Today, continuity and safety of electricity supply operation also apply to industrial applications, where any unplanned shutdown can cause significant economic losses. For these reasons, the most durable solution of electricity distribution is sought with the maximum possible level of security against fault and at the same time with the possibility of preventive maintenance..
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.
