Multipoint Ground System

Custom Search

Back
Figure 1-51. Typical Multi-Deck Building Plan (Side View)

Up
Grounding, Bonding, and Shielding for Electronic Equipments and Facilities

Table 1-22 Size of Equipment Ground Cables

MIL-HDBK-419A

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.

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).

Where steel frame construction is not used, install a supplemental network consisting of large

copper cables conforming to Table 1-22.

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:

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.

1-118