Just one central earthing point (CEP) - and how do I deal with multiple feedings?

Indeed this is presently one of the open gaps in the new EMC strategies of EN standards that standardisation boards are now striving to fill in. A dedicated DKE working board is presently dealing with this special issue. It used to be common practice and was even compulsory to earth the star connection of any transformer or generator feeding into a low voltage supply system. In case of multiple feedings each of them was earthed »via the shortest possible connection to the nearest bonding bar or similar device«, as the codes wanted it.

Under the heavy impacts of return currents not only from unbalanced but prevalently from harmonic loads, together with the sensitivity of the electrical appliances fed from such system today, this turns out not to be best practice any more. When there are multiple connections from the neutral to the protective / earthing / bonding conductors, then a substantial part of the return currents will flow via the earthing system instead of taking the dedicated route along the neutral line. This must be avoided for several reasons. Therefore the wiring scheme for a multiply fed system was decided to be modified in EN 50174-2, edited Sept 2001. However, erratically the old picture with separate star point earthing had been included again. Only the errata sheet EN 50174-2:2002-03 provides the correct picture. Same practice is presently being suggested to include in IEC 60364-3.

Stefan Fassbinder

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