Siemens S7-300 Installation And Operating Manual page 238

Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
Wiring scheme H: 4-wire transducer, external sensor supply with module redundancy
An undervoltage at the transducer cannot be detected, i.e., a transducer with undervoltage
detection may be required.
● It is necessary to incorporate the external elements into the application-specific safety
Note
1L+, 2L+, and 3L+ can be fed from one power supply. The maximum permissible common
mode voltage U
Depending on the internal structure of the sensor, a short circuit from 2L+ to Mn+ (sensor
with measuring circuit referring to 2M) or from Mn- to 2M (sensor with measuring circuit
referring to 2L+) can destroy the input resistors (see documentation for the utilized sensor).
For this reason, a suitable measure must be taken to protect the input resistors (e.g. fuse
on the module).
Figure 9-29
For information about the Zener diodes, refer to chapter "Calculation of the residual supply
voltage at the transducer (Page 239)".
238
WARNING
examination. This means appropriate external elements (e.g., Zener diodes) are needed
to achieve the respective Safety Integrity Level.
must be observed.
CM
WARNING
4-wire transducer, external sensor supply with module redundancy
Installation and Operating Manual, 01/2010, A5E00085586-10
Fail-safe signal modules