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5.2.6  Filters.  The source of power and the type of equipment used will
define what filtering, if any, is required.  Practices in the past include
filtering of all technical power.  If all BLACK runs have been properly
installed and the conduits are properly grounded and adequately separated
from RED runs, the need for such filtering is eliminated or greatly reduced.
The designer should engineer the power installation so that if additional
filtering is indicated by instrumented tests, a retrofit can be accomplished
with minimal effort.  Designers are cautioned that filtering power lines to
equipment containing filters must be avoided.  Filters in series effectively
operate as a single composite filter with entirely different characteristics,
which may present other problems or may fail to operate as required (see
figure 24 and 5.5).  If the source of power is totally contained in the CS,
power filtering may not be needed.  If MGs or no-break systems are used,
filters may not be needed if sufficient isolation exists between the feed and
load to prevent PLC.  This situation would have to be confirmed with
instrumented tests by a cognizant TEMPEST agency of the department or
activity.  If other power sources are used, then the type of equipment
supporting the mission dictates the need for filtering.  Ideally, each
equipment processing RED information would be filtered.  In such cases, no
additional filtering should be required, and the technical power would be
considered BLACK power.  Such filtering is preferred because the filter then
matches the equipment.  All TEMPEST approved equipment meet the criteria.
Filters are included in the design of some commercial equipment.  These
filters, however, probably will not conform to the attenuation requirements
of MIL-F-15733.  Such equipment should be retrofitted, if possible.  Bulk
filtering is indicated when RED processing equipment does not include
filters, or contains filters which do not conform to standards.  Such
nonconforming filters should be removed.  Bulk filtering involves installing
filters of appropriate size on the feeder lines to a power panel.  The
filters should be located at the point of entrance to the LEA.  Power on the
load side of the filter is considered RED power.


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