Siemens Simatic S7-400 - Manual (Page 259)

Analog Modules
5-43
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
5.8 Connecting Sensors to Analog Inputs
Introduction
You can connect different sensors to the analog input modules depending on the
measuring method; voltage and current sensors, and resistors.
This section contains general information that is generally applicable to all the
connection options for sensors described in the sections that follow.
Cables for Analog Signals
To reduce electrical interference, you should use twisted-pair shielded cables for
the analog signals. The shield of the analog signal cables should be grounded at
both cable ends.
If there are potential differences between the cable ends, an equipotential bonding
current can flow over the shield, which leads to an interference of the analog
signals. In such a case, you should ground the shield at one end of the cable only.
Non-Isolated Analog Input Modules
In the case of the non-isolated analog input modules there is an electrical
connection between the reference point of the measuring circuit M
ANA
and chassis
ground.
You use non-isolated analog modules if there are few or no potential differences
between the measuring sensors and chassis ground.
Isolated Analog Input Modules
With the isolated analog input modules there is no electrical connection between
the reference point of the measuring circuit M
ANA
and chassis ground.
You use isolated analog input modules if a potential difference U
ISO
can occur
between the reference point of the measuring circuit M
ANA
and chassis ground. By
means of an equipotential bonding conductor between the M
ANA
terminal and
chassis ground, make sure that U
ISO
does not exceed the permitted value.
Limited Potential Difference U
CM
Only a limited potential difference U
CM
(common mode voltage) may occur
amongst the M– measuring lines of the input channels and between the leads and
the reference point of the measuring circuit M
ANA
. In order to prevent the
permissible value from being exceeded, you must take different actions, described
below, depending on the potential connection of the sensors.