Quantcast Multipoint Ground System

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MIL-HDBK-419A
a.
In steel frame buildings, make all structural members of the building (e.g., building columns, wall
frames, roof trusses, etc.) electrically continuous by bonding each joint and interconnection with a welded,
brazed, soldered, or high-compression bolted connection. Where direct bonds of these types are not possible,
bridge the joint with a l/0 AWG stranded copper cable both ends of which are brazed, welded, or bolted in
place. This does not include rebars.
b.
Connect the bonded structural steel network to the earth electrode subsystem with 1/0 AWG copper
cables.  The distance between adjacent connections from the building structure to the earth electrode
subsystem should not exceed 15 meters (50 feet).
c.
Where steel frame construction is not used, install a supplemental network consisting of large
copper cables conforming to Table 1-22.
d.
Equipment cabinets, electrical supporting structures, and utility pipes are to be connected to this
structural steel or copper cable grid (equipotential plane) with #6 AWG copper wire. This interconnecting wire
should be as short as feasible, preferably not over 24 inches to minimize high frequency reactance. (Electrical
supporting structures include all the conduit, raceways, switch and breaker panels, and other hardware (not
energized) commonly associated with the communication electronic facility.)
1.5.1.1 Multipoint Ground System. The multipoint ground system requires the existence of an equipotential
ground plane for the system. Such an equipotential plane exists in a building with a metal floor or ceiling grid
electrically bonded together, or in a building with a concrete floor with a ground grid embedded in it, connected
to the facility ground.  Equipment cabinets are then connected to the equipotential plane. Chassis are
connected to the equipment cabinets and all components, signal return leads, etc., are connected to the chassis.
The equipotential plane is then terminated to the earth electrode subsystem to assure personnel safety and a
low impedance path for lower frequency signals.
At higher frequencies, the large conducting surface, embedded in the floor or the metallic raised floor under
the equipments to be grounded, presents a much lower characteristic impedance than a signal wire, even if both
were improperly terminated. This is true because the characteristic impedance (Zo) is a function of L/C. As
capacity to earth increases, Zo decreases. Normally, the capacity of a metallic sheet to earth is 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 Z  o . If the dimensions
of a metallic sheet increase extensively (as in the case of a conducting subfloor), the characteristic impedance
approaches a very low value. In this case, even if improperly terminated, the 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. With this reference plane bonded to earth, the following advantages are obtained:
a.
Any "noisy" cable or conductor connected to the receptor through or along such a ground plane will
have its field contained between the conductor and the ground plane. The noise field can be "shorted out" by
filters and bond straps because the distance between these "transmission line" conductors is very small.
Shorting out the noise field has the desirable effect of keeping noise current from flowing over the receptor
case and along any antenna input cables.
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