TN, TT and IT systems define how neutral and exposed parts connect to earth.
TN-S, TN-C-S, TT and IT systems define how the source neutral and exposed conductive parts are earthed. The earth fault loop impedance must be low enough to trip protection quickly. Earth electrodes need low resistance; IT systems tolerate a first fault.
Hospitals use IT systems for operating theatres.
Earthing systems determine how fault current flows and how safe a building is for people, so the system type must be chosen carefully.
TN, TT and IT systems define how neutral and exposed parts connect to earth.
Earthing systems determine how fault current flows and how safe a building is for people, so the system type must be chosen carefully.
The first letter describes the supply earth and the second describes the earth of exposed metal, with T for earth, N for neutral and I for isolated.
TT systems rely on residual current devices, because the earth loop impedance is high and the overcurrent device may not trip quickly.
Typical applications include selecting a TN-S system for a commercial building, designing a TT earth for a rural house with its own electrode and choosing an IT system for a hospital operating theatre to limit fault current.
Study "Electrical wiring and standards" first (topic 100 of 106 in Electrical Engineering). It covers the groundwork this topic builds on.
Touch and step voltages. Equipotential bonding. Earth resistivity.
Hospitals use IT systems for operating theatres.
The next topic is "Lightning and surge protection", which builds directly on this one.
It is classed as Advanced in Electrical Engineering (topic 101 of 106). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Electrical Engineering.
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