Grounding and Interfacing 1191any voltage that might exist between equipment and
other earth-grounded objects, such as water pipes,
during the fault event. Power engineers refer to voltage
differences created by these fault events as step or touch
potentials. The neutral (white) and line (black) wires are
part of the normal load circuit that connects the voltage
source to the load. The green wire or conduit is intended
to carry fault currents only.
NEC also requires safety grounding of wiring race-
ways and equipment cabinets, including rack cabinets.
Per Article 250-95, safety grounding wires, which may
be bare or insulated, must have a minimum size of #14
copper for a 15 A or #12 copper for a 20 A branch
circuit to assure rapid circuit breaker action. This
grounding path must be bonded to the safety grounding
system, not to building steel or a separate earth ground
system! Separate earth grounds cannot provide safety
grounding!! As shown in Fig. 32-16, soil resistance is
far too high to guarantee tripping of a circuit breaker
under fault conditions.^13 With safety grounds in place,
potentially deadly equipment faults simply cause high
currents from power line hot to safety ground, quickly
tripping circuit breakers and removing power from
those branch circuits. Safety grounding in many resi-
dential and commercial buildings is provided through
metal conduit, metallic J-boxes, and saddle-grounded or
SG outlets. Technical or isolated grounding will be
discussed in Section 32.7.
When trying to track down and correct system noise
problems, it easy to assume that power outlets are wired
correctly. Low-cost outlet testers, which generally cost
less than $10.00, will find dangerous problems such as
hot-neutral or hot-ground reversals and open connec-
tions. Because they check for correct voltages between
the pins, and both neutral and ground are normally at
0 V, they cannot detect a neutral-ground reversal. This
insidious wiring error can create nightmarish noise
problems in an audio system. Finding the error by visual
inspection of outlets is one possibility, but this could get
labor intensive if the number of outlets is large. For
large systems, and even those that can’t be powered
down, a sensitive, noncontact, clamp-on current probe
can help identify the forks in the road when trouble-
shooting.^14 Code requires that neutral and safety ground
be bonded only at the power service disconnecting
means that is generally at the main breaker panel.
Serious system noise problems can also occur when an
extraneous neutral-to-ground connection exists else-
where in the building wiring. A special test procedure
can be used to determine this condition.^15
NEVER, NEVER use devices such as three-prong-to
two-prong ac plug adapters—a.k.a. ground lifters—to
solve a noise problem! Such an adapter is intended to
provide a safety ground (via the cover plate screw to a
grounded saddle outlet and J-box) in cases where
three-prong plugs must be connected to two-prong re-
ceptacles in pre-1960 buildings, Fig. 32-17.Consider two devices with grounding ac plugs that
are connected by a signal cable. One device has a
ground lifter on its plug and the other doesn’t. If a fault
occurs in the lifted device, the fault current flows
through the signal cable to get to the grounded device.
It’s very likely that the cable will melt and catch fire!Figure 32-15. Fault protection is provided by safety ground
to neutral bond.Utility power
transformerBreaker panelGround rodShortEarthL
NL
N
GBLK
WHI
GRNN120 V G
120 VDefective
equipmentFault currents range
from 100 A to 1000 A
Trip times range from
under 10 ms to 2.5 sFigure 32-16. Fault protection is not provided by an earth
ground connection!Figure 32-17. This is intended to provide a safety ground.Ground rodYikesHigh Impedance“Quiet” groundCircuit breaker will not trip!
Utility power
transformerBreaker panelGround rodShortEarthL
NL
N
GBLK
WHI
GRNN120 V G
120 VDefective
equipmentCONNECT TAB TOCAUTION
GROUNDED SCREWADAPTER
15 A 125 V