Siemens Simatic S7-300 - Isolated or Non-Isolated Modules

Configuring
S7-300 Programmable Controller Hardware and Installation
4-18 A5E00105492-01
4.8.4 Isolated or non-isolated modules?
Isolated modules
In configurations with isolated modules, the reference potentials of the control
circuit (M
internal
) and load circuit (M
external
) are electrically isolated (see Figure).
Application
Use isolated modules for:
• All AC load circuits
• DC load circuits with separate reference potential
Examples:
– DC load circuits whose sensors have different reference potentials (for
example if grounded sensors are located at some considerable distance
from the control system and no equipotential bonding is possible)
– DC load circuits with grounded positive pole (L+) (battery circuits).
Isolated modules and grounding conception
You can use isolated modules, regardless of whether or not the control system's
reference potential is grounded.
Example:
The figure below shows a sample configuration: One CPU 31xC or 312 IFM with
isolated modules.