or bolted construction. The unused method should be
deleted from the project specification. Welded
construction will usually consist of continuous
1.897 mm (14 gauge) thick steel plate and angles to
form the enclosure. Thicker material may be used if
it is more cost-effective or required for structural
reasons. Welded construction is used when a
shielded facility requires a long maintainable
service life of high-level protection, 100 dB
attenuation, or HEMP protection, 100 dB. Bolted
construction is associated with a lower level (50
dB) of maintained shielding effectiveness. Bolted
construction will usually consist of modular panels
bolted together with metal strips or channels.
Panels are commonly plywood with steel sheets
laminated to one or both sides. Bolted construction
is used when a shielded facility's service life is
short, 10 years or less, or the system is required
to be demountable for change of location. This
system requires more maintenance than a welded
system and requires access to the panels. The EM
shield layout may restrict attenuation testing of
the enclosure. It is desirable for large facilities
to place the shield at least 1 meter (3.3 feet)
inside the exterior walls, although cost and
construction restrict this consideration. The floor
shielding can be tested by SELDS test but not by
IEEE Std 299 if it is on grade. The facility layout
must be carefully planned to allow for testing and
shield maintenance.
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The EM shielded enclosure shall be installed in accordance with this
specification, the drawings, and the recommendations of the manufacturer
and EM shielding specialist. Shielding steel shall be handled and
installed without damage to the shielding steel. Penetrations of the
shield, other than those indicated on the drawings, will not be permitted,
including fasteners and mounting bolts, without prior written authorization
from the Contracting Officer.
3.2.1
Surface Preparation
Contacting surfaces shall be cleaned and buffed to ensure firm contact with
shielding steel.
3.2.2
Control of Warping
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NOTE: Steel plates exposed to sunlight and changing
environmental conditions increase warpage and
buckling.
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Warping of steel shielding plates during installation and welding shall be
kept within 1 mm in 1 meter 1/8 inch in 10 feet. Embeds, drive pins,
and/or anchor bolts or ties shall be used to hold plates in place during
welding. Other techniques such as skip welding shall also be used to
reduce warpage. The system chosen shall be fully coordinated and approved
by the Contracting Officer. Fasteners, drive pins, and other shield
SECTION 13095A
Page 45
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