S9407-AB-HBK-010 Datasheet, PDF (161/276 Pages) Glenair, Inc. – HANDBOOK OF SHIPBOARD ELECTROMAGNETIC SHIELDING PRACTICES

S9407-AB-HBK-010, Rev. 2
7.3.3 Low-Carbon Steel Enclosures
Low-carbon steel enclosures shall be fabricated from either hot- or cold-rolled steel, in
accordance with Federal Specification QQ-S-698. The thickness of the steel shall be a minimum of
11 gauge (0.1196 inch) using the manufacturers' standard gauge for sheet steel. Annealing of low-
carbon steel enclosures is not required. A good example of a steel enclosure is presented in figure 7-
2 (sheets 1 through 8) which is a copy of drawings 2630-102-01 and 2630-102-02 entitled "Terminal
Box for Shielded Applications." This greatly reduced copy is included for reference purposes rather
than for construction.
7.3.4 Mumetal Enclosures
Figure 7-1 shows that mumetal enclosures will provide greater shielding effectiveness than
low-carbon steel enclosures of the same thickness. Mumetal enclosures can provide the same
shielding effectiveness as low-carbon steel enclosures but with less metal thickness. The thickness of
a typical mumetal enclosure is 0.062 inch for the box and 0.075 inch for the cover. The shielding
effectiveness of mumetal is extremely sensitive to shock. If a mumetal enclosure which has been
annealed is later formed, dropped, machined, or otherwise subjected to mechanical shock, the
enclosure must be reannealed to restore its shielding effectiveness.
An approved design for mumetal terminal boxes can be obtained from BUSHIPS Drawing No.
SSB(N)616-404-2091704, "Mumetal Connection and Pull Boxes; Assembly and Details."
7.3.5 Annealing of Mumetal Enclosures
To develop optimum magnetic properties, mumetal must be annealed after all fabricating
operations have been completed. Contamination by oxygen, sulfur, or carbon must be prevented
during the annealing cycle. In specific applications in which a part cannot be fully annealed due to
physical reasons, a compromise treatment may be given. Less than full annealing, however, will
obviously result in less than optimum magnetic properties. The method of annealing in a dry
hydrogen atmosphere is presented in the following paragraphs.
7.3.5.1 Preparation for Heat-Treatment of Mumetal
a. Complete all mechanical operations (such as drilling, punching, grinding) prior to
annealing.
7-6