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Piping and Conduit Penetrations.  Numerous penetrations in an
enclosure are required for electrical conduits (power, telephone, alarm,
etc.), and for mechanical piping (chilled water, fire sprinkling vents and
drains).  The TEMPEST requirement for an electrically isolating section six
inches in length, within two inches from the shielding surface tends to
complicate the selection of the penetrating piping materials.  Schedule 80 PVC
with standard fittings are readily available with reasonable temperature and
pressure ratings (2 in., 295 psi at 68 deg. F, or 51 mm, 2034 KPa at 20 deg.
C, for example).  For higher temperatures, schedule 80, PVDF can be used
(rated at 300 deg. F or 149 deg. C).  The penetration through the shielding
must be metal, and continuously bonded electrically around the periphery of
the penetration to the shielding (welded or brazed) as seen in Figures 3 and
Clamped or threaded penetrations, and those using RF gaskets are not projected
to provide long term electrical continuity because of oxidation and corrosion
of closure contact surfaces under normal environments.  These nonpermanent
types of penetrations should be prohibited in the written specifications.
The metal piping penetrations will provide waveguide below cutoff attenuation
for frequencies below their cutoff frequency.  For piping larger than 1/2 in.
(12.7 mm) I.D. and with NSA 65-6 requirements for attenuation at 10 GHz, a
waveguide filter insert may be required, as seen in Figure 5.  Where the
piping has continuously welded, brazed or soldered joints in the run from the
penetration to the equipment within the shielded enclosure, or where the
piping has tight threaded couplings, the filter insert may not be needed.  For
short runs to the penetrations typical of the air vents and roof or floor
drains, the insert is usually required.  The waveguide attenuation piping
insert will be a honeycomb comprised of steel or brass, tin-dipped, or else
stainless steel or brass disk with clusters of drilled holes, as seen in
Figure 6.  The filter insert unit will be soldered, brazed, or welded to the
piping.  Large diameter piping for fire sprinkler systems within the enclosure
are typically assembled with a nonconductive gasketed joint, and will require
a filter insert.  Generally the piping penetration in the shielding is first
fitted with a brazed or welded collar of steel to provide a transition between
the typically thin shielding material and the heavier walled piping, as seen
in Figures 3 and 5.  The collar also allows for cutting of the necessary
penetration hole in the field for welding on site without close tolerance
requirements on the hole dimensions.  The waveguide filter piping insert will
cause a pressure drop in the flow of liquids which must be considered in their
design.  National Fire Protection Association regulations may require the
addition of refractory materials around electrical isolation sections of
piping used with TEMPEST requirements (typical of the fire sprinkling


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