Coupling of magnetic fields from power cables to grounding loops

The coupling of magnetic fields from three-phase power cables to loops in the grounding system is investigated. These loops are formed on one side by the protective earth or safety conductor of the power cable and on the other side by the grounding system or equipotential bonding to which the safety conductor is connected. The inductive coupling between the current carrying conductors and the safety conductor results in induced voltages in this conductor and therefore also in the loops. This is even true in the case of the same absolute values of current in all the three phases. The induced voltages may in some cases lead to significant currents in these loops and therefore also to significant magnetic field strengths which may disturb sensitive equipment. The induced voltage and the resulting current is shown to be strongly dependent on the geometry of the cable safety conductor with respect to the current carrying conductors. Recommendations are made for the type of power cable to be used in installations where stray magnetic fields could be problematic