Siemens S7-300 Installation And Operating Manual page 228

Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
External sensor supply
The figures in chapter "Applications of SM 336; F-AI 6 x 0/4 ... 20 mA HART (Page 229)"
show how you can supply power to the sensors via an external sensor supply (for example,
from another module).
If there is a short circuit from L+ to Mn+, the input resistors will be destroyed.
You can avoid this problem through proper wiring and use of the internal sensor supply.
When an external sensor supply is used, other suitable measures are necessary to protect
the input resistors (e.g. fuse on the module).
The stability of the external sensor supply must conform to the desired safety requirement
class SIL 2, 3. To ensure that the sensor functions problem-free, we recommend one of the
following options:
 Use of a redundant external sensor supply
 Monitoring of the external sensor supply for undervoltage/overvoltage, including
Isolated transducers
The isolated transducers are not bonded to local earth potential. These transducers can be
operated with floating potential. Local conditions or interference may cause potential
differences U
and the reference point of measuring circuit M.
Note
It is advisable to wire M- to M in order to prevent common mode voltages in excess of the
permitted value for U
interference.
Non-isolated transducers
The non-isolated transducers are bonded to local earth potential. Always interconnect M with
earth potential. Local conditions or interference may cause potential differences U
or dynamic) between the locally distributed measuring points.
If common mode voltage exceeds the permitted value for U
equipotential bonding conductors between the measuring points.
228
WARNING
WARNING
or
shutdown of the sensor supply when a fault is detected (single-channel for SIL 2; dual-
channel for SIL 3)
(static or dynamic) between the measuring lines M- of the input channels
CM
when operating the equipment in areas subject to heavy EMC
CM
, you must provide for
CM
Installation and Operating Manual, 01/2010, A5E00085586-10
(static
CM
Fail-safe signal modules