What is the key feature of an unearthed system?
As input to unearthed 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 earth with a low resistance, no high fault current flows in the event of a short circuit to exposed conductive part or an earth 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 earth.
As required by the standards, an insulation monitoring device is mandatory in an IT system.
What is an IT system?
In TN systems, the star point of the supplying transformers is connected to earth through a low impedance and the exposed conductive parts of the electrical installation are connected to the protective earth (PE) of the system. In TT systems, the star point is also connected to earth through a low impedance but the exposed conductive parts of the electrical installation are earthed independently from the system earthing. In IT systems, however, all active parts are insulated from earth or connected to earth through a high impedance. The high impedance can be applied for metrological reasons, provided that electrical safety is not jeopardised. The earthing of the exposed conductive parts of the electrical installation is carried out either individually or collectively.