System earthings worldwide and evolutions

Review of standardised earthing systems

Electrical power was actually used in 1900. Today electrical installation standards are highly developed and cover all major aspects for a safe installation. In LV, the reference standard is IEC 364 (see appendix no. 1). Standard makers have paid particular attention to the measures to be implemented to guarantee protection of personnel and property (part 4 of the above-mentioned standards).

This concern has resulted in the standardisation of three Earthing Systems. Before reminding readers of these three systems, a concise historical review will certainly be of use.

General

In all industrialised countries, LV networks and loads are earthed for safety reasons to guarantee protection against electric current for persons. The objectives are always the same:

• fixing the potential of live conductors with respect to the earth in normal operation;
• limiting voltage between the frames of electrical equipment and the earth should an insulation fault occur;
• implementing protection devices which remove the risk of Electric Shocks or electrocution of personne;
• limiting rises in potential due to MV faults.

Influence of MV earthing systems

While the first three objectives listed above fall into the range of LV earthing systems, the fourth has considerable repercussions on safety of personnel and property in LV. Thus, at MV/LV substation level, a MV phase/frame fault or a fault between MV and LV windings may present a risk for equipment and users of the LV network.

In public and industrial MV, except in certain special cases, the neutral is not distributed and there is no protective conductor (PE) between substations or between the MV load and substation. A phase/earth fault thus results in a single-phase short-circuit current limited by earth connection resistance and the presence of limitation impedances, if any (zero sequence generator).

AUTHOR: Schneider Electric experts | Bernard LACROIX, Roland CALVAS