Siemens Simatic S7-200 System Manual page 31

Guidelines for Grounding the S7-200
The best way to ground your application is to ensure that all the common connections of your S7-200 and
related equipment are grounded to a single point. This single point should be connected directly to the
earth ground for your system.
For improved electrical noise protection, it is recommended that all DC common returns be connected to
the same single-point earth ground. Connect the 24 VDC sensor supply common (M) to earth ground.
All ground wires should be as short as possible and should use a large wire size, such as 2 mm
(14 AWG).
When locating grounds, remember to consider safety grounding requirements and the proper operation of
protective interrupting devices.
Guidelines for Wiring the S7-200
When designing the wiring for your S7-200, provide a single disconnect switch that simultaneously
removes power from the S7-200 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.
You might want to provide 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
optimum protection against electrical noise. Typically, grounding the shield at the S7-200 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 S7-200 because the sensor supply is already current-limited.
Most S7-200 modules have removable connectors for user wiring. (Refer to Appendix A to determine if
your module has removable connectors.) 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 to not over-tighten the screws. The maximum torque for the connector screw is
0.56 N-m (5 inch-pounds).
To help prevent unwanted current flows in your installation, the S7-200 provides isolation boundaries at
certain points. When you plan the wiring for your system, you should consider these isolation boundaries.
Refer to Appendix A for the amount of isolation provided and the location of the isolation boundaries.
Isolation boundaries rated less than 1500VAC must not be depended on as safety boundaries.
Tip
For a communications network, the maximum length of the communications cable is 50 m without using
a repeater. The communications port on the S7-200 is non-isolated. Refer to Chapter 7 for more
information.
2 2
to 0.3 mm (14 AWG to 22 AWG). Use shielded wires for
Installing the S7-200 Chapter 3
2
3
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