Operation Principle Of The Irdh275 - ABB ACS800 Series Hardware Manual

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Operation principle of the IRDH275

The internal structure of the IRDH275 is presented below with a simplified circuit
diagram in case of an earth fault in V-phase.
When there is no earth fault, the measuring voltage U
cause direct current to flow through earth. A resistive earth fault closes the circuit for
direct current. This direct current (I
from the total current (
calculated as follows:
where
The insulation resistance is indicated on the display or an external ohmmeter after
the response time.
Note 1: The IRDH275 reacts to all earth faults in IT systems which are galvanically
connected to each other. The earth fault detection is not based on system imbalance
but the IRDH275 detects symmetrical and unsymmetrical earth faults as well.
Note 2: The internal resistance of the IRDH275 is approximately 120 kohm. This
allows direct current to flow through the IRDH275 from phase to earth. This is why it
is not allowed to connect two IRDH275 units in the same IT system or in systems
galvanically connected to each other. The IRDH275 would react to the other
IRDH275 as to an earth fault.
Earth fault monitoring in IT (ungrounded) systems (+Q954)
Bender
IRDH275
Earth fault in V-phase
) flowing through the IRDH275. The fault resistance is
i
M
U
m
R =
- R
f
i
I
m
R fault resistance
=
f
U measuring voltage
=
m
I filtered direct component of i
=
m
R internal resistance (120 kohm) of the Bender
=
i
i
m
R
i
U
m
) caused by the measuring voltage is filtered
m
(equals to the current flowing through the earth fault)
M
Earth fault
R
f
(direct voltage) does not
m