Installation
4.4 Wiring guidelines
S7-1200 Programmable controller
78 System Manual, V4.2, 09/2016, A5E02486680-AK
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. For more information, see Surge immunity (Page 1359) in the General technical
specifications section.
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.
Wire and cable should have a temperature rating 30 °C higher than the ambient temperature
around the S7-1200 (for example, a minimum of 85 °C-rated conductors for 55 °C ambient
temperature). You should determine other wiring type and material requirements from the
specific electrical circuit ratings and your installation environment.
Use shielded wires for optimum protection against electrical noise. Typically, grounding the
shield at the S7-1200 gives the best results. You should ground communication cable
shields to S7-1200 communication connector shells using connectors that engage the cable
shield, or by bonding the communication cable shields to a separate ground. You should
ground other cable shields using clamps or copper tape around the shield to provide a high
surface area connection to the grounding point.
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 V DC sensor supply from the S7-1200 because the sensor
supply is already current-limited.
All S7-1200 modules have removable connectors for user wiring. To prevent loose
connections, ensure that the connector is seated securely and that the wire is installed
securely into the connector.
To help prevent unwanted current flows in your installation, the S7-1200 provides isolation
boundaries at certain points. When you plan the wiring for your system, you should consider
these isolation boundaries. Refer to the technical specifications (Page 1425) for the amount
of isolation provided and the location of the isolation boundaries. Circuits rated for AC line
voltage include safety isolation to other circuits. Isolation boundaries between 24 V DC
circuits are functional only, and you should not depend on these boundaries for safety.
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