Siemens SIMATIC S7 System Manual page 63

Isolation guidelines
The AC power supply boundaries and I/O boundaries to AC circuits have been designed and
approved to provide safe separation between AC line voltages and low voltage circuits.
These boundaries include double or reinforced insulation, or basic plus supplementary
insulation, according to various standards. Components which cross these boundaries such
as optical couplers, capacitors, transformers, and relays have been approved as providing
safe separation. Only 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.
The sensor supply output, communications circuits, and internal logic circuits of an S7-200
SMART with included AC power supply are sourced as SELV (safety extra-low voltage)
according to EN 61131-2.
To maintain the safe character of the S7-200 SMART low voltage circuits, external
connections to communications ports, analog circuits, and all 24 V DC nominal power supply
and I/O circuits must be powered from approved sources that meet the requirements of
SELV, PELV, Class 2, Limited Voltage, or Limited Power according to various standards.
Safe use of power converters
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.
Use only high-voltage-to-low-voltage power converters that are approved as sources of
touch-safe, limited-voltage circuits.
Grounding guidelines
The best way to ground your application is to ensure that all the common and ground
connections of your PLC and related equipment are grounded to a single point. This single
point should be connected directly to the earth ground for your system.
All ground wires should be as short as possible and should use a large wire size, such as
2 mm
When locating grounds, remember to consider safety grounding requirements and the proper
operation of protective interrupting devices.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
WARNING
(14 AWG).
2
Installation
3.4 Wiring guidelines
63