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274
AC
POWER
GROUNDING
CHECKS
To check
for
proper AC
power
receptacle grounding,
measure the AC voltages
at
the location shown in
the
following figure. This check does not detect a poor
quality ground (high resistance
to
earth).
Grou:1-o
d
-.t===O
'\
n--,
\L...--Neutral
Hot
~
Front
View
The voltage between neutral and ground should be less
than 2 volts AC.
The voltage between neutral and hot should be
approximately 110 Vac
to
120 Vac. Also, the voltage
between ground and
hot
should be approximately
110
Vac
to
120 Vac.
Check
that
all chassis ground connections are clean and
tight
in all devices on the system. Chassis grounding is
either a braided cable
or
a green and yellow wire.
5100 ground locations:
Bottom cover, center right side
- CRT mounting screen
Power supply fan
motor
Raceway (only
if
an
I/O
device installed)
Power switch
AC
power
box
- Tape unit
motor
- AC capacitor in tape unit
5103 ground locations:
Left
front
corner
- Right
front
corner
AC line cord
- Flat cable shield clamp
Left rear
of
forms tractor
5106 ground locations:
- Tape unit
motor
AC capacitor
- Cooling fan
Power switch
AC line cord
- Maple block
Flat cable shield clamp
AC
Power
Considerations
Checking
for
proper AC line voltage is a task
that
you
are familiar with. However, other aspects
of
the AC
power
source are also very important. The AC line
voltage should
not
vary by more than
±1
0 % except
for
500
ms transients
of
+
15%
to
-18%.
A type
of
AC
power
disturbance becoming more
frequent is fractional phase loss. This is a result
of
phase controlled triacs
or
SCRs controlling motors,
ovens, or other loads. These devices turn on their loads
during each phase and deplete the sine wave
of
energy.
AC
Power
Terms
Current carrying ground: This is the neutral line
that
is
connected
to
the neutral bus.
Neutral bus: This bus bar is inside the
power
panel.
The neutral bus should be tied
to
this bus. The neutral
bus should be tied
to
the ground bus only at the main
distribution panel.
Main distribution panel: This is the
first
power
panel
inside
of
the customer's building. This panel is fed
directly from the
power
company lines. The ground bus
in this panel is serviced by the service entrance ground.
Ground bus: This bus bar is in the power panel. The
ground wire
for
the 51oo's
power
source should connect
to
this bar, along
with
a wire into the panel
from
an
approved earth ground source.
Earth ground: The definition depends upon local
electrical building codes. Usually,
an
earth ground is
supplied in
two
forms:
1.
2.
A metal pipe running into the earth and
containing running water. Stagnant
water
or
sump lines are
not
good earth ground sources.
Lines broken by nonmetal connections
do
not
supply a
good
earth ground.
A metal stake driven into the ground. The
length
of
the stake and the depth
that
it
must
be driven into the ground depend on local
codes.
Power 2-55
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