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 MIL-HDBK-419A
Care must also be taken to ensure sixty hertz power currents and other high amplitude lower frequency currents
flowing through the facility ground system do not conductively couple into signal circuits and create intolerable
interference in susceptible lower frequency circuits.
5.3.3.1 Equipotential Plane.
The importance of equipotential ground planes cannot be overemphasized for proper equipment operation, as
well as for EMI and noise/static suppression. An equipotential ground plane implies a mass, or masses of
conducting material which, when bonded together, offers a negligible impedance to current flow. Connections
between conducting materials which offer a significant impedance to current flow, can place an equipotential
plane at a high potential with respect to earth. High impedance interconnections between metallic members
subject to large amounts of current due to power system faults can be extremely hazardous to personnel and
equipment. The RFI effect of an equipotential plane or system must however be carefully considered, and it is
important to understand that grounding may not, in and of itself, reduce all types of RFI. On the contrary,
grounding a system may in some instances increase interference by providing conductive coupling paths or
radiative or inductive loops.
Many of the deficiencies of the wire distribution system can be overcome by embedding a large conducting
medium, in the floor under the equipments to be grounded. For existing facilities this system may be installed
above the equipment to be grounded. A large conducting surface presents a much lower characteristic
impedance than that of wire because the characteristic impedance
is a function of L/C, hence as capacity
to earth increases,
decreases. The capacity of a metallic sheet or grid to earth is much higher than that of
wire. If the size of the sheet is increased and allowed to encompass more area, the capacitance increases.
Also, the unit length inductance decreases with width, which further decreases
If the dimensions of a
metallic sheet increase extensively (as in the case of conducting floor), the characteristic impedance
approaches a very low value. In this case, the characteristic impedance would be quite low throughout a large
portion of the spectrum. This, in turn, would establish an equipotential reference plane for all equipments
bonded to it.
Although it is not necessary from a functional point of view, terminating the surface to an earth connection
presents the following advantages:
Personnel safety is not dependent on long cable runs for protection against power faults.
a.
Low impedance is provided for power and radio frequencies.
b.
Grounding buses in a communication facility where higher frequencies are present, act as lossy transmission
lines and therefore must be treated as such. Due to this phenomena single-point grounds and multipoint grounds
employing ground buses are high impedance grounds at higher frequencies. To be effective at the higher
frequencies, the multipoint ground system requires the existence of an equipotential ground plane.
Equipotential Planes are sometimes considered to exist in a building with a metal floor or ceiling grid
electrically bonded together, or in a building with the ground grid embedded in a concrete floor connected to
the structural steel and the facility ground system. Equipment cabinets are then connected to the
equipotential plane. Chassis are connected to the equipment cabinets and all components, signal return leads,
5-26
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