Siemens Simatic S7-300, Инструкция по установке и эксплуатации

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Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
Fail-safe signal modules
238
Installation and Operating Manual, 01/2010, A5E00085586-10
Wiring scheme H: 4-wire transducer, external sensor supply with module redundancy
WARNING
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
examination. This means appropriate external elements (e.g., Zener diodes) are needed
to achieve the respective Safety Integrity Level.
Note
1L+, 2L+, and 3L+ can be fed from one power supply. The maximum permissible common
mode voltage U
CM
must be observed.
WARNING
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).
60
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Figure 9-29 4-wire transducer, external sensor supply with module redundancy
For information about the Zener diodes, refer to chapter "Calculation of the residual supply
voltage at the transducer (Page 239)".