Technical
AC Line
Circuit 2 ground
Power
1 Hz
1 GHz
QS1502-HoR01
Hands-On Radio
by Mark Spencer, WA8SME
H. Ward Silver, NØAX, n0ax@arrl.org
Grounding and Bonding Systems
Along with effective receiving and transmitting, a station’s design must address three
important electrical requirements: ac power
safety, lightning protection, and equipmentto-equipment bonding. Figure 1 shows that
the frequency ranges involved are quite different, requiring different solutions.
As you learned in the previous “Hands-On
Radio” column, however, few parts of an
amateur station exist in isolation, particularly as frequency goes up. Thus, the measures taken to satisfy these requirements can
affect and even compromise each other if
performed improperly. Standard methods of
protection from shock hazards and lightning
can be found in the National Electrical
Code (NEC).
Low-Frequency AC Safety
Reducing the detailed language of the NEC
to simple terms, you must connect all exposed metal from ac-powered equipment to
a central, common ground. All equipment
electrically connected to an ac-powered device should have a permanent safety connection even if the equipment is unpowered,
like an antenna tuner or audio switch.
(Don’t depend on removable cables for
safety connections.)
This connection is usually referred to as the
ac safety ground, and the connection is made
through the “third wire” of your ac power
wiring; the bare or “green” wire. Figure 2
shows how power and enclosure grounds are
connected through the ac ground wire to the
breaker panel’s ground bus.
If a short circuit develops between the ac
wiring and the enclosure of a piece of
equipment, the resulting fault current in the
ac safety ground trips a circuit breaker in the
hot conductor. Leakage current is mostly
due to filter capacitance and parasitic capacitance between the phase (hot) ac con-
1
Experiment #145
RF Signal Energy
Peak Energy
100 kHz to 10 MHz
Lightning
Peak concern
50-60 Hz
AC Power
1 kHz 1 MHz
Figure 1 — The frequency ranges of concern to
amateurs for ac power, lightning, and RF signal
energy.
ductor and the equipment chassis or
enclosure.
Ground Fault Circuit Interrupter (GFCI)
breakers go one step further and monitor the
balance of current on the hot and neutral
lines of an ac circuit. If an imbalance is detected, it is assumed that the missing current
is flowing on the equipment chassis or enclosure, where it can present a shock hazard, and the GFCI trips to remove power.
Since the ac safety ground is concerned
Neutral
AC
Panel
Gnd
Bus
Power
Supply
Circuit 1 ground
with currents at the power line frequency
and its first few harmonics, the length of the
connection doesn’t matter much. At 60 Hz,
a 50-foot conductor is only 3 millionths of a
wavelength long. Similarly, resistance in
the ac safety ground path or an imbalance of
a few ohms between circuits doesn’t matter
much from the perspective of safety.
(Imbalances might be significant for signallevel connections.) The important thing is
that hazardous current takes a path that
doesn’t include you!
Lightning Protection
Another area of concern also addressed by
the NEC is that of lightning protection.
The goal of lightning protection wiring is
two-fold. First, a low-impedance path to the
Earth and between earth connections is provided for the high currents involved, often
thousands of amps. Second, bonding keeps
all equipment and circuits at close to the
same voltage so that voltage differences and
current flowing between them are minimized.
Lightning energy is distributed mostly between 100 kHz and 10 MHz — far higher
in frequency than ac power — so that wiring inductance dominates the performance
of the protection system. Voltage across a
QS1502-HoR02
RF bonding bus
12V Radio
Computer
Amplifier
2
1
Notes appear on page 69.
68 February 2015 ARRL, the national association for Amateur Radio
Figure 2 — AC safety grounding routes fault and leakage currents that present shock hazards away
from a human operator. Most safety grounding is done through a home’s ac wiring.
®
www.arrl.org
Reprinted with permission; copyright ARRL.
Heavy Bonding
Ground Rod
Conductor
Antenna
Cables
Antenna Cables
Phone
Service
Cable TV
Service
AC Power
Service
Tower Grounding
System
Figure 3 — A lightning protection system attempts to dissipate charge into the Earth outside the
home or station. All earth connections are bonded together to provide a low-impedance path for
lightning, and to minimize voltage differences and current flow between equipment and circuits.
