S9407-AB-HBK-010 Datasheet, PDF (160/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
7.3.2.1
Example Demonstrating Shielding Principles as Applied to Enclosure
Requirements
The following example of an interference problem illustrates a typical requirement for a
shielding enclosure.
A sonar system shows that interference is present in the signal pass-band. The interference
is 40 dB above the equipment design performance sensitivity, seriously degrading operational
capability. Tests show that external interference is magnetically induced into the system via a brass
junction box having a wall thickness (t) of 11 gauge (0.120 inch).
From figure 7-1, at a frequency of 2 kHz, the skin depth (δ) for brass is 0.115 inch. From
equation (3) of paragraph 7.3.2 the attenuation provided by the brass junction box is
A (dB) = 8.686 X( ^0.120 ) = 9.06 dB
0.115
A junction box made of some other material which provides an attenuation of 40 dB greater
than that provided by the brass junction box is required for adequate shielding. Hot- or cold-rolled, 11-
gauge sheet steel is readily available, and is of the minimum allowable thickness approved for
construction of shielding enclosures.
From figure 7-1, at a frequency of 2 kHz, the skin depth (δ) for hot- or cold-rolled steel is
approximately 0.010 inch. From equation (3) of paragraph 7.3.2 the attenuation provided by a steel
junction box having a wall thickness of 11 gauge (0.120 inch) is
A (dB) = 8.686 X( ^0.120 ) = 104 dB
0.010
This steel junction box will provide more shielding than that required to protect the sonar
circuits. The 104 dB of attenuation is in excess of the 9.06 dB provided by the brass junction box and
the 40 dB of interference by the amount determined as
Excess A (dB) = 104 dB -(9.06 dB + 40 dB) = 55 dB .
7.3.2.2 Materials for Enclosures
A comparison of the shielding effectiveness in terms of "skin depth" of various ferrous and
nonferrous metals is presented in figure 7-1. Note that stainless steel and monel are not effective
shields for the frequencies plotted, and brass, aluminum, and copper are undesirable as shielding
materials against 60-Hz and 400-Hz power frequencies. Permalloy 45 and mumetal are more
effective as shields against low frequency magnetic fields than unannealed low-carbon steel of the
same thickness. Other materials not shown in figure 7-1, such as Permalloy 78.5 (78.5% Ni, 21.5%
Fe) and Hipernom (80% Ni, 4.2% Mo, 15% Fe) possess skin depth characteristics between the values
shown for Permalloy 45 and mumetal. These materials may also be used for constructing shielding
enclosures. However, the shielding effectiveness of these medium- and high-permeability alloys is
affected by mechanical shock. If an enclosure made of one of these annealed metals is machined,
dropped, or otherwise subjected to mechanical shock, the enclosure must be reannealed. Therefore,
in light of the difficulty in assuring protection from the hazards of mechanical shock, the use of
enclosures made from these materials is discouraged.
Of the shielding materials noted in figure 7-1, mumetal and low-carbon steel shall be used in
ships. Of the two, low-carbon steel (hot- or cold- rolled) is preferred because it is not as shock-
sensitive as mumetal.
7-5