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 conducted to determine the radiated and conducted emission levels and the
radiation and conducted susceptibility levels of the equipment proposed for
installation in the site. This portion of the evaluation is the
responsibility of the sponsor and occasionally involves access to classified
information (frequency ranges, field strengths, pulse widths and rise times).
This information is usually available only to the user. The results of these
investigations shall be used to calculate the required radiated shielding
effectiveness and the required conducted attenuation. All pertinent
information should be set forth in the BESEP.
3.4
Base Electronic System Engineering Plan (BESEP). The basic document
used by SPAWARSYSCOM for planning shore electronic facilities is the BESEP.
It translates the operational requirements into a detailed technical plan to
meet the general requirements. In addition to general requirements, equipment
and facility layout and type of construction, the BESEP must clearly define
any special electrical requirements, security requirements, the EMC evaluation
and the electromagnetic (EM) shielding required. The BESEP is prepared by
SPAWARSYSCOM and with the assistance of NAVFAC if requested. It is the
responsibility of the sponsor to request that SPAWARSYSCOM prepare the BESEP
and ensure that the EMI shielding requirements and cost are included in the
Facility Study (DD 1391).
3.5
Types of Construction for EMI Shielding. The material and type of
construction should be determined during the planning phase. Various highly
conductive materials are available as the shielding material; these include
copper, aluminum or steel. The shielding effectiveness required to meet the
TEMPEST criteria can generally be constructed cost effectively with steel
sheets. The thickness of the material is usually not the most critical part
of the shield. Two layers of 26 gauge galvanized steel will generally provide
sufficient SE. The most critical part of the EMI shielding is its
discontinuities, such as penetrations, methods used to join the sheets into a
complete enclosure and the door closing and sealing means provided. The two
primary methods of construction of shielded enclosures are demountable
(bolted) and welded. For small size enclosures the bolted types are usually
less expensive. When the enclosure is large enough to require numerous
structural member penetrations, the welded type, which provides superior long
term shielding effectiveness, may become cost effective.
3.5.1
Demountable Enclosures. Demountable enclosures are constructed with
modular panels and seam plates with standard 4 ft wide panels available in 8,
10, or 12 ft standard lengths from several manufacturers. The panels consist
of 3/4-in. plywood or particle board laminated between two zinc coated 24 or
26 gauge steel sheets. The panels are connected together with a framing and
joining system that provides both support for the self-standing enclosure and
clamps the panels together with a fairly constant seam pressure that provides
a uniform electrical continuity between the panels. The joining clamps are
usually fastened together with 1/4-20 screws on 3 or 4-in. centers, torqued to
about 80-in. pounds. The screws are typically provided with blind nuts so
that the screw does not result in a full penetration of the shielding which
could deteriorate with oxidation to result in an antenna extending on both
sides of the shield.
The bolted type of the shielded construction provides good initial
shielding effectiveness when mating surfaces are clean and bright to result in
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