Siemens SIMATIC ET 200AL, System Manual

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Connecting transducers
4.3 Wiring analog inputs without MANA connection
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
69
4.3 Wiring analog inputs without MANA connection
The reference potentials of the measuring inputs and the central ground are electrically
isolated from each other in analog input modules without an M
ANA
connection.
Limited potential difference U
ISO
(insulation voltage)
Always verify that the maximum potential difference U
ISO
between the reference points of the
measuring inputs and 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 reference points of the measuring inputs and the central
grounding point.
Limited potential difference U
CM
(common mode)
Always ensure that the permitted potential difference U
CM
between the reference points of
the measuring inputs 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 permitted value U
CM
is not exceeded, install equipotential
bonding cables between the reference points of the measuring inputs or, with the
ET 200eco PN and ET 200pro, between the reference points of the measuring inputs and
ground.