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 must provide a continuous, uniform, and constant pressure against the shield-
ing elements of the panels. Screws are used in the mechanical clamping sys-
tem to control the pressure. The screws must be torqued to the proper torque
specified by the shielding manufacturer. The shield panels surfaces that mate
must be free of dirt, oil, corrosion products and other contaminates. The
edges of the panels and the channel hardware must be free from dings, dents
and all kinds of deformations. The seams are also a critical area and require
careful inspection.
5.3.1.3
Foundations and Ceilings. Both systems must be constructed on the
proper foundations. The floor on which the enclosure is being installed must
be adequate to support the weight of the enclosure and must be must be leveled
to the tolerances required by the enclosure manufacturer. The ceiling should
also be free from excessive sag.
5.3.2
Penetrations. Penetrations should be made only in the areas where
approved and should be kept in as small of area possible. On the welded steel
enclosures all penetrations including pipes, conduits and other metal items
shall be sealed by continuously welding around the periphery of the item at
the point of penetration. A collar welded to the penetrating item may be used
to provide a better means of welding to shielding. On the demountable
enclosure the plans may allow the penetration to be sealed by a clamping
system. Penetrations of the shielding by screws, nails, bolts and other items
shall be prohibited.
5.3.3
Doors. One of the most critical and vulnerable parts of shielded
enclosure is the door. On all welded steel enclosures the door frames should
be continuously welded to the shielded enclosure. On all shielded enclosures
the doors latching and hinging mechanism should operate smoothly and in the
latched position provide uniform pressure on the fingerstock.
5.3.3.1
Doorframe Contact Surfaces. The doorframe should be installed
square to allow the contact finger to apply uniform pressure between the door
and doorjamb and to prevent damage to contact fingers. The contact fingers
are critical to the shielding effectiveness, damaged, dirty or oily contact
fingers and and contact fingers that do not provide uniform contact around the
door can cause a significant decrease in shielding effectiveness. Contact
fingers should be easily replaceable and should be cleaned and checked
periodically for damage and cleanliness. Also, painting the fingerstrips
could drastically reduce the shielding effectiveness.
5.3.4
Waveguide-Type Air Vents. The waveguide-type beyond cutoff air
vents are required to allow HVAC supply air to enter the enclosure and return
air to exit the enclosure. The air vent must have a shielding effectiveness
that is equal to or better than the shielding effectiveness of the overall
enclosure. To maintain the required shielding effectiveness the frame of the
air vent must make good and continuous contact with the shield. The
recommended method for attaching the vent to the shield is to continuously
weld the vent frame to the shield using MIG welding method. The size of the
air vents and ducts must be chosen to provide an adequate volume of air,
minimize the static pressure drop across the air vent, minimize the acoustical
noise level, and keep the air velocity below 1000 fpm.
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