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filter that is identical to the one under test in place of the two buffer
networks. The additional filter is installed in the wall of a shielded test
chamber, beside the filter under test, and is connected in series with the
test filter and either an ac or a dc current source.  A low voltage ac current
source can be used to drive the two series-connected filters in a short
circuit load arrangement, with the peak load current controlled by the current
source (typically a transformer with adjustable taps).  The filter-out, direct
reference signal is fed through the shielded test chamber wall by means of a
bulkhead adaptor, including series blocking capacitors provided to protect the
source and receiver from the current source during the filter-in measurement.
The filter-in signal is next fed through the filter under test and measured
for comparison to the reference signal.  The isolation provided by the
additional filter in series with the impedance of the current source and the
inductance provided by the connecting cables will normally be sufficient to
result in a meaningful filter measurement test.
Facility Grounding System.  The facility ground system is typically
composed of four subsystems as described in MIL-HDBK-419; the earth electrode
subsystem, the lightning protection subsystem, the electrical fault subsystem,
and the signal reference subsystem.  The earth electrode subsystem is the
grouping of driven rods, buried mesh grids, or buried radial conductors
utilized to provide a low resistance to earth (less than 10 ohms) for
attachment by the lightning subsystem, the fault protection subsystem, and the
signal reference subsystem.  In areas where high earth resistivity makes it
difficult to achieve less than ten ohms with a reasonably sized buried earth
electrode system, it may be necessary to add chemical treatment in the form of
ion-producing salts such as magnesium sulphate, copper sulphate or calcium
chloride to the soil surrounding the buried electrodes or conductors.  Such
treatments can be very effective in reducing resistance of values of the
electrode systems, but they must be provided with a replenishment schedule to
overcome the leaching effects of groundwater runoff.
Lightning Protection Subsystem.  Lightning protection grounding
includes roof mounted air terminals with attached down conductors to the earth
electrode system.  Location and height of air terminals necessary to provide
adequate zones of protection is treated extensively in NFPA 78, Lightning
Protection Code, and MIL-HDBK-419 and they vary considerably with the size and
slope of the roof structure, height of the building or the height of
protrusions extending above the roof level.  Air terminals are composed
typically of either suspended horizontal conductors a minimum of 6 ft above
the highest projection, vertical masts with a 45 deg. cone of protection for
important buildings, or a connected system of vertical metal (copper, aluminum
or bronze) rods up to 3 ft in length, with a spacing of less than 50 ft (on
flat roofs), and connected to the grounding system with a minimum of two down
Fault Protection Subsystem.  Fault protection requires the grounding
of all exposed metallic equipment frames which contain electrical power wiring
to the earth electrode grounding system so that any faulting of energized
conductors to frame will result in a low impedance ground return path for
positive actuation of fusing and breaker systems.  A green wire ground system
of wiring for electrical circuits is required, and metal conduits may not be
relied upon for the ground return conductor.  The ac neutral is normally
grounded at the service transformer and at the first disconnect means, in


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