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low resistance in seam closures and joints.  With proper tensioning and
torqueing of bolts and penetrations, this type of shielding construction is
usually able to provide the required 100 dB performance required by NSA 65-6,
National Security Agency Specification for RF Shielded Enclosures for
Communications Equipment:  General Specification.
However, in a bolted seam closure, the metal to metal contact occurs
on high points (on a microscopic basis).  With time, these points can oxidize,
especially when the seams are not sealed to prevent exchange of air, and there
is a resultant degradation in shielding performance.  The use of RF mesh
gasket materials under seam or penetration closures normally results in faster
degradation than the seam closures, and their use should be prohibited in the
specifications.  Two types of bolted penetration panels which provide for
welded or brazed piping or conduit penetration treatment are shown in Figures
3 and 4.  This same type of bolted penetration panel should be used for the
single entry ground stud penetration, seen in Figure 14.
3.5.2
Welded Enclosures.  This type of construction is usually employed
with TEMPEST shielding requirements.  For small vaults a 1/4-in. steel plate
satisfies Defense Intelligence Agency (DIAM) 50-3, Physical Security Standards
for Sensitive Compartmented Information Facilities, and often is combined with
a larger bolted enclosure.  For enclosures larger than about 1,000 ft2 the
welded enclosure looks more cost effective, and can be free standing, isolated
on all six sides with dielectric underlayment and structural hangers, or can
be integrated into the building beam and column structures.  Where the BESEP
dictates electrical isolation and single-point ground for TEMPEST requirements
the welded enclosure should of the free standing design with penetrating
columns enclosed in shielding as seen in Figure 8 and with a tubular steel
backing for all six sides of sheet steel (typically 14 gauge or thicker).  The
tubular steel framework is typically laid out on a 4 by 4 ft grid, or 2 by 4
ft, corresponding to 4 by 8 ft sheets of steel, to control warping and
buckling of the welded steel sheets, and to provide structural rigidity for
wall and ceiling.  Tubular frame members are typically 3 by 3 or 4 by 4 in.,
1/8-in. thick steel.  The sheet steel seams should be continuously welded
using MIG welding to prevent voids, gas pockets and incomplete fusion, and to
avoid electrical discontinuities in the seams.  All welds should be backed up
with steel, either with overlapping of the sheets, welding against a tubular
frame member, or against a backing strip of equal thickness to the sheets, in
order to minimize possible electromagnetic leaks.  Examples of seam
configurations, column and beam junctures, floor-wall intersections, column
tops and bases, are seen in Figures 7 through 9 and Figures 16 through 25 for
both welded and bolted construction.  Examples of several types of bolted and
welded seam and door closures are seen in Figure 1.  Floor drain and wall
penetrations with a waveguide insert attenuator disc are seen in Figures 3, 5
and 6.
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