Professional Audio Mixing Console Operating Manual

Figure 4-2. Testing a 2-wire AC Outlet and a
3-Prong to 2-Prong Adaptor
4.2.4Improperly Wired AC Outlets:
Lifted Grounds
A "lifted ground" condition exists if the ground or
If you detect any voltage between the larger slot
(white wire) in an outlet and the ground-terminal
(round prong, green wire) when there is no load on that
line, you should contact a licensed electrician to check it
out and correct the situation.
green wire from the outlet’s safety ground is discon-
nected or missing. In older wiring, the heavy green wire
was sometimes omitted from internal wall wiring in
favor of letting the metal flex conduit or pipe suffice as
the ground path from the electrical service entrance.
This method of grounding is generally acceptable, as
long as the metal conduit in the wall is intact and all
the screws holding the joints together are secure.
However, a single loose screw in a conduit joint inside a
wall can remove the safety ground from the next outlet
box in the line, and from all the subsequent boxes on
that same line.
WARNING: In AC power wiring, black is
hot, and white is neutral-the opposite of
most audio signal
wiring and speaker
wiring. It is safer to consider all AC wiring
as potentially lethal. It is possible some-
one miswired the system, or that a short
circuit has developed. Test the voltages
yourself, and be safe.
Page 4-3
4.2.5Improperly Wired AC Outlets:
Lifted Neutral
If the neutral becomes lifted at a power outlet, it is
possible that items plugged into the outlet will be fed
the full 220 to 240 volts available from the power
service instead of the desired 110 to 120 volts.
Such outlets may operate, but the voltage can swing
from 0 volts to 220 or 240 volts AC (or whatever the
maximum voltage at the service entrance), creating a
shock hazard and possibly damaging your equipment.
Although the white wires (neutral) and
the green wires (ground) in the AC wiring
are technically at the same potential
(voltage), and should measure the same
potential using a voltmeter, the ground
prong connections at the outlets should be
connected to the grounding bar that was
driven into the earth as an additional
safety precaution in case something
should happen to the wires running from
the service entrance transformer to the
building or within the equipment itself. If
a short should occur within the equip-
ment, hopefully the electricity will find its
Figure 4-3. Schematic of an Outlet
With a Lifted Neutral
If the PW4000 is plugged into one socket of the two
outlets with lifted neutral, and a rack of signal process-
ing equipment or power amplifiers is plugged into the
other, fuses would probably blow upon turning on the
system, and some of the sound equipment could be
destroyed.