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 MIL-HDBK-419A
5.3.4 Floating System.
The floating ground system is completely insulated from the building or from any wiring that may be a source of
circulating currents. The effectiveness of floating ground systems depends on their true isolation. In large
systems, it is difficult to provide required isolation to maintain a good quality floating ground. Insulation
breakdown occurs easily because static charges, fault potentials and lightning potentials may accumulate
between the floating ground and other accessible grounds, such as external power line neutrals, water pipes,
etc. Due to the personnel hazards from the difference of potential between the floating ground and building
ground, this system Is not recommended.
The preferred grounding method is to have an equipotential plane bonded to the earth electrode subsystem and
building structure steel at multiple points with the structural steel also bonded to the earth electrode
subsystem. In those facilities which do not have structural steel, multiple copper downleads should be
connected from the equipotential plane to the earth electrode subsystem.
5.4 SITE APPLICATIONS.
Because of the interference threat that stray power currents present to audio, digital, and control circuits (or
others whose operating band extends down to 60 Hz or below), steps must be taken to isolate these large
currents from signal return paths. Obviously, one way of lessening the effects of large power currents is to
configure the signal ground system so that the signal return path does not share a path common with a power
return. This can be accomplished by making sure that the grounding conductor (green wire) of the power system
is always run in the same cable, conduit, duct, or raceway with the phase and neutral conductors to the first
service disconnect and then bonded to the earth electrode subsystem.
The first step in the development of an interference-free signal reference subsystem for an equipment Or a
facility is to assure that the ac primary power return lines are interconnected with the safety grounding
network at only one point. Isolation of ac power returns from the signal reference subsystem is a major factor
toward reducing many noise problems. Additional steps should also be taken to minimize other stray ac
currents such as those resulting from power line filters. (One way of reducing these currents is to limit the
number of filter capacitors in an installation by using common filtered ac lines wherever possible or by locating
the filters as near as possible to the power service entry of the facility.)
To meet the safety requirements while minimizing the effects of power currents flowing with signal currents
through a common impedance, a single connection * between the power distribution neutral and the earth
electrode subsystem is necessary. This single connection eliminates conductive loops in which circulating
(power) currents can flow to produce interference between elements of the signal reference network.
*This connection to the earth electrode subsystem should be made from the first service disconnect. Care
should be taken to ensure that the signal reference, fault protection, and lightning protection subsystems are
bonded to the earth electrode subsystem at separate ground rod locations.
5-28
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