Siemens SIMATIC S7-400 Reference Manual page 206

Analog modules
5.9 Connecting sensors to analog inputs
Connecting non-isolated sensors
Depending on local conditions or interference, potential differences U
can occur between the locally distributed measuring points. If the permitted value for U
exceeded, there must be equipotential bonding conductors between the measuring points.
1
3
Figure 5-6
(1)
(2)
(3)
M +:
M -:
M
ANA
U
ISO
Note
Do not use non-isolated 2-wire transducers and non-isolated resistance-type sensors!
206
M+
M-
M+
M-
U
CM
M
ANA
U
ISO
Connecting non-isolated sensors to an isolated AI
Non-isolated sensors
Chassis ground
Equipotential bonding conductor
Measuring line (positive)
Measuring line (negative)
: Reference potential of the analog measuring circuit
: Potential difference between M
2
and chassis ground
ANA
Reference Manual, Ausgabe 11/2016, A5E00850736-08
(static or dynamic)
CM
S7-400 Automation System Module Data
is
CM