Electromagnetic shielding

The purpose of electromagnetic shielding is to protect electronic (or electrical) installations against the formidable effects of certain electromagnetic couplings. Shielding increases the electromagnetic immunity of equipment; it is also reversible, since it can reduce the amplitude of undesirable radiation. In the face of electromagnetic interference, shielding acts as a physical boundary, isolating components sensitive to interference or confining radiating sources to a restricted volume. For a variety of reasons, mainly linked to the physical nature of the materials that make up the shielding and the technological constraints imposed by their manufacture or installation, this boundary is not totally impermeable. Residual interference can therefore penetrate the zone protected by the shielding.

As specified in the usual definitions recalled in the first paragraph of this article, shielding is assigned an efficiency. This may be an amplitude ratio or a linear parameter homogeneous with an impedance. In most cases, shielding protection is a combination of shielded cables, shielded enclosures and connectors. The following paragraphs examine the physical causes of imperfect shielding, and the methods used to calculate or measure its effectiveness. Finally, we look at the additional protection that often accompanies the reducing action of shielding, in particular filter cables, amplitude limiters and surface-effect shielding.