96BInstallation
3.4 Wiring guidelines
S7-1200 Programmable controller
58
System Manual, 11/2011, A5E02486680-05
To maintain the safe character of the S7-1200 low voltage circuits, external connections to
communications ports, analog circuits, and all 24 V nominal power supply and I/O circuits
must be powered from approved sources that meet the requirements of SELV, PELV, Class
2, Limited Voltage, or Limited Power according to various standards.
WARNING
Use of non-isolated or single insulation supplies to supply low voltage circuits from an AC
line can result in hazardous voltages appearing on circuits that are expected to be touch
safe, such as communications circuits and low voltage sensor wiring.
Such unexpected high voltages could cause electric shock resulting in death, severe
personal injury and/or property damage.
Only use high voltage to low voltage power converters that are approved as sources of
touch safe, limited voltage circuits.
Guidelines for grounding the S7-1200
The best way to ground your application is to ensure that all the common and ground
connections of your S7-1200 and related equipment are grounded to a single point. This
single point should be connected directly to the earth ground for your system.
All ground wires should be as short as possible and should use a large wire size, such as 2
mm
2
(14 AWG).
When locating grounds, consider safety-grounding requirements and the proper operation of
protective interrupting devices.
Guidelines for wiring the S7-1200
When designing the wiring for your S7-1200, provide a single disconnect switch that
simultaneously removes power from the S7-1200 CPU power supply, from all input circuits,
and from all output circuits. Provide over-current protection, such as a fuse or circuit breaker,
to limit fault currents on supply wiring. Consider providing additional protection by placing a
fuse or other current limit in each output circuit.
Install appropriate surge suppression devices for any wiring that could be subject to lightning
surges.
Avoid placing low-voltage signal wires and communications cables in the same wire tray with
AC wires and high-energy, rapidly switched DC wires. Always route wires in pairs, with the
neutral or common wire paired with the hot or signal-carrying wire.
Use the shortest wire possible and ensure that the wire is sized properly to carry the required
current. The CPU and SM connector accepts wire sizes from 2 mm
2
to 0.3 mm
2
(14 AWG to
22 AWG). The SB connector accepts wire sizes from 1.3 mm
2
to 0.3 mm
2
(16 AWG to 22
AWG). Use shielded wires for optimum protection against electrical noise. Typically,
grounding the shield at the S7-1200 gives the best results.
When wiring input circuits that are powered by an external power supply, include an
overcurrent protection device in that circuit. External protection is not necessary for circuits
that are powered by the 24 VDC sensor supply from the S7-1200 because the sensor supply
is already current-limited.
