HT instruments HT7052 - Manual (Page 15)

HT7052
EN - 13
POLARIZATION INDEX (PI)
PI is the ratio of Insulation Resistance values measured after 1 minute and after 10
minutes. The DC test voltage is present during the whole period of the measurement (an
Insulation Resistance measurement is also running). On completion of the test the PI ratio
is displayed:
 
 
min 1
min 10
R
R
iso
iso PI 
General applicable values:
PI value Tested material status
1 to 1.5 Not acceptable (older types)
2 to 4 (typically 3) Considered as good insulation (older types)
>4(very high insulation resistance) Modern type of (good) insulation systems
When determining Riso (1min) pay close attention to the capacitance of the object under
test. It has to be charged-up in the first time section (1 min). Approximate maximum
capacitance using:
 
 
 
V
s
F
U
t
C
10 3
max
.
 
where:
t = period of first time unit (e.g. 1min)
U = test voltage
To avoid this problem, increase the DIAG. Starting time parameter in CONFIGURATION
menu, because start of timer in the DIAGNOSTIC TEST functions depends on the test
voltage. The timer begins to run when test voltage reaches the threshold voltage, which is
product of the DIAG. Starting time and nominal test voltage (Unominal)