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 required levels of shielding effectiveness (attenuation over the required
frequency range of performance).
It is recommended that a test module be specified on projects with
large complex shielded enclosures. The test module should be an 8 ft x 8 ft x
8 ft (2.4 m x 2.4 m x 2.4 m) representation of the contractor's shielding
techniques which must be constructed, tested, and approved prior to any
fabrication or construction of the shield itself. The test module shall
include all typical seams and joints and all typical penetrations which will
be installed in the facility.
All TEMPEST isolation requirements must be clearly specified and the
isolation materials identified. The shielding specification and all other
sections of the specification must be thoroughly edited to assure that
homogeneous metal-to-metal surfaces (electrochemically compatible) exist
throughout the shielding system. Any novel design features such as a new type
of door or hatch should also be tested in the test module.
4.9
Testing. The testing requirements of the EMI filtering system must
be thoroughly analyzed and defined in the specifications, including test
frequencies, test procedures, operation temperature ranges, current loads, and
test points. When filtering is required on power, signal, control and alarm
conductors, the filters must be constructed and tested to the requirements of
MIL-F-15733E, Filter, Radio Interference, General Specification for, including
the temperature rise, voltage drop, harmonic distortion, current loading, RF
radiation, and insertion loss tests described in NFGS-16650. The insertion
loss must be tested in conformance with MIL-STD-220A, Method of Insertion-Loss
Measurement, modified to include extended buffer networks to 14 kHz, and to
include filter current loads in excess of 100 amps. The limits of acceptable
performance and type of testing must be clearly stated in the specification to
avoid problems with rejection of products which do not meet the minimum
performance when tested. Similarly, the testing requirements of the EMI
shielding system must be thoroughly analyzed and defined in the
specifications.
4.9.1
Shielding Effectiveness Leak Detection System (SELDS). One test
that should be used on both the Test Module and EMI Shielding System is the
Shielding Effectiveness Leak Detection System (SELDS). The SELDS testing
procedure is useful as a means of confidence testing of seams and penetrations
during the construction process. An approximately 100 kHz source is used to
drive a loop configured and placed to excite the portion of the shielding of
concern from one side in such a way as to minimize the leakage around the
shielding section. Then a hand held receiver, with a 140 dB dynamic range and
a small ferrite loop probe are used to explore the seams and penetrations of
the section of shielding under test for indications of signal penetrations
from the opposite side of the shielding from the source loop. When
indications of leakage are located, a representative MIL-STD-285 type magnetic
field shielding effectiveness measurement can be made using a 12 in. (305 mm)
diameter shielded loop source and the ferrite probe loop spaced at a distance
of 24 in. (609 mm) for a reference reading. There is a step attenuator
available on the SELDS receiver providing 100 dB in 10 dB steps. After the
reference reading is made, the source and receiver loops are positioned 12 in.
from the shielding surface, and opposite each other with the same loop
orientation as used in the reference reading, and the step attenuator is
60
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