Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.14 Diagnostics
Analog value processing
44
Function Manual, 06/2014, A5E03461439-AC
Short-circuit
Activating the "Short circuit" checkbox activates short-circuit diagnostics for an analog
channel. The diagnostics function is triggered by channel overload.
Possible causes:
●
Wiring error (e.g. short circuit between conductors at the connections or in the cable)
● Use of faulty or incorrect actuators (e.g. internal short circuit or low input resistance as a
result of actuator failure)
The analog signal output is used for short circuit detection. If the voltage is too low, reliable
short-circuit diagnostics are not possible. In this case, the diagnostics function is deactivated
without triggering a change of the diagnostics status. The voltage limit up to which the
module will diagnose a short circuit can be found in the technical data for the module.
SIMATIC modules have a special protective circuit to prevent short circuit. The short circuit
current is limited internally in the module. The level of short circuit current is specified in the
technical data of your analog module.
Note
Overload
Please note that overload exposes the modules are exposed to a greater thermal load. This
can affect the output channels. You should therefore avoid continuously running analog
output modules in overload.
Reference junction
This type of diagnostics can only be selected for analog input modules for "Thermocouple"
measuring. Selecting the "Reference junction" check box diagnoses an error on the
reference channel for compensating the reference junction temperature.
The temperature at the reference junction of a thermocouple is measured at the reference
channel of the module using an external thermal resistor (RTD). If an error occurs, for
example as a result of a wire break, the temperature measured at the reference junction of
the thermocouple will no longer be compensated. The reference temperature may then be
outside the valid range.
Additional information on connecting thermocouples and thermal resistors and on how they
operate can be found in Thermocouples and Wiring transducers.