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.