What is an IT system?
In TN systems, the star point of the supplying transformers is connected to ground through a low impedance and the exposed conductive parts of the electrical installation are connected to the protective ground (PE) of the system. In TT systems, the star point is also connected to ground through a low impedance but the exposed conductive parts of the electrical installation are grounded independently from the system grounding.
In IT systems, however, all active parts are isolated from ground or connected to ground through a high impedance. The high impedance can be applied for metrological reasons, provided that electrical safety is not jeopardized. The grounding of the exposed conductive parts of the electrical installation is carried out either individually or collectively.
What is the key feature of an ungrounded system?
As input to ungrounded IT systems, either a transformer or an independent power source, such as a battery or a generator are used.
Since no active conductor is connected to ground with a low resistance, no high fault current flows in the event of a short circuit to exposed conductive part or a ground fault.
The result is a low fault current the size of which depends on the insulation resistances and the capacitance of the conductors and system components to ground.
As required by the standards, a ground-fault monitoring device is mandatory in a floating system.