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(Tussman s Law)
To protect a harness containing several wire pairs against radiated interfering signals, a tin-plated braided shield is used, made of copper-clad steel. The mass proportion of various metals in the braid are steel 60%, copper 37%, and tin 3%. The thickness of the braid is t = 1 mm. The braid is 80% solid material and 20% air (by volume). When slipped over the harness, it has an average radius of 10 mm. Estimate the magnetic shielding effectiveness at 50 Hz and 100 Hz.
Solution
Calculation. Since we are dealing with low-frequency magnetic fields, the shielding effectiveness corresponds to the equation of case 1b of Table 9.3, which is recalled here:
The problem here is to evaluate the equivalent conductivity and permeability of the braid, which is an inhomogeneous material.
The equivalent relative permeability of the braid (with respect to copper) is calculated taking into account the proportions of various materials:
As the permeability of air is negligible, the last term of the sum can be discarded:
Table 9.1 gives Cu = Sn = 1, Fe = 1000 and upon substituting these values in the latter expression, we find
In a similar way, the equivalent relative conductivity (with respect to copper) is
According to Table 9.1 ? Fe = 0.1, ? Cu = 1, and ? Sn
