1302 Chapter 34
There are two primary reasons for careful grounding
and shielding in an audio system. The first reason is
safety. A poorly grounded system, especially outdoors,
may be a shock hazard. The second reason is to reduce
pickup of external noise. That external noise expresses
itself in the form of hums and buzzes and other noises
including radio station pickup.
34.5.1.9.1 Grounding for Safety
In U.S. electrical systems, the third (round) prong on the
ac cable of any piece of audio equipment is the ac safety
ground. When plugged into a properly wired ac recepta-
cle, the third prong of the ac cable connects the chassis
of the audio device to the ac ground through the third
prong of the ac receptacle.
This is the ideal situation from a safety viewpoint.
Under these conditions, there is almost no combination
of events that could cause a shock hazard from a single
audio device by itself. It is unfortunate (from a safety
viewpoint) that any audio device is seldom used by
itself; there are always other pieces of equipment
involved, and most of the time, these are also ac
powered. In addition, one or both of the following may
be encountered:
- Older audio equipment (in particular, guitar ampli-
fiers) with two-wire ac plugs and ground or hum
switches. - Older, two-wire ac receptacles or improperly wired
three-wire ac receptacles.
34.5.1.9.2 Improperly Wired ac Receptacles
An improperly wired outlet, Fig. 34-65, may have its
two ac wires reversed (polarity reversal), or it may have
a disconnected ground. Any fault in the wiring of the ac
receptacle is potentially hazardous, and, thus, the best,
and perhaps the only safe way to deal with an improp-
erly wired ac receptacle is to simply refuse to use it until
it has been properly repaired by a licensed electrician.
A simple three-prong outlet tester can indicate many
of these problems and is a useful addition to any audio
technician’s tool kit. Note that the three-prong outlet
tester cannot detect a ground-neutral swap. Neither can
it detect a high-impedance ground. Both of these are
hazardous conditions.
Another worthwhile measurement is the actual ac
receptacle voltage, especially in an unfamiliar facility.
Voltages that are too high or too low could cause
improper operation, or even damage the equipment;
too-high voltages could also pose a shock hazard. Most
audio equipment will work fine on an ac outlet with
voltages as low as about 110 Vac and as high as about
120 Vac. Newer equipment may be designed to auto-
matically adjust for voltages as low as 100 Vac and as
high as 240 Vac. Check the specifications for the equip-
ment if there is any doubt.34.5.1.9.3 Two-Wire ac ReceptaclesAll new ac installations in the United States use mod-
ern three-wire ac receptacles with a third ground prong.
The problem with two-wire ac receptacles is that they
don’t have that important third ground prong. Thus, to
use one of these two-wire receptacles, it’s necessary to
adapt it to the three-wire ac plug on a more modern
piece of audio equipment using a two-wire to three-wire
ac adapter. When the two-wire ac outlet is wired prop-
erly and a low-impedance grounded screw is available,
these adapters can maintain a safe ground for the
three-wire audio equipment.
To make this two-wire adapter work properly,
connect the loose (ground) wire with a connector or the
ground lug of the adapter to a grounded screw on the
two-wire ac receptacle. To check the safety of the
two-wire to three-wire connection, first, connect the
loose wire on the adapter to the screw on the two-wire
receptacle; then plug the two-wire adapter into the
two-wire receptacle. Next, plug a three-wire ac outlet
tester into the adapter. If the screw is grounded, the ac
outlet tester will so indicate. (Most three-wire ac outlet
testers either have a “good” light or they don’t light at
all on a good receptacle.) If the screw is not grounded,
the outlet tester will so indicate. In this case, connect the
loose wire from the adapter to some other grounded
screw in order to maintain a safe ground. If the outlet
tester shows a good ground but reversed polarity on the
two-wire to three-wire adapter, simply reverse the
adapter in the receptacle.
Remember that the three-prong outlet tester cannot
detect a ground-neutral swap, a hazardous condition.
Also, it is possible for the three-prong outlet tester to
indicate a good ground when the ground connection is
actually a high-impedance ground (poor connection).
For these reasons, it is strongly recommended that,
whenever you are connecting any three-wire
ac-powered equipment to any two-wire ac receptacle,
you use a portable GFI (ground fault interruption)
device to protect the equipment and the users.