Sound System Design 130334.5.1.9.4 Older Two-Wire Audio EquipmentSome newer equipment, especially consumer-type
equipment, may come with a two-wire ac cable. This
newer equipment may be as safe as if it had a three-wire
(grounded) ac cable. A good example of such a piece of
(nonaudio) equipment with a two-wire ac cable is a
double-insulated power tool (drills, saws, and so on).
One way to judge the safety of a piece of audio equip-
ment with a two-wire ac cable is to look for a UL
(Underwriter’s Laboratories) sticker. Listings from
other recognized safety agencies may also be used to
judge the safety of a piece of equipment.
It’s the older, two-wire audio equipment, however,
that can be potentially hazardous. The details of how a
shock hazard can develop are complex, but dealing with
this problem in an audio system is straightforward. The
shock hazard, if there is one, will probably develop
between the chassis of an older, two-wire guitar ampli-
fier and the chassis of a microphone.
The chassis of the microphone is connected to the
chassis of the microphone-preamplifier-mixer through
the shield of the connecting cable. Thus, if the mixer is
properly grounded, the chassis of the microphone is
properly grounded, too, and neither the microphone nor
the mixer will present any safety hazard. The guitar
amplifier (or other two-wire equipment), however, is,
potentially, not properly grounded. That means that a
hazardous ac voltage could be present on the chassis of
the guitar amp or on the strings of a guitar, which are
connected to the chassis of the amplifier through the
shield of the guitar cord.
Although it is possible to test for this problem, it’s
also important to protect the user with a portable GFI
(ground fault interruption) device on the guitar
amplifier.34.5.1.9.5 Grounding for Safety OutdoorsThe most common safety problems outdoors are
improperly wired portable ac outlets and wet ground or
wet portable stages (and, of course, rain). Check the
wiring carefully, using the same techniques as if the sys-
tem were indoors. Consider canceling a performance if
rain begins. If a performance must proceed on wet
ground or in the rain, the best way to avoid shock haz-
ards to the performers is to use wireless microphones
and wireless instrument transmitters. These same out-
door problems, of course, can develop indoors on a
damp floor. Portable GFI (ground fault interruption)
devices are strongly recommended for any equipment
used in an outdoor or damp indoor situation.Figure 34-65. Ac receptacle problems. Courtesy Yamaha
International Corp.
”Hot” (Black)”Hot” (Black)”Hot” (Black)”Hot””Hot””Hot” #1”Hot” #2”Hot”220 V220 V110 V110 V110 V”Neutral” (White)”Neutral” (White)”Neutral” (White)”Neutral””Neutral””Neutral”No Ground
connectedDimmerBuilding service
transformerBuilding service
transformer”GND” (Green)”GND” (Green)”GND” (Green)F. A 110 Vac outlet connected to a light dimmer
circuit, a dangerous and illegal connection.A. Properly wired 110 Vac outlet.B. 110 Vac outlet with disconnected
ac ground
wire creating potential shock hazard.C. 110 Vac outlet with polarity (hot and neutral) reversed
creating shock hazard and causing possible noise.D. 110 Vac outlets with open neutral. Outlets will
operate with voltage varying from 0 to 220 Vac,
creating shock hazard and causing possible
equipment damage.E. 110 Vac outlet with a 220 Vac circuit connected
to it. This is a highly dangerous and illegal connection.110 V