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2.6.5
Structural Penetrations.  A major shielding penetration is the
supporting column for ceiling or roof beams.  This can be treated by several
different methods.  For TEMPEST enclosures the best electrical isolation
between the shielding and the column is obtained by either isolating the
column within the shielding, as seen in Figure 7, or by totally enclosing the
penetrating column in the shielding material with sufficient distance or
nonconducting material provided to result in the necessary electrical
isolation as shown in Figures 8 and 9.  A minimum of 1-in. of air space is
recommended between the column and the enclosing shield.  If single point
grounding of the shield is not a requirement, the penetrating column may be
treated by continuously welding penetrating members such as reinforcing steel
or steel beams to the floor or ceiling shielding sheets.  Transition collars
or sections are typically required to weld thin shielding sheets to heavier
metal beam penetrations as shown in Figure 7.  With the large shielded
enclosures built as an outer or inner liner for the building construction, the
beam structure members are typically required to penetrate the shielding in
many places.  The design of the shielding must accommodate the expansion and
contraction of large metal surfaces pinned at the penetrations to large
structural members that are fixed in position.  Expansion joints may be
required in the shielding membrane for both vertical and horizontal motion so
that connection with heavier members don't result in cracking of welds during
thermal expansion and contracting, especially while the construction is
exposed to the exterior environment.
2.7
Conducted EMI Isolation.  As stated earlier, the shielded enclosure
must provide attenuation of both radiated and conducted electromagnetic field
energies.  Shielding surface discontinuities such as cracks, holes, poorly
tightened bolted seams, or metal piping penetrations may act as slot antennas,
loop sources or transmission lines radiating or conducting electromagnetic
energies from the inside of the shielding membrane to the outside.  The
discontinuities required in the construction of a six sided enclosure with its
necessary entrances and penetrations are the usual source of electromagnetic
leaks.  These leaks will cause differences of potential to occur on the outer
surface of the shielding. When metal penetrations such as piping and conduits
are not provided with an electrical isolating section on the exterior of the
shielding which disrupts electrical connection to possible ground returns, the
differences of potential caused by the leaks will cause currents to flow in
the attached conductors.  These induced currents may then be transported or
propagated into uncontrolled areas where unauthorized access is possible.
Where conduits carrying wiring pass from one shielded enclosure into adjacent
enclosures and pass through uncontrolled space, they should be provided with
electrically isolating sections on at least one end, and electrical filters on
each shielding penetration.  If the conduit with contained wires passes from
one shielded enclosure to another through controlled space, it may be possible
to use conduit with welded joints, no electrical filters, but a single point
ground on only one of the enclosures.
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