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of steel panels.  Where a modular, bolted enclosure is utilized, then bolt
spacings and torque requirements shall be included. Seismic restraint and
expansion joint design shall also be provided when required.  Special
precautions may be required to provide electrical isolation between a TEMPEST
EMI shielded enclosure and all structural steel and the building foundation
and framing so that the single point grounding criteria will not be violated.
When the TEMPEST isolated enclosure is required to have seismic restraint, the
anchoring means between the shielding and the structure must maintain the
electrical isolation as shown in Figure 27.  For specific guidance on TEMPEST
isolations, see Figures 4, 5, 7, 8, 13, 21, 22 and 23.
Mechanical Design.  EMI shielded enclosures require ventilation or
air conditioning depending on the enclosure heat load.  To maintain the
required shielding, the mechanical design must provide waveguide below cutoff
vents in all supply, return, and exhaust HVAC ducting which penetrates the
shield.  The vents are generally a honeycomb type arrangement of the proper
geometry and material to provide the same SE as the basic shielding material
without adversely affecting the air flow rate.  The static pressure drop
created by these vents, must be included in the HVAC calculations and the duct
air velocity must be kept low enough to minimize the acoustical level.  The
air velocity should be kept below 1000 fpm.  The vent's framework must be
attached to the shielded enclosure using a method that provides a good and
continuous electrical connection. The recommended method to attach the vent to
the shield is to continuously weld the vent framework to the shield using MIG
welding method.
Minimizing and Shielding Mechanical Penetration.  The number of
mechanical penetrations, such as piping, vents and drain penetration, should
be minimized.  The larger pipes, vents or drains may require waveguides below
cutoff inserts to maintain the required shielding effectiveness.  All
penetrations should be continuously welded to the enclosure at the point of
penetration.  To facilitate welding pipes, vents, drains or air filters to the
enclosure it may be necessary to weld a sheet of larger gauge metal to the
shielding panels at the point of penetration.  To minimize areas of potential
RF leakage, penetrations to the shield should be localized to one area, as
much as possible.  All required penetrations must be clearly identified and
detailed on the plans.
Dimensions and Materials for Shielding Mechanical Penetrations.
TEMPEST shielding requires electrical isolations of all pipes, vents, drains
and ducts within two 2 in. of the shield on the outside of the enclosure.  The
electrical isolation can be provided by inserting a section of nonconductive
material with a minimum length of 6 in. in each pipe, vent, drain or duct that
enters or exits the enclosure.
Fire Protection Design.  Fire protection design considerations in
EMI shielded enclosures include electrical filters for all conductors, special
penetration details for all pipe penetrations for the fire protection system,
and adequate emergency exits.  All conductors that penetrate the shielded
enclosure must be filtered.  Pipe penetration required for the fire sprinkler
system should be design similar to the penetrations required for the
mechanical system.  Larger than 1/2-in. diameter piping may require waveguide
attenuation filter inserts to provide 100 dB attenuation at 10 GHz.  The


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