A basic signal ground point or plane is an important requirement for reliable, interference-free equipment
operation. Unfortunately, the ideal ground plane does not exist and some difference of potential will always
exist between different ground points. The following sections present techniques and procedures for minimizing
this potential difference in both lower and higher frequency equipments.
188.8.131.52 Lower Frequency Equipment.
184.108.40.206.1 Signal Ground Network Configuration. Equipment shall be manufactured with a single-point, signal
ground network in lower frequency equipments, i.e., equipment operating at 30 kHz or less. Isolate the lower
frequency signal reference points and planes established internal to the equipment from the equipment case.*
(The functional requirements of the internal circuitry of the equipment will determine the actual methods and
techniques which must be used in establishing the individual reference points and planes. Because of the wide
variability of design requirements, the final selection must be left to the individual designer. The designer
should become thoroughly familiar with the fundamental design principles contained in Volume I, Chapters 5 and
220.127.116.11.2 Signal Ground Terminals.
Equipment shall be manufactured with an insulated signal ground terminal on each equipment case
as illustrated by Figure 3-l to provide a path for interconnecting the signal reference inside the equipment to
the facility lower frequency ground network.
The ground terminals can be a pin in a connector, a screw or pin on a terminal strip, an insulated
wire or an insulated stud, a jack, or a feedthrough. If an insulated wire is used, it should be at least No. 16
AWG copper or larger to provide adequate mechanical strength; for No. 16 AWG, its length should not exceed
1.5 meters (5 feet). If a longer ground wire is required in order to connect to the lower frequency signal ground
network in the facility, the size of this wire should be increased. To determine the necessary wire size, first
compute its required cross-sectional area from:
Required area in circular mils = Length required in running feet x 500 cmil per foot.
Using either Table 5-1 in Volume I or a standard wire table, determine the standard AWG size having the
required (or larger) cross-sectional area. For example, assume the required wire length is 3 meters (10 feet).
The minimum necessary cross-sectional area is then
10 ft x 500 cmil/ft = 5000 cmil.
*Common battery systems typically are designed with the signal ground connected to chassis or cabinet ground
which is also connected to one of the dc supply buses. It is recommended that such systems be isolated from
the structure and from the racks and cabinets of other lower frequency equipments and systems. All interfaces
between common battery systems and other equipments and systems should be balanced. Shield grounding must
be controlled to ensure that the desired isolation be maintained.