Industry

These characteristics are met by an IT distribution system type. IT power supply systems are isolated from the ground and they go into a grounded status in the event of an insulation fault (becomes a TN type system). That is, where standard grounded system would stop supplying power (there would be a short circuit and, as a result, a response of the fuse element), IT system only becomes a grounded network (so-called first fault case) and still supplies energy. Only in the event of another insulation fault (i.e., second fault) the power supply is disconnected into the overcurrent. Therefore, it can be said that IT power supply systems are more reliable than TN networks in terms of energy supply continuity. More about this issue is described in the standard HD 60364-4-41:2017.

In order to clearly define whether the network is still isolated (it is still IT type) or is already in the event of the first fault, special measuring devices are used - insulation monitoring devices.

These are active devices that monitor the current status of the insulation resistance between the power supply system and the reference point (in industry usually PE conductor - potential of the earth). If the insulation monitoring device detects a drop in the insulation value below the set limit, it reports a fault. Signalling relays are usually used for this report. Each insulation monitoring device is equipped with signalling relays.

The response to such a detected fault is then individual and depends on the specific application. In practice, the principles of maintenance scheduling, operation limitation, or direct immediate system disconnection from the power supply are usually used. However, these conditions depend on the application and should be subject to local operating regulations.