1192 Chapter 32
Also consider that consumer audio and video equip-
ment is responsible for about ten electrocutions every
year in the United States. In a typical year, this equip-
ment causes some 2000 residential fires that result in
100 civilian injuries, 20 deaths, and over $30 million in
property losses, Fig.32-1816,17
Some small appliances, power tools, and consumer
electronics are supplied with two-prong (ungrounded)
ac plugs. Sometimes called double insulated, these
devices are specially designed to meet strict UL and
other requirements to remain safe even if one of their
two insulation systems fails. Often there is a one-shot
thermal cutoff switch inside the power transformer or
motor windings to prevent overheating and subsequent
insulation breakdown. Only devices that carry a
UL-listed label and originally supplied with
ungrounded ac plugs should ever be operated without
safety grounding. Devices originally supplied with
grounding three-prong plugs must always be operated
with the safety ground properly connected!
32.4.3 Signal Grounding and EMC
EMC stands for electromagnetic compatibility, which is
a field concerned with interference from electronic
devices and their susceptibility to the interference
created by other devices. As the world becomes increas-
ingly wireless and digital, the general electromagnetic
environment is becoming increasingly hostile. Engi-
neers working in other disciplines, most notably infor-
mation technology or IT—where signal/data
frequencies are very high and narrowband—tend to
minimize our difficulties in making audio systems
robust against hostile electrical environments. In fact,
high-quality audio systems are unique among elec-
tronic systems in two ways:- The signals cover a very broad, nearly 5 decade,
range of frequencies. - The signals can require a very wide, currently over
120 dB, dynamic range.
Adding to the difficulty is the fact that ac power
frequencies and their harmonics also fall within the
system’s working frequency range. As you might
suspect, grounding plays a pivotal role in controlling
both emissions and susceptibility in both electronic
devices and systems. In general, the same principles and
techniques that reduce emissions will also reduce
susceptibility. Grounding schemes generally fall into
one of three categories:- Single point or star grounding.
- Multipoint or mesh grounding.
- Frequency selective transitional or hybrid grounding.
At frequencies below about 1 MHz (which includes
audio), virtually all experts agree that star grounding
works best because system wiring is electrically short
compared to the wavelengths involved. At these low
frequencies, the dominant noise coupling problems arise
from the simple lumped parameter behavior of wiring
and electronic components. This includes the resistance
and inductance of wires, the noise currents resulting
from capacitances between utility power and system
grounds, and magnetic and capacitive coupling effects.
On the other hand, at higher frequencies, system
wiring can become electrically long and transmission
line effects, such as standing waves and resonances,
become the dominant problems. For example, because a
25 ft (7.5 m) audio cable is a quarter wavelength long at
10 MHz, it becomes an antenna. At frequencies of
100 MHz or higher, even a 12 in (30 cm) wire can no
longer be considered low impedance path. To be effec-
tive at these frequencies, therefore, grounding schemes
must emulate a flat metal sheet having extremely low
inductance called a ground plane. In practice, this can
usually only be approximated with a multipoint ground
system using wires. The wire lengths between points
must remain well under a quarter-wavelength so, as
frequency increases, larger numbers of increasingly
shorter wires must be used to create the mesh. Ulti-
mately, only a real ground plane can produce
low-impedance ground connections at very high
frequencies. Even a ground plane is not a perfect or
equipotential—i.e., zero volts at all points—ground.
Because it has finite resistance, significant voltageFigure 32-18. Interconnect cables can carry lethal voltages
throughout a system if just one ground lifted device fails.