Function
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IRDH375_D00124_05_M_XXEN/01.2020
3.4 Function
The ISOMETER® IRDH375 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.
If the reading is below the selected response values Alarm 1/Alarm 2, the as-
sociated alarm relays respond and the alarm LEDs "Alarm 1/2" light up and the
measuring value is indicated on the LC display (in the event of DC insulation
faults, the faulty supply line is indicated). If the terminals R1/R2 are bridged
(external RESET button [NC contact] or wire bridge), the fault indication will be
stored. Pressing the RESET button, resets the insulation fault message, provid-
ed that the currently displayed insulation resistance is at least 25% above the
actual response value when the reset is carried out. The fault memory behav-
iour can also be set in the "ISO SETUP" menu, by selecting the sub menu
"Memory: on/off".
The connections for external kΩ display supplied by the current output
0…400 μA or 0(4)…20 mA (IRDH375B) at M+/M- are galvanically isolated.
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