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 TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
returns a measurement of the conductors' induc-
frequencies. Modem systems achieve clock and data
tance plus the grounding system's contact resis-
rates at 4 megahertz and higher. Clock rate is the
tance. If the ground resistance reading is high at
rate at which a word or characters of a word (bits)
the low frequencies applied by test instrument, it
are transferred from one internal computer element
will be much higher at the higher frequencies. The
to another. Data rate is the rate at which data is
manufacturers of some electronic systems call for
transferred (bauds or bits per second) between com-
system grounding resistance of one ohm or less.
puters. At these frequencies, due to the impedance,
This low resistance is many times more difficult to
a regular ground wire acts as conductor for only a
achieve than the 25 ohm maximum grounding resis-
few feet. Compare the frequency and wavelength of
tance of a made electrode for power systems (NEC
these systems with those used for 60 Hz power.
article 250-84). To put that in perspective; aircraft
Electricity is conducted on a wire at very nearly the
do not maintain an earth ground, but do maintain a
speed of light (186,000 miles/sec.). Dividing the
low impedance between on-board electronic devices
speed by the frequency gives the full wavelength.
by using the aircraft skin and framework as a zero
For 60 Hz, one wavelength is about 3,100 miles.
voltage equipotential plane.
Communications and radar personnel know that in-
(4) Loop-flow. A low resistance to ground in the
teresting things begin to happen at one fourth of the
input power system is no promise of trouble-free
wavelength. The voltage and the current no longer
performance. It is necessary to understand that the
have the same relationship at this point on the wire.
earth is not a magic dumping area where unwanted
The quarter wavelength for 60 Hz is about 775
signals and currents simply disappear. Currents al-
miles. However, the quarter wavelength for ten
ways flow in complete circuit loops that may include
megahertz is only 24.5 feet. This is not the worst
various portions of the earth, the grounding elec-
case; the change in the current and voltage relation-
trodes, the grounding conductors, equipment bonds,
ship along a wire occurs gradually over the distance
and the equipment enclosures.
travelled. To maintain a close relationship between
(5) Isolated ground system. Loop currents flow-
the voltage and current at all points along the con-
ing through one portion of the earth into another
ductor's path, it cannot be much longer than l/20th
usually include a substantial amount of induced
of a wavelength. Therefore, effective ground conduc-
high frequency common mode noise. Many design-
tor length for a ten megahertz signal is only about
ers have tried to solve the noise problem with a
4.9 feet. This does not mean electronic grounding
single point, isolated ground system. This system
systems cannot be longer than four or five feet. The
uses an insulated ground wire from the load to the
important conductor is the equipment grounding
service entrance panel board. All isolated ground
(bonding) conductor, which may be a copper cable,
outlets are of a special design such that the ground
strap, sheet, or braid. It is this particular conductor
wire is isolated from the normal connections to the
which limits how far the electronic or computer
metal mounting frame and electrical outlet box. The
equipment may be placed from the signal reference
isolated ground system is actually a very high im-
grid (equipotential plane) or system.
pedance at high frequencies. This high impedance
(2) Noise interference. Coupled and induced
does attenuate this noise, but causes problems as
electrical noise is also a problem at higher fre-
high frequency voltages build up over its length,
quencies. This effect is rarely a concern for systems
due to the high frequency current through the im-
operating at the 60 Hz powerline frequency. Very
little, if any, current is induced or coupled to the
pedance of the conductor (IZ). Most manufacturers
now include surge protection with their isolated
ground conductors at low frequency. At high fre-
quency, relatively more current is induced into the
ground receptacles to protect the equipment from
ground conductors through shields, cable trays, con-
the high voltages that develop at high frequencies
duit, and the enclosures used to house the electronic
across these types of receptacles (common and
system- As a result, these conductors must deal
transverse mode). All exposed metal parts still re-
with more noise current than 60 Hz systems. In
quire the equipment ground conductor. Therefore,
addition, they must hold the reference voltage very
two ground conductors are required: the equipment
near zero at all points on the equipotential system.
safety ("dirty") ground, and the isolated system
(3) Power system grounding. The input power
("clean") ground. All of the equipment grounds are
system ground resistance is important because it
routed and bonded in the normal way. All isolated
keeps the system voltage at nominal values. This
ground conductors must be brought back to one
"resistance" is not only a simple resistance measure-
point in the subpanel. The subpanel isolated ground
ment but also a frequency dependent impedance
bus must not be bonded to the subpanel enclosure.
measurement. The best test instruments (para
This ground bus must be isolated and only con-
14-5) actually apply an alternating current which
nected with insulated conductor(s) to the service
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