Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.10 Common mode interference (UCM)
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
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2.10 Common mode interference (UCM)
Definition
Common mode interference denotes interference voltages and currents in the lines
connecting electrical devices and system components. They affect both the positive and
negative line with the same phase angle and amplitude. The interference signal needs an
additional current path to affect the utility signal. This current path is usually provided by
common reference potential, e.g. earth or ground connection, of the utility signal and
interference source.
Common mode interference in analog modules
Common mode interference occurs in analog input and output modules if their reference
potential is different from the reference potential of the connected sensor or actuator.
Common mode interference is most likely to occur when analog modules are operated with
grounded sensors. The size of the overall system means that voltage differences may
develop between machine components with grounded sensors and the analog module
reference. These voltage differences affect both the positive and negative signal path
equally, which is why they are termed common mode interference.
In operation with non-grounded sensors, the occurrence of common mode interference is not
as obvious as in grounded operation. Nevertheless, capacitive or inductive coupling can also
create voltage differences in such cases which have the effect of common mode
interference. Depending on the conditions in grounded and non-grounded operation,
common mode voltage can occur as direct or as alternating voltage.