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 5.3.5
RFI filters. RFI Filters are required for all conductors that
penetrate the shielded enclosure, this includes electrical power, telephone,
fire alarm, intrusion detection systems, and all control wiring. The required
insertion loss of the filters should be equal to or greater than the shielding
effectiveness required for the EMI 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. Electrical power filters
over 100 A can be large, heavy, and can generate a significant quantity of
heat. Sufficient space should be allowed for the filter units and they should
be accessible for maintenance and testing after construction. Sufficient
support and adequate ventilation must also be provided.
5.3.6
Grounding. The grounding system is an important subsystem to the
shield enclosure. Therefore, the grounding system should be carefully
installed in accordance with specifications and the details on the plans.
5.4
Testing. The contractor should submit a test plan and
qualifications of an independent testing laboratory to the ROICC for review
and approval prior to beginning work of the shielded enclosure. The plan must
address specific dates and durations that testing will be conducted during the
overall construction period so that the expert government witness may be
scheduled to observe the testing, and so that repairs may be made to the
shield, and retests conducted, before the building finish materials are
installed. The test plan must include the test laboratory's qualifications,
equipment listings (including calibration dates and antenna factors),
equipment setup sketches, test frequencies, test locations, and proposed test
report format. The testing laboratory must have recent experience in SELDS
and MIL-STD-285/NSA 65-6 shielded enclosure testing. Finally the test plan
must indicate the proposed dates and duration of the lowest and highest
frequency tests following installation of the building finish materials so
that an expert government witness may be available for these final acceptance
tests (if required by the contract specifications). The final test must be
performed prior to the finishes being placed on the interior of the enclosure
but after the installation of all services, power/telephone/signal lines, RF
filters, and waveguide vents. Ensure that the test plan includes all the
frequencies that are required in the specification. The test frequencies that
are required by MIL-STD-285 are much narrower than the frequency range
required by many of the shielded enclosures that are being constructed.
5.4.1
Leak Detection Systems. Shielded enclosure leak detection system
(SELDS) is be used to locate defects in the seams of the enclosure during
construction. The system uses an oscillator and a handheld magnetic field
detector called a "sniffer," with a probe to get into the interior corners
and other difficult spots. The SELDS does not provide a value of shielding
effectiveness but provides a relative comparison of the seam with other
portions of the shielding. The test device detects the magnetic fields
associated with discontinuities in the shield, such as poor electrical
continuity in seams, and does not measure the shielding effectiveness of the
shield. SELDS testing can be conducted at various stages of construction and
it isn't necessary to wait until the construction is complete. Therefore, the
contractor should be using the "sniffer" to test the seams during
construction. Discontinuities should be marked during SELDS testing, and
repaired by the contractor.
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