Siemens Simatic S7 Series System Manual page 51

Wiring guidelines
When designing the wiring for your PLC, adhere to the following guidelines:
● Provide a single disconnect switch that simultaneously removes power from the CPU
power supply, from all input circuits, and from all output circuits.
● Provide overcurrent 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 connector accepts wire sizes from 2 mm
22 AWG). Use shielded wires for optimum protection against electrical noise. Typically,
grounding the shield at the CPU 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 V DC sensor supply from the CPU because the
sensor supply is already current-limited.
● All S7-200 SMART 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. Siemens recommends that the wire insulation is removed
from the wire approximately 6 mm to ensure a proper connection. To avoid damaging the
connector, be careful that you do not over-tighten the screws. The maximum torque for
the CPU and EM connector screw is 0.56 N-m (5 inch-pounds). The maximum torque for
the SB connector screw is 0.33 N-m (3 inch-pounds)
Isolation boundaries
To help prevent unwanted current flows in your installation, the 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 565) for the amount of
isolation provided and the location of the isolation boundaries. Isolation boundaries rated
less than 1500 V AC must not be depended on as safety boundaries.
Guidelines for lamp loads
Lamp loads are damaging to relay contacts because of the high turn-on surge current. This
surge current will nominally be 10 to 15 times the steady state current for a tungsten lamp. A
replaceable interposing relay or surge limiter is recommended for lamp loads that will be
switched a large number of times during the lifetime of the application.
S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
Installation
3.4 Wiring guidelines
to 0.3 mm (14 AWG to
2 2
51