Siemens SIMATIC S7 System Manual page 64

Installation
3.4 Wiring guidelines
Wiring guidelines
When designing the wiring for your S7-200 SMART CPU, provide a single disconnect switch
that simultaneously removes power from the 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.
Use wire and cable with a temperature rating 30 °C higher than the ambient temperature
around the S7-200 SMART CPU (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 S7-200 SMART CPU gives the best results. You should ground communication
cable shields to S7-200 SMART CPU 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 S7-200 SMART CPU because the
sensor supply is already current-limited.
All S7-200 SMART CPU 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 S7-200 SMART CPU
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 679) 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.
64
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART