The Basics of Electrical Shielding

Electrical shielding is the imposition of a metal or composite barrier between one or more sources of electrical noise and their victims with the objective of reducing or eliminating electrical interference. Examples of the barrier are the case or housing of equipment; shields covering interconnecting cables between equipment; large cabinets, racks, or consoles; shielded (screen) rooms; and entire shielded buildings or vehicles.

The principal measure of a shield's performance is the shielding effectiveness. It's defined by the equation below where SE_dB

SE_dB = 20 log₁₀ (F_b/F_a)

is the shielding effectiveness in decibels, F_b is the electric (or magnetic) field strength before imposition of the barrier, and F_a is the electric (or magnetic) field strength after imposition of the barrier.

Shielding is obtained by the combination of reflection loss and absorption loss. The former is due to the impedance mismatch between the wave impedance of an oncoming wave and the surface impedance of the interposed barrier. Absorption loss corresponds to the attenuation due to the skin effect at higher frequencies, and is dependant on the frequency, conductivity, permeability, and thickness of the barrier. Shielding effectiveness is the sum of both losses.

Most intentional shields are made of metal to ensure high reflection losses. Even thin metals, such as household aluminum foils (with a thickness of about 1.5 mils or 0.038 mm), offer shielding effectiveness in excess of 100 dB. At low frequencies, these foils become electromagnetically transparent (i.e., they don't attenuate magnetic fields). Thus, if there is a shielding problem due to the selection and makeup of the metal barrier, it's likely to occur only for low-frequency magnetic fields.

To obtain significant shielding to magnetic fields at low frequencies, the metal barrier must be very thick or composed of a highly-permeable material such as Mumetal, Supermalloy, or Hypernom. Often such shields are fabricated in two or more layers, frequently laminated, to obtain good shielding properties per unit of weight.

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