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4 Wiring the Device
4.1
Wiring Guidelines
The following are general guidelines for wiring an S7-I/O
device.
Users should:
•
Follow all applicable electrical codes when wiring and
operating equipment connected to the S7-I/O device.
•
Use the shortest wiring runs possible.
•
Avoid placing I/O wiring near power conductors.
•
Equip I/O wiring with surge suppression devices if used
in areas subject to lightning surges.
•
Provide a single disconnect switch that removes power
from the device and all inputs and outputs.
•
Provide overcurrent protection to the device, inputs,
and outputs. Each output can be fused for further pro-
tection. External overcurrent protection for the inputs is
not required when the 24 VDC internal supply is used.
•
The DC sensor supply from the base module may be
used to supply power for the base module inputs, DC
Siemens Energy & Automation, Inc.
4
Wiring the Device
inputs on an expansion module, or relay coils on an AC
relay output expansion module. The DC sensor supply
is short-circuit protected.
•
Connect all ground terminals to the closest available
earth ground to provide the greatest immunity from
noise. If possible, connect all ground terminals on the
base and expansion units to a common point. Use 14
AWG (1.5 mm²) wire.
The following guidelines apply to using a DC power supply
with the DC base module.
•
Ensure the DC power supply has sufficient surge
capacity to maintain voltage during sudden load
changes. Additional external capacitance may be
required.
•
For ungrounded DC power supplies, connect a resistor
and capacitor in parallel from the power source com-
mon to ground. The resistor (1 MΩ typical) provides a
leakage path to prevent static charge accumulation.
The capacitor (4700 pf typical) provides a drain for
high-frequency noise. A grounded DC system may also
be created by connecting the power source common
directly to ground. See Figure 4.1 below.
AC Power
Line
Neutral
Ground
DC Power Supply
AC
DC
4700 pf
1 MΩ
Typical
Typical
Figure 4.1 DC Supply connection
Base Module
11
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