Siemens Simatic M7-400 Reference Manual page 261

Non-Isolated Sensors
The non-isolated sensors are connected with the local ground potential (local
ground). When using non-isolated sensors, you must connect M
ground.
Connecting Non-Isolated Sensors
Caused by local conditions or interference, potential differences U
dynamic) can occur between the locally distributed individual measuring points. If
the potential difference U
equipotential bonding conductors between the measuring points.
Figure 5-8
Note
Do not use non-isolated two-wire transmitters and non-isolated resistance sensors!
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
exceeds the permissible value, you must provide
CM
Non-isolated
sensors
U
CM
Equipotential
bonding conductor
Connecting Non-Isolated Sensors to an Isolated AI
Analog Modules
ANA
CM
M+
M–
M+
M–
M
ANA
U
ISO
Chassis ground
to chassis
(static or
5-45