Designing low voltage supply systems for electromagnetic compatibility

Electrical safety of low voltage supply systems is the subject of many national and international standards. Installation rules providing electromagnetic compatibility were introduced during the last decade, but still are not well known to installers and system designers.

From the perspective of electromagnetic compatibility, there must be no operating currents, not even stray currents (as defined in IEC 60364-5-54) flowing through the earth wire and earthing and equipotential bonding system.

In an earthed supply, this requirement can only be achieved by a TN-S system. This is the only way to prevent galvanic couplings with other circuits and cable shields and to reduce the sum of currents in one cable or bus system roughly to zero, thus minimising the line frequency magnetic fields. Practical measures necessary in order to reach these objectives are discussed.