Substation auxiliary circuits often operate as unearthed IT systems, in which the first earth fault does not disconnect the supply but remains unknown until a second fault. The RN 5897/010 insulation monitor of the VARIMETER IMD family continuously measures the insulation resistance of AC, DC and mixed systems against the protective conductor, signals the drop at a pre-alarm and an alarm threshold and, with a coupling device, also covers systems with higher voltages. The trigger output starts the fault location system.
The auxiliary supply and control circuits are supervised by the MK 9300N multifunction measuring relay: phase sequence and phase failure, asymmetry, voltage outside the window, current, frequency and power factor in one module with a display. Both products have test data according to the DNV guidelines in their datasheets, which confirms resistance to vibration, humidity and interference in switchgear.
Selection starts with the voltage of the monitored system: it decides whether the insulation monitor works directly or through the RL / RP 5898 coupling device, and the leakage capacitance of the system determines the response time. For the measuring relay the circuit current is checked against the current path of the module and a version with one or two relay outputs is chosen.
The monitoring offer is completed by the RN 9877 phase monitor, supplied from the monitored phases without auxiliary voltage, the UG 9400 measuring relay with Modbus RTU interface, which passes measurements and alarms to the supervisory system, and the UH 9830 battery symmetry monitor for standby supply strings.
