In health sectors and heavy industries, it is essential to monitor the ungrounded IT networks' status to make sure that network is safe and operating continuously. The insulation condition of the IT network is monitored by the insulation monitoring devices (IMD), which at an event of any insulation resistance decrease below the pre-selected Rcrit level informs the operators by acoustic or optical alarms.
Industrial and commercial sectors that can use the insulation monitoring devices (IMD) offered by Hakel in ungrounded IT power supply systems are as follows:
Underground and surface mine
IT Power Supply Network
An insulated system called IT power supply system has all active components isolated from the earth or one network point grounded via high impedance. Components of the electrical installation which are inactive are grounded. Reliable operations and human safety can be guaranteed with an ungrounded system. It is therefore used in above mentioned industrial and commercial sectors. The main benefit of the ungrounded system is that the device connected to this system remains operational even after the first fault (so-called earth fault). The phase voltage of the undamaged phase (or phases in the three-phase system) to the value of the delta voltage at the time of first fault. If the components that are inactive are properly grounded, this system can work safely. There will no occurrence of bigger than safe current levels. In this case, information about the failure must be given to the relevant responsible person and the first fault must be eliminated immediately. The power supply system must be immediately disconnected after the second fault (double earth fault). The ungrounded system is monitored by the insulation monitoring devices or residual current relays, which indicate decrease in insulation level below the set value.