Bond if Near the
Service Entrance
conductor is equal to its inductance multiplied by the rate-of-rise of the lightning’s
current pulse in amps/second. For example,
a straight wire 1 meter long has an inductance of about 1 μH. If the lightning’s current rises 20 kA in 2 μs, the voltage between
the ends of the wire is 1 μH × 20 kA / 2 μs
= 10,000 V! Voltage transients large enough
to do significant damage can also be in-
duced on your wiring by the magnetic field
of nearby strikes as well as being conducted
to your wiring by ac power, phone, or cable
TV systems.
Figure 3 shows the basic approach for lightning protection. AC safety ground wiring is
included, as shown in Figure 2. The additional service entrance ground connection
from the ac power distribution panel to a
ground rod establishes a local earth connection for lightning protection of ac-powered
equipment and appliances. In the shack,
ham equipment-to-equipment bonding is
also shown with its required connection to
the ac safety ground.
Any earth connections at the ac power
panel, other service entries, or an earth connection to the shack RF bonding bus must
be bonded together by heavy copper wire or
wide copper strap to provide a mechanically secure, low impedance connection.
Using #6 AWG or heavier wire is recom-
Entry
Panel
AC
Power
Panel
Service Entrance Ground
Conductor Extends to All
Earth Connections
mended and a minimum size may be specified by your local building codes.
All antenna cables (including feed lines and
rotator control cables) should be routed
through an entry panel where they can be
protected by lightning arrestors to route
high-voltage, high-current surges to an
earth connection. This minimizes the voltage between cables and the resulting current that would flow through the equipment
connections as a result. Other services may
have their own entry panels and earth connections.
Each separate earth connection for safety,
or to manage RF energy, creates a path
through the house for lightning and can result in significant voltages from lightning
surges or ac power faults. To minimize
these voltages, bond all external earth connections together. The ground rods and
bonding conductor provide a low impedance path bypassing the house for lightning’s charge and keep equipment close to
the same voltage at low frequencies. The
goal is to dissipate as much lightning energy
as possible outside while minimizing large
voltage differences and current surges that
damage equipment inside.
The extra ground system shown next to the
tower helps dissipate charge, too. This is
Ham Radio
Equipment
Phone
Cable
AC Safety Ground
House Exterior
RF
Bonding
Bus
QS1502-HoR03
Know the Difference
What we often refer to as “grounding” is really three different functions:
ac power safety, lightning protection,
and managing RF energy. Knowing
the difference will help you build and
maintain a safe and effective station.
particularly important if your tower is not
located close to the house and its bonded
earth connections. If the tower is used as a
vertical antenna, a radial ground screen can
help spread out charge from lightning along
with its RF function. What a great reason to
create another antenna!
Tying It All Together
Let’s review. All of your exposed connections to equipment powered from the ac
line, or connected to such equipment, need
to be connected to the ac safety ground. All
external earth connections must be bonded
together outside your home with heavy
wire. If possible, use a single, grounded
entry panel for all signal connections entering your house. If multiple entry points are
used for different services, connect those
panels or lightning arrestors to the outside
bonding conductor. Finally, inside your
shack, bond the equipment together with
short, heavy connections to a common bus.
The Safety chapter in The ARRL Handbook
and the ARRL’s Technical Information
Service web page on safety (www.arrl.org/
) both contain lists of useful refer-
safety
ences, guidelines, and tutorials.
website (www.w8ji.com/station_ground.
htm) discusses station grounding and shows
examples of an entry system and wiring
practices. K9YC has published a slide-show
tutorial on grounding (www.audiosystems
group.com/GroundingAndAudio.pdf)
covering a variety of concerns.
Notes
1
The NEC is a good set of reference standards
but your local city or county building department may have additional requirements
based on the specific circumstances in your
area. The authority having jurisdiction (AHJ),
such as a city or county building department,
determines what is required at your location.
2
NEC Article 810, “Radio and Television
Equipment,” Section III covers “Amateur and
Citizen Band Transmitting and Receiving
Stations — Antenna Systems.” Bonding
requirements are discussed in 810.58, and
towers are discussed in 810.15.
3
The ARRL Handbook, 92nd edition, ARRL,
Chapter 28.
3
W8JI’s
Reprinted with permission; copyright ARRL.
QST® – Devoted entirely to Amateur Radio www.arrl.org February 2015 69
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