Analog modules
9.3 SM 336; F-AI 6 x 0/4 ... 20 mA HART
Fail-safe signal modules
228
Installation and Operating Manual, 01/2010, A5E00085586-10
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).
WARNING
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).
WARNING
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
or
Monitoring of the external sensor supply for undervoltage/overvoltage, including
shutdown of the sensor supply when a fault is detected (single-channel for SIL 2; dual-
channel for SIL 3)
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
CM
(static or dynamic) between the measuring lines M- of the input channels
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
CM
when operating the equipment in areas subject to heavy EMC
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
CM
(static
or dynamic) between the locally distributed measuring points.
If common mode voltage exceeds the permitted value for U
CM
, you must provide for
equipotential bonding conductors between the measuring points.