The IT system earthing (unearthed neutral) in LV

Specific earthing system

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, maintenability 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.

Protection of persons against electric shocks

Use of Safety by Extra Low Voltage (< 25 V) -SELV- is the most drastic solution since it eliminates the electrical hazard. However it is applicable only in low power distribution. Regarding everyday use of electricity, a number of studies have identified the causes of electric shocks and provided specific solutions for each one.

Electric shocks have two causes, namely:

• direct contact, i.e. a person or an animal touching an exposed live conductor;
• indirect contact, i.e. a person touching the metal frame of an electrical load on which an insulation fault has occurred.

Protection against direct contact

To provide protection against direct contact, insulation and/or distancing measures are taken. These measures can be reinforced in final distribution by additional protection in the form of a high sensitivity Residual Current Device (RCD).

Protection against indirect contact

With respect to protection against indirect contact, between an accidentally energised frame and the earth, the basic solution is to earth all the load frames via the protective conductors. However, this measure does not rule out the existence of a contact voltage hazardous for persons if it exceeds the conventional limit safety voltage UL defined by standard IEC 60479. This contact voltage depends on the earthing systems defined in the international standard IEC 60364.

AUTHOR: Schneider Electric experts | François JULLIEN, Isabelle HERITIER