low-impedance path from the air terminal to earth must also be provided. These requirements are met by
either (1) an integral system of air terminals, roof conductors, and down conductors securely interconnected to
provide the shortest practicable path to earth; or (2) a separately mounted shielding system, such as a metal
mast or wires (which act as air terminals) and down conductors to the earth electrode subsystem.
c . Fault protection subsystem. The fault protection subsystem ensures that personnel are protected
from shock hazard and equipment is protected from damage or destruction resulting from faults that may
develop in the electrical system. It includes deliberately engineered grounding conductors (green wires) which
are provided throughout the power distribution system to afford electrical paths of sufficient capacity, so that
protective devices such as fuses and circuit breakers installed in the phase or hot leads can operate promptly.
If at all possible the equipment fault protection conductors should be physically separate from signal reference
grounds except at the earth electrode subsystem. The equipment fault protection subsystem provides grounding
of conduits for signal conductors and all other structural metallic elements as well as the cabinets or racks of
d . Signal reference subsystem. The signal reference subsystem establishes a common reference for
C-E equipments, thereby also minimizing voltage differences between equipments. This in turn reduces the
current flow between equipments and also minimizes or eliminates noise voltages on signal paths or circuits.
Within a piece of equipment, the signal reference subsystem may be a bus bar or conductor that serves as a
reference for some or all of the signal circuits in the equipment. Between equipments, the signal reference
subsystem will be a network consisting of a number of interconnected conductors. Whether serving a collection
of circuits within an equipment or serving several equipments within a facility, the signal reference network
will in the vast majority of cases be a multiple point/equipotential plane but could also, in some cases, be a
single point depending on the equipment design, the facility, and the frequencies involved.
1.5.2 Grounding and Power Distribution Systems. For safety reasons, both the MIL-STD-188-124A and the
National Electrical Code (NEC) require the electrical power systems and equipments be intentionally grounded;
therefore, the facility ground system is directly affected by the proper installation and maintenance of the
power distribution systems. The intentional grounding of electrical power systems minimizes the magnitude and
duration of overvoltages on an electrical circuit, thereby reducing the probability of personnel injury, insulation
failure, or fire and consequent system, equipment, or building damage.
a . Alternating currents in the facility ground system are primarily caused as a result of improper ac
wiring, simple mistakes in the ac power distribution system installation, or as a result of power faults. To
provide the desired safety to personnel and reduce equipment damage, all 3-phase wye wiring to either fixed or
transportable communication facilities shall be accomplished by the 5-wire or conductor distribution system
consisting of three phase or "hot" leads, one neutral lead and one grounding (green) conductor. A single building
receiving power from a single source requires the ac neutral be grounded to the earth electrode subsystem on
the source side of the first service disconnect or service entrance panel as well to a ground terminal at the
power source (transformer, generator, etc.). This neutral shall not be grounded at any point within the building
or on the load side of the service entrance panel. The grounding of all C-E equipment within the building is
accomplished via the grounding (green) conductor which is bonded to the neutral bus in the source side of the
service entrance panel and, in turn, grounded to the earth electrode subsystem. In addition to the three phase
or "hot" leads and the neutral (grounded) conductor, a fifth wire is employed to interconnect the facility earth
electrode subsystem with the ground terminal at the power source.