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 MIL-HDBK-419A
b.
Filters at the interface terminals of equipment can operate more effectively when both terminals of
their equivalent "transmission line" are available. As in a, above, a large conducting surface makes it possible
to contain the field carried by the offending conductor, in such a way that it can be more easily prevented from
traveling further.
c.
A large conducting surface will also provide isolation between any rooftop antennas and from cable
runs below it.
1.5.1.1.1 Types of Equipotential Planes. Conducting media that can be utilized for the equipotential plane are
(a) a copper grid embedded in the concrete floor or raised metal floor such as computer floor, (b) a subfloor of
aluminum, copper, phospher bronze screen or sheet metal laid underneath the floor tile or carpet, or (c) a
ceiling grid above the equipment. The grid openings should not be larger than 1/20 wavelength at the highest
frequency of concern up to four inches. As a design objective (DO) the grid openings should not be larger than
four inches. The following equipotential planes may be utilized on new facilities or those facilities undergoing a
major rehab, or upgrading of communications electronics equipments.
1.5.1.1.1.1 Copper Grid Embedded in Concrete. Since a large solid conducting surface is not economically
feasible for some installations, a ground reference plane, made up of a copper grid, or copperclad construction
mesh with 4 inch openings may be embedded in the concrete with ground risers installed to the surface of the
concrete as shown in Figure 1-54. The mesh is commercially available in AWG wire sizes Nos. 6, 8, 10, and 12.
It is normally furnished in 3.7m (12 foot) rolls, but can be obtained in various widths up to 5.5m (18 feet). See
Figure 1-52. Where sections of mesh are joined together, there should be a one foot overlap and bonded
together every two feet by welding, brazing, or manufactured connectors that are connected to the grid and
give grounding access at the floor surface. See Figures 1-53, 1-54, 1-55, and 1-56. Normally, if the grid is
embedded in a concrete floor, the latter method provides the easiest grounding source. The equipotential plane
shall be welded to the main structural steel of the building at multiple locations. Where frame buildings are
utilized the plane is connected to the earth electrode subsystem at multiple locations using l/0 AWG copper
conductors. If metal floor systems are used (metal floors with concrete poured over the floor) then the floor
system itself can be used as the equipotential plane. In fact, this would be the preferred method of establishing
the plane.
1.5.1.1.1.2 Equipotential Plane Under Floor Tile or Carpet. An equipotential plane can be realized by
installing a metal sheet or roll of either aluminum, copper, or phospher bronze under the floor tile or carpet.
This sheet may be either thin gauge solid metal or window screen type material bonded to the floor with mastic
and tile or carpet installed on top of it. In existing facilities where equipments are already installed, the plane
need not be installed under the equipment cabinets, but must be bonded to the cabinets on all four sides. The
plane shall be bonded to the main structural steel members of the building at multiple locations. The structural
steel shall in turn be bonded to the earth electrode subsystem.
1.5.1.1.1.3 Overhead Equipotential Plane. Where it is not practicable to install a plane on the floor around the
equipment, it is possible to install an overhead equipotential plane in or on the ceiling of the equipment room.
This can be accomplished by installing either thin metal sheets or screen either above or on the ceiling. Care
must be taken to keep bonding straps from the equipment to the plane as short as possible. Generally phospher
bronze screen is used in this application because it is light, durable, and easy to work. The plane must be
connected to the building steel which in turn is bonded to the earth electrode subsystem.
1-120
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