The IT system earthing (unearthed neutral) in LV

Although all Earthing Systems offer users the same degree of safety, they do not all have the same operating characteristics. This is why, in certain countries, a specific earthing system is stipulated by legislation or standards according to buildings. For example, in France the IT system is compulsory in hospital operating theatres, and the TN-C is forbidden in premises where there is a risk of explosion. These stipulations apart, dependability objectives (safety, availability, reliability, maintainability and proper operation of low current communication systems) determine which earthing system should be chosen for which installation. The aim of this “Cahier Technique” is to describe the advantages and areas of application of the IT earthing system. After a brief introduction of the electrical hazard and the various earthing systems, the first fault situations, followed by the double fault specific to the IT system, are studied, and the advantages and disadvantages of this particular earthing system are developed. This “Cahier Technique” also offers solutions for the surge limiter with the various types of possible overvoltages. Finally, a choice table is provided for all earthing systems, based on criteria for safety, availability, electromagnetic compatibility and operators’ professional requirements.