Quantcast Electrical Noise in Communications Systems

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MIL-HDBK-419A
To eliminate or reduce undesired noise or hum, multiple facilities supplied from a single source shall ground the
neutral only at the power source and not to the earth electrode subsystem at the service entrance point. Care
should be taken to ensure the neutral is not grounded on the load side of the first disconnect service or at any
point within the building. The grounding (green) conductor in this case is not bonded to the neutral bus in the
service disconnect panel. It is, however, bonded to the facility earth electrode subsystem at the service
entrance panel. The fifth wire shall be employed to interconnect the earth electrode subsystem with the ground
terminal at the power source.
The secondary power distribution wiring for a 240 volt single phase system consists of two phase or "hot" leads,
a neutral (grounded) and a grounding (green) conductor while the three conductor secondary power distribution
system is comprised of one phase, one neutral, and one grounding lead. In both cases, the neutral shall not be
grounded on the load side of the first service disconnect. It shall, however, be grounded to the ground terminal
at the power source and to the earth electrode subsystem if one power source supplies power only to a single
building.
The ac wiring sequence (phase, neutral, and equipment fault protection) must be correct all the way from the
main incoming ac power source to the last ac load, with no reversals between leads and no interconnection
between neutral and ground leads. Multiple ac neutral grounds and reversals between the ac neutral and the
fault protection subsystem will generally result in ac currents in all ground conductors to varying degrees. The
NEC recognizes and allows the removal or relocation of grounds on the green wire which cause circulating
currents. (Paragraph 250-21(b) of the NEC refers.) Alternating current line filters also cause some ac currents
in the ground system when distributed in various areas of the facility; this is due to some ac current passing
through capacitors in the ac line filters when the lines are filtered to ground. Power line filters should not
induce more than 30 milliamperes of current to the fault protection subsystem.
b .  DC power equipment has been found to be a significant electrical noise source that can be minimized
through proper configuration of the facility, the physical and electrical isolation of the dc power equipment
from communications equipment, and filtering of the output. Certain communications equipment with inverter
or switching type power supplies also cause electrical noise on the dc supply leads and the ac input power leads.
This noise can be minimized by the use of decentralizing filters at or in the equipment. The location, number,
and termination of the dc reference ground leads are also important elements in providing adequate protection
for dc systems and, at the same time, minimizing electrical noise and dc currents in the ground system.
1.5.3 Electrical Noise in Communications Systems. Interference-causing signals are associated with
time-varying, repetitive electromagnetic fields and are directly related to rates of change of currents with
time. A current-changing source generates either periodic signals, impulse signals, or a signal that varies
randomly with time. To cause interference, a potentially interfering signal must be transferred from the point
of generation to the location of the susceptible device. The transfer of noise may occur over one or several
paths. There are several modes of signal transfer (i.e., radiation, conduction, and inductive and capacitive
coupling).
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