Siemens SIMATIC ET 200AL, System Manual

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Connecting transducers
4.2 Wiring analog inputs with MANA connection
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
67
4.2 Wiring analog inputs with MANA connection
The reference potentials of the measuring inputs and the central ground are electrically
isolated in analog input modules with M
ANA
connection.
Limited potential difference U
ISO
(insulation voltage)
Always verify that the admissible potential difference U
ISO
between the reference point of the
analog ground M
ANA
and the central ground is not exceeded.
Potential difference U
ISO
may be caused by: Maximum line lengths exceeded.
If you want to ensure that the permitted value U
ISO
is not exceeded, install an equipotential
bonding cable between the terminal M
ANA
and the central grounding point.
Limited potential difference U
CM
(common mode)
Always verify that the admissible potential difference U
CM
between the reference point of the
measuring inputs and the analog groundM
ANA
is not exceeded.
Potential differences U
CM
may be caused by:
● Environments with EMC disturbance
● Use of grounded transducers
● Use of long cable lengths
Measuring errors / malfunctions can occur if the admissible potential difference U
CM
is
exceeded.
Some modules can detect an invalid potential difference U
CM
and report this error with an
entry in the diagnostics buffer of the CPU.
If you want to ensure that the maximum value U
CM
is not exceeded, interconnect the
reference points of the measuring inputs and the analog ground M
ANA
with an equipotential
bonding cable.