Function
15
isoEV425_D00126_09_M_XXEN/03.2019
3.2
Functional description
The ISOMETER® measures the insulation resistance
R
F
and the system leakage
capacitance
C
e
between the system to be monitored (L1/+, L2/-) and earth
(PE). The RMS value of the nominal system voltage
U
n
between L1/+ and L2/-
as well as the residual voltages
U
L1e
(between L1/+ and earth) and
U
L2e
(be-
tween L2/- and earth) are also measured.
From a minimum value of the nominal system voltage, the ISOMETER® deter-
mines the faulty conductor L1/+/L2/-, which shows the distribution of the in-
sulation resistance between conductors L1/+ and L2/-. The distribution is
indicated by a positive or negative sign preceding the insulation resistance
measurement. The value range of the faulty conductor is ±100 %:
The partial resistances can be calculated from the total insulation resistance
R
F
and the faulty conductor (R %) using the following formula:
Also from a minimum value of the nominal system voltage, the ISOMETER® de-
termines the insulation resistance
R
UGF
from the residual voltages
U
L1e
and
U
L2e
. It is an approximate value for one-sided insulation faults and can be used
as a trend indicator in cases where the ISOMETER® has to adapt to an
R
F
and
C
e
relation that varies considerably.
It is possible to assign the detected fault or the faulty conductor to an alarm
relay via the menu. If the values
R
F
or
U
n
violate the response values activated
in the "AL" menu, this will be indicated by the LEDs and relays "K1" and "K2"
according to the alarm assignment set in the "out" menu. In addition, the
operation of the relay (n.o./n.c.) can be set and the fault memory "M", activat-
ed.
Display
Description
-100 % one-sided fault at conductor L2/-
0 % symmetrical fault
+100 % one-sided fault at conductor L1/+
Fault at conductor L1/+
R
L1F
= (200 % *
R
F
)/(100 % + R %)
Fault at conductor L2/-
R
L2F
= (200 % *
R
F
)/(100 % – R %)