As you know three earthing systems are used such as defined in IEC 60364 are:
- Exposed-conductive parts connected to neutral -TN-
- Earthed neutral -TT-
- Unearthed (or impedance-earthed) neutral -IT-
The purpose of these three systems is identical as regards protection of persons and property mastery of insulation fault effects. For choosing the best grounding system, it is important to know our requirements and resources:
- need for continuity of service,
- Whether or not there is a maintenance service,
- Fire hazard.
-continuity of service and maintenance service: the IT will be chosen,
-continuity of service and no maintenance service: no fully satisfactory solution: prefer the TT whose discrimination on tripping is easier to implement and which minimizes damage with respect to the TN.
The installation of additional output is easily achieved without the necessity of further calculations.
- Continuity of service not essential and component maintenance service: prefer the TN-S (rapid repairs and extensions performed according to rules),
-continuity of service not essential and no maintenance service: prefer the TT,
- Fire hazard: IT if maintenance service and use of 0.5 A RCD or TT.
Allow for the special features of network and loads:
- Very long network or, even more important, leakage current: prefer the TN-S,
- use of replacement or standby power supplies: prefer the TT,
- Loads sensitive to high fault currents (motors): prefer the TT or IT,
- Loads with low natural insulation (furnaces) or with large HF filter (large computers): prefer the TN-S,
- Supply of control and monitoring systems: prefer the IT (continuity of service) or the TT (enhanced equipotentiality of communicating devices).