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 MIL-HDBK-419A
3.2.1.1.7
Shield Grounding (Black Only).
The shields of lower frequency signal lines should be grounded at only one end to the signal ground
a.
network internal to the equipment as shown in Figure 3-3. The ground connection may be made at either the
source or the load end (3-2), (3-3), (3-4). In general,
(1)
shields of sensitive data lines should be grounded at the load end;
(2)
shields of high level* signal lines should be grounded at the source end; and
(3) shields on lines from high impedance dc sources such as strain gauges, thermocouples, etc.,
should be grounded at the source end (3-5). (Further details on shield grounding for data acquisition systems are
given in Section 3.5.2.)
All individual shields of lower frequency signal lines within a cable bundle must be insulated from
b.
each other to minimize cross coupling. Further, these individual shields must be isolated from the overall
bundle shield, equipment chassis and enclosures, junction boxes, conduit, cable trays, and all other elements of
the facility ground system. When cables are long, extra attention must be directed toward maintaining the
isolation of the individual shields at the ungrounded end and at all intermediate connectors throughout the cable
run.
At terminating equipments, the shields of individual lower frequency signal lines may be carried into
c.
the case or cabinet on separate pins or may be grounded together to be carried in (or out) on a common
connector pin, depending upon the characteristics of the equipment involved. If the common pin arrangement is
used, it must not compromise the single-point grounding principle. It is advisable to use one pin for low level
signal shields with a different pin used for high level signal lines. These individual shields should be terminated
to the lower frequency signal ground network. See g through i below for overall shield grounding.
The pigtail between the shield breakout and the connector pin should be as short as physically
d.
practical.
In multiconductor cables some of the individually shielded signal lines will be grounded at one end
e.
while other shields will be grounded at the other end. Careful attention must be given to the installation of
such cables to prevent grounding of shields at both ends.
If pickup or radiation problems arise because the shield is longer than λ/10 at critical frequencies
f.
(see Volume I, Section 6.2.2.4), the shield may be divided into segments in the manner illustrated in Figure 3-6.
Each shielded section should be grounded at one end only.
*High level versus low level is a matter of degree and will depend upon the characteristics of the particular
system under consideration. A suggested rule of thumb is that if the voltage levels of two signals differ by a
factor greater than 10 to 1, then the larger should be treated as high level relative to the smaller.
3-6
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