Handbook for Sound Engineers

1190 Chapter 32

fantasy can lead to noise disaster in their audio equip-
ment and system designs.
Grounding has several important purposes and most
often a single ground circuit serves, intentionally or
accidentally, more than one purpose. We must under-
stand how these ground circuits work and how noise
can couple into signal circuits if we expect to control or
eliminate noise in audio systems.

32.4.1 Earth Grounding

An earth ground is one actually connected to the earth
via a low-impedance path. In general, earth grounds are
necessary only to protect people from lightning. Before
modern standards such as the National Electrical Code
(NEC or just Code) were developed, lightning that
struck a power line was often effectively routed directly
into buildings, starting a fire or killing someone. Light-
ning strikes are the discharge of giant capacitors formed
by the earth and clouds. Strikes involve millions of volts
and tens of thousands of amperes, producing brief
bursts of incredible power in the form of heat, light, and
electromagnetic fields. Electrically, lightning is a
high-frequency event, with most of its energy concen-
trated in frequencies over 300 kHz! That’s why, as we
discussed in Section 32.2.4, wiring to ground rods
should be as short and free of sharp bends as possible.
The most destructive effects of a strike can be avoided
by simply giving the current an easy, low-impedance
path to earth before it enters a building. Because over-
head power lines are frequent targets of lightning, virtu-
ally all modern electric power is distributed on lines
having one conductor that is connected to earth ground
frequently along its length.
Fig. 32-14 shows how ac power is supplied through
a three-wire split single-phase service to outlets on a
typical 120 Vac branch circuit in a building. One of the
service wires, which is often uninsulated, is the
grounded neutral conductor. Note that both the white
neutral and the green safety ground wires of each
branch circuit are tied or bonded to each other and an
earth ground rod (or its equivalent grounding electrode
system) at the service entrance as required by Code.
This earth ground, along with those at neighboring
buildings and at the utility poles, provide the easy paths
for lightning to reach earth.
Telephone, CATV, and satellite TV cables are also
required to divert or arrest lightning energy before it
enters a building. The telco-supplied gray box or NIU
provides this protection for phone lines as x grounding
blocks do for CATV and satellite dishes. NEC Articles
800, 810, and 820 describe requirements for telephone,

satellite/TV antennas, and CATV, respectively. All

protective ground connections should be made to the

same ground rod used for the utility power, if the

ground wire is 20 ft or less in length. If longer, separate

ground rods must be used, and they must be bonded to

the main utility power grounding electrode with a #6

AW G w i r e.^11 Otherwise, because of considerable soil

resistance between separate ground rods, thousands of

volts could exist between them when lightning events

occur or downed power lines energize the signal lines.

Without the bond such events could seriously damage a

computer modem, for example, that straddles a

computer grounded to one rod via its power cord and a

telephone line protectively grounded to another.^12

32.4.2 Fault or Safety Grounding

Any ac line powered device having conductive exposed

parts (which includes signal connectors) can become a

shock or electrocution hazard if it develops certain

internal defects. Insulation is used in power trans-

formers, switches, motors, and other internal parts to

keep the electricity where it belongs. But, for various

reasons, the insulation may fail and effectively connect

live power to exposed metal. This kind of defect is

called a fault. A washing machine, for example, could

electrocute someone who happened to touch the

machine and a water faucet (assumed grounded via

buried metal pipes) at the same time.

NEC requires that 120 Vac power distribution in

homes and buildings use a three-wire system as shown

in Fig. 32-15. To prevent electrocution, most devices

have a third wire connecting exposed metal to the safety

ground pin of these outlets. The outlet safety ground is

routed, through either the green wire or metallic

conduit, to the neutral conductor and earth ground at the

main breaker panel. The connection to neutral allows

high fault current to flow, quickly tripping the circuit

breaker, while the earth ground connection minimizes

Figure 32-14. Simplified residential ac power from feeder

to outlet.

Utility power

transformer

Breaker panel

Ground rod

Source

Load

Earth

L

N

L

N

G

BLK

WHI

GRN

H

L

120 V G

120 V

Other phase of the incoming power , meter,

main breaker, etc. omitted for clarity