I
10
(if the voltage is constant)
where:
R
10
= megohms insulation resistance
at 10 minutes
R
R
1
= megohms insulation resistance
at 1 minute
I
I
1
= insulation current at 1 minute
I
10
= insulation current at 10 minutes
Table I shows the polarization index for the
four curves of
four curves of
indicate, the difference in polarization index
between dry and moist insulation is more
pronounced at higher temperatures and
therefore, the hot test is more sensitive. The
increase in polarization index with dryness of
insulation is also illustrated by the diverging 1-
and 10-minute curves in
between dry and moist insulation is more
pronounced at higher temperatures and
therefore, the hot test is more sensitive. The
increase in polarization index with dryness of
insulation is also illustrated by the diverging 1-
and 10-minute curves in
13. Test Procedures and Records.
The
test procedure for making dielectric ab-
sorption tests is:
sorption tests is:
a. Hot resistance test - following
shutdown from full-load operation of at
least 4 hours, or until temperature is
stabilized:
shutdown from full-load operation of at
least 4 hours, or until temperature is
stabilized:
(1) Disconnect the equipment to be
tested from other equipment by opening
breakers, opening disconnecting
switches, or unbolting connections. Dis-
connect any potential transformers or
other devices connected between
phases and ground, or neutral and
ground. Any external circuits which
cannot be readily disconnected should
be recorded in the test record.
tested from other equipment by opening
breakers, opening disconnecting
switches, or unbolting connections. Dis-
connect any potential transformers or
other devices connected between
phases and ground, or neutral and
ground. Any external circuits which
cannot be readily disconnected should
be recorded in the test record.
(2) For at least 10 minutes, ground the
winding to be tested to drain off any
charge.
winding to be tested to drain off any
charge.
(3) Remove the ground connection,
connect the insulation resistance
tester, and begin taking dielectric
absorption readings. Take read-
ings at 1 - minute intervals for 10
minutes. Make tests between entire
connect the insulation resistance
tester, and begin taking dielectric
absorption readings. Take read-
ings at 1 - minute intervals for 10
minutes. Make tests between entire
Table I
Curve
Condition of
Temperature
Polarization
figure 2
insulation
degrees Celsius
Index, R
10
/R
1
A
Moist
36
1.88
B
Moist
75
2.50
C
Dry
75
3.11
D
Dry
36
2.16
IEEE Standard No. 43 indicates the recommended minimum values of polarization index for a-c
(alternating current machines) are:
(alternating current machines) are:
For class A insulation - 1.5
For class B insulation - 2.0
For class B insulation - 2.0
For class F insulation - 2.0
5 (FIST 3-1 12/91)