Siemens SIMATIC S7-200 System Manual page 35

Warning
Use of non-isolated or single insulation supplies to supply low voltage circuits from an AC line
can result in hazardous voltages appearing on circuits that are expected to be touch safe, such
as communications circuits and low voltage sensor wiring.
Such unexpected high voltages could result in death or serious injury to personnel, and/or
damage to equipment.
Only use high voltage to low voltage power converters that are approved as sources of touch
safe, limited voltage circuits.
Guidelines for Grounding the S7-200
The best way to ground your application is to ensure that all the common and ground 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
shielded wires for 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 that you do 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 1500 VAC 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.
Installing the S7-200
2 to 0.3 mm 2 (14 AWG to 22 AWG). Use
Chapter 3
2
21