Bender ISOMETER IRDH275, Manual

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Function
20 IRDH275_D00122_03_M_XXEN/05.2017
3.3 Product description
The ISOMETER® type IRDH275 monitors the insulation resistance of IT systems.
It is suitable for universal use in 3(N)AC, AC/DC and DC systems. AC systems
may include extensive DC supplied loads, such as converters or thyristor-con-
trolled DC drives. The device automatically adapts itself to the existing system
leakage capacitance.
Suitable coupling devices are available to extend the nominal voltage range U
n
.
The IRDH275B can be used in combination with a control and indicating de-
vice, e.g. PRC1470 version 2 or higher, on the BMS (BMS = Bender Measuring
Device Interface) bus.
3.4 Function
The ISOMETER® IRDH275 is connected between the unearthed system (IT sys-
tem) and the protective conductor (PE).
The response values and other function parameters are set via the function
keys. The parameters are indicated on the LC display and are stored in a non-
volatile memory (EEPROM) after the setting is completed.
A microprocessor-controlled pulsating AC measuring voltage is superim-
posed on the IT system to be monitored (
measuring principle*).
The measuring cycle consists of positive and negative pulses of the same am-
plitude. The period of these pulses depends on the respective system leakage
capacitances and the insulation resistances of the IT system to be monitored.
An insulation fault between the IT system and earth closes the measuring cir-
cuit. From the measured current value, the microprocessor calculates the in-
sulation resistance which is indicated on the LC display or the external kΩ
measuring instrument.
The measuring time is determined by the system leakage capacitances, the in-
sulation resistance, and the system-related interference disturbances. System
leakage capacitances do not influence the measuring accuracy.