Physical links of shielded cables to terrestrial networks

The question of the connection protocols of an installation or electronic equipment to ground networks is often highly controversial when it comes to choosing the protection strategy adopted against electromagnetic disturbances. In other words, should an installation, and in particular a shielded cable, be connected to earth at a single point or at several points? In view of the extreme diversity of situations encountered in practice, this supplementary text to the [D 1 320] dossier devoted to electromagnetic shielding endeavors to clarify this difficult question on the basis of deliberately simple examples.

After a review of the properties of the earth line and its links with the power supply network, we look at the impact of different types of disturbance produced by magnetic fields, electric fields and, in other cases, by undesirable currents flowing in the ground.

To make it much easier to discern the effect of ground connections on the amplitude of residual voltages collected at the ends of a shielded cable, disturbances are reduced to point sources with sinusoidal amplitude variations as a function of time. The use of equivalent electrical circuits simplifies the physical analysis, from which we extract a few formulas containing the most influential parameters in terms of electromagnetic compatibility. This approach is then perfected by taking into account the contribution of propagation phenomena.

The use of electromagnetic reciprocity allows us to analyze the behavior of shielded cables that produce residual signals.

A reminder of some elements of transmission line theory should make it easier to analyze the synthesis of equivalent circuits and to take propagation phenomena into account.