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BENDER ISOMETER® IRDH275
Insulation monitoring device for unearthed
AC, AC/DC and DC systems (IT systems)

Product description
The BENDER ISOMETER® of the IRDH275 series is designed to monitor the insulation resistance of
unearthed main circuits (IT systems) AC, AC/DC 0…793 V resp. DC 0…650 V. The AMPPlus
measurement method meets the particular requirements of modern power supplies
which often include rectifiers, converters, thyristor-controlled DC drives and directly connected
DC components. In these systems often high leakage capacitances against earth occur
due to interference suppression measures. The IRDH275 automatically adapts itself to the
existing system conditions.
In combination with a coupling device, the devices can also be used for higher voltages.
An external supply voltage allows de-energised systems to be monitored too. For door
mounting into distribution panels, refer to type IRDH375(B).
Application
Function
When the insulation resistance between the system conductors and earth falls below the
set response value, the alarm relays switch and the alarm LEDs light up. Two separately
adjustable alarm relays allow to distinguish between prewarning and alarm. The measured
value is indicated on the LC display or an externally connectable measuring instrument.
In this way any changes, for example when circuits are connected to the system, can be
recognised easily. The fault message can be stored. The fault memory can be reset by
pressing the reset button. By pressing the test button, the function of the device as well
as the connections to system and earth can be tested. Pressing the Info button provides
additional information, such as the existing system leakage capacitance or device settings.
The function of the device and the system and earth connections are continuously monitored.
When a fault occurs, the system fault relay switches and the alarm LED “system fault”
lights up. The parameterisation of the device can be carried out via the LC display or the
function buttons integrated in the front plate.
Device version IRDH275B
Device version IRDH275B includes the following additional functions:
• History memory with real-time clock to store all alarm messages with date and time stamp
• Electrically isolated RS-485 interface (BMS protocol) for communication with other
Bender devices
• Isometer disconnecting relays for the operation of several ISOMETER®s in coupled
IT systems
• Current output 0(4 )…20 mA (electrically isolated)
Use in coupled IT systems
Only one ISOMETER® may be active when several IT systems are coupled. Isometer disconnecting
relays and the control inputs F1/F2 integrated in version IRDH275B guarantee
that only one ISOMETER® is active at any one time.
Measurement method
The IRDH275(B) uses the patented AMPPlus measurement method. This
measurement method allows concise monitoring of modern power
supply systems, also in case of extensive, directly connected DC components and high
system leakage capacitances.
Standards
The ISOMETER® of the IRDH257 series complies with the requirements of the device standards:
DIN EN 61557-8 (VDE 0413-8), EN 61557-8, IEC 61557-8, IEC 61326-2-4, DIN EN 60664-1
(VDE 0110-1), DIN EN 60664-3 (VDE 0110-3), ASTM F1669M-96 (2007), ASTM F1207M-96 (2007)

Device features
• Insulation monitoring for unearthed AC,
AC/DC systems 0…793 V, DC 0…650 V
• Nominal voltage extendable via coupling
device
• Two separately adjustable response values1
kΩ…10 MΩ
• AMPPlus measurement method
• Automatic adaptation to the system
leakage capacitance
• Info button to display device settings
and system leakage capacitance
• Self monitoring with automatic alarm
• Automatic self test, selectable
• Connection for external kΩ indication
• Test and reset button
• External test/reset button can be connected
• Two separate alarm relays with two
potential-free changeover contacts
• N/O or N/C operation, selectable
• Backlit LC display
• RS-485 interface

• AC, DC or AC/DC main circuits
• AC/DC main circuits with directly connected
DC components, such as rectifiers,
converters, and thyristor-controlled
DC drives
• UPS systems, battery systems
• Heaters with phase control
• Installations including switch-mode
power supplies
• IT systems including high leakage
capacitances
• Coupled IT systems