The Insulation Monitoring Relay IR195 measures the network insulation resistance of ungrounded DC current networks to earth. Symmetrical earth faults are not detected, however this is of no significance for the fault conditions normally encountered. Threshold value RAN is factory set and non-variable. The IR 195 works according to the contact opening principle. Application of the aux. voltage causes the output relay to switch to the work position and a LED marked "ON" is lit, indicating that network insulation resistance RE>RAN. If the network insulation resistance drops below RAN the LED (ON) extinguishes, another LED marked "earth fault" is lit and the output relay deenergizes. With a jumper between terminal "1" and "2" IR 195 is programmed for memory, without link for hysteresis function. This link may be replaced by an external NC contact reset switch Correct functioning of the relay can be verified with a built-in test key.