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 number of required penetrations, electrical and mechanical, should be
minimized (refer to para. 4.5.1.1).
4.7
Electrical Design Considerations. Important electrical design
considerations should include the electromagnetic continuity of the EMI
shielding system, filtering of conductors, and grounding of the electronic
equipment and EMI shield. A shielding specialist familiar with electrical
conductivity and permeability must provide guidance for the overall shielding
design by establishing required methods of penetration connections, seam
connections, door connections, and surface preparations to eliminate all
discontinuities of conductivity and permeability throughout the system.
4.7.1
Design Allocations for Filters. The electrical designer is
responsible for providing filters on all power, signal, and communication
cables which penetrate the shielded enclosure. All filters should be located
close to the shielded enclosure. The ideal location for the filter units is
on the surface of the shielded enclosure which should be shown on the plans.
The number of power filter units should be minimized but multiple filter units
may be required if the amperage requirement is large. Sufficient space should
be allowed for the filter units and they should be accessible for maintenance
and testing after construction. The location of the power filters should also
be shown on the one line diagram, to ensure that the filters are properly
connected in the electrical system. The electrical characteristics of the
filters must be clearly identified including the voltage rating, amperage
rating and number of filters per filter unit. Each power filter unit should
provide a filter for each conductor including the neutral in a common
enclosure. The required insertion loss of the filters must be specified and
must equal or be greater than the shielding effectiveness required for the EMI
shielded enclosure. Care should be taken during design to avoid potential EMI
sources within the controlled environment, such as dc power sources or
The design must clearly define the grounding systems required in the
shielded enclosure. Details should be included on the design showing the
single point ground system, if it required and for separate signal reference
ground subsystem, if it is required, in addition to the fault/personnel
protection ground system required by the National Electric Code. The
electrical designer is referred to MIL-HDBK-419, Grounding, Bonding and
Shielding Electronic Equipments and Facilities, for special grounding and
bonding requirements for EMC enclosures.
All metallic electrical conduits which penetrate a TEMPEST shield
must be isolated within 2 in. of the exterior of the shield by a nonmetallic
conduit section at least 6 in. long to prevent conduction of information from
the shielded enclosure.
4.8
Specification Design Considerations. The contract specifications
for an RFI shielded facility also require special consideration. NAVFAC Guide
Specifications (NFGS)-13765, Radio Frequency Shielded Enclosures, Demountable
Type, NFGS-13766, Radio Frequency Shielded Enclosures, Welded Type, and
NFGS-16650, Radio Frequency Interference Power Line Filters, are available but
require careful editing to delineate the specific project requirements. Both
the shielding specification and the filter specification must indicate the
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