Siemens SIMATIC S5-115U, User Manual

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S5-115U Manual Analog Value Processing
Certain precautionary measures must be taken in order to make sure that potential difference
V
CM
is not exceeded. Different measures are required for isolated and non-isolated transducers.
When isolated transducers are used, the measuring circuit can assume a potential to earth that
exceeds the permissible potential difference U
CM
(refer to the maximum values for the various
modules).
To prevent this, the transducer's negative potential must be connected to the module's reference
potential (reference bus).
Example: Measuring temperature on a busbar with an isolated thermocouple.
In a worst-case situation, the measuring circuit can assume a potential that would
destroy the module; this must be prevented through the use of an equipotential
bonding conductor (see Figure 10-3).
Possible causes:
•
Static charge
• Contact resistors through which the measuring circuit assumes the potential of the
busbar (e.g. 220 V AC).
When using non-isolated transducers, the permissible potential difference U
CM
between the
inputs and the reference bus must not be exceeded.
Example: Measuring the temperature of the busbar of an electroplating bath with a non-iso-
lated thermocouple. The difference between the potential of the busbar and the
reference potential of the module is max. 24 V DC. A 460 analog input module with
floating input (permissible U
CM
is 60 V AC/75 V DC) is to be used.
Figure 10-3. Connecting Transducers
Range
card for
four
inputs
Range
card for
four
inputs
-
-
+
+ +
-
M -
M+
M -
M+
Equi-
potential
bonding
conductor
U
CM
U
CM
U
E
U
E
Module
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Isolated
transducer Non-isolated
transducer
EWA 4NEB 811 6130-02b
10-5