IRDH375B-435 Bender

ISOMETER® IRDH375 Insulation monitoring device for unearthed 
AC, AC/DC and DC systems (IT systems)
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 values 
1 kΩ…10 MΩ 
• AMPPlus 
measurement method
• Automatic adaptation to the system 
leakage capacitance
• Info button to display device settings 
and the system leakage capacitance
• Continuous self monitoring, with automatic
 alarm message
• Automatic self test, selectable
• Connection for external kΩ indication
• Test and reset button 
• Connection external test and reset button
• Two separate alarm relays with two 
potential-free changeover contacts 
• N/O or N/C operation
• Alarm relay for system fault (N/C operation)
• Backlit LC display
• RS-485 interface
• Plug-in terminals 
Approvals
for IRDH375…-4…5
Product description
The ISOMETER® of the IRDH375(B) 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 IRDH375(B) 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.
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 earth connections are 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 IRDH375B
Device version IRDH375B 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
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 IRDH375(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 IRDH375 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)
• AC, DC or AC/DC main circuits
• AC/DC main circuits with directly 
connected DC components, such as 
rectifiers, converters, and thyristorcontrolled
 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