With the correct wiring, how can you avoid exceeding the common-mode voltage (Ucm) and subsequent measured value overflow or underflow in the analog input module 6ES7 331-1KF0.-0AB0?

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When operating the module, a maximum voltage of 2V is specified for the difference of potential Ucm (common-mode voltage) between the "M-" input terminals.
This data is available in the manual S7-300 Module Data, chapter 6.
Link to manual: Entry ID 8859629
The common-mode voltage can be slightly exceeded with open inputs and when potential-free measuring transducers are connected. Therefore you must take the following measures for isolated 4-wire measuring transducers and potential-free voltage sensors:

Connect M- of the inputs to each other and to the module ground.
Disable unused channels and likewise connect M- to the module ground.
In the case of disabled channels on the front connector, if long, open cables are connected, we recommend a jumper between M+ / S- and M-. This avoids sources of interference from these open cables.

Measurements for determining the error voltage between the "M-" terminals for determining the Ucm are not practical, because even high-ohm measuring devices cause equipotential bonding.

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Entry belongs to product tree folder(s):

Automation Technology Automation Systems Industrial Automation Systems SIMATIC PLC Advanced Controller S7-300 I/O modules Analog input/output modules SM331 input module

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