Wiring An S7-1200 System - Siemens SIMATIC S7 Functional Safety Manual

Fail-Safe CPU and signal module (SM) installation
4.3 Control system wiring guidelines
4.3.3

Wiring an S7-1200 system

Guidelines for wiring the S7-1200 system
When designing the wiring for your S7-1200, provide a single disconnect switch that
simultaneously removes power from the fail-safe 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.
Note
The standard I/O channels integrated in the S7-1200 fail-safe CPUs are not designed to be
used for the fail-safe I/O function
In your project, you can use the integrated fail-safe CPU I/O with standard code blocks, but
you must use the S7-1200 fail-safe DI and DQ signal modules as the fail-safe digital I/O
channels with safety code blocks.
Use the shortest wire possible and ensure that the wire is sized properly to carry the required
current. The CPU and SM connectors accept wire sizes from 2 mm
22 AWG). Wire strip length is 6.4 mm.
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 VDC 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 avoid damaging the connector, be careful that you do not
over-tighten the screws. The maximum torque for the CPU and SM connector screw is 0.56
N-m (5 inch pounds).
100
to 0.3 mm (14 AWG to
2 2
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA