Functional Safety; Safety Function - Siemens SITRANS P DS III Operating Instructions Manual

Functional safety

8.1

Safety function

The safety function of SITRANS P DS III with PROFIsafe protocol (ordering option -Z C21) is
based on the measurement of pressure.
The pressure is converted to a digital measured value and transmitted via PROFIsafe
communication. Fault tolerance allowances must be made, because the measured value
transferred to the automation system by the transmitter can deviate from the physical value.
This fault tolerance is calculated as follows:
Total tolerance (safety function) = ± [application-specific measurement error + 2 % safety
accuracy].
Transmitter safety accuracy: the maximum effect of a non-critical individual error on the
measured value.
The measured value is transferred along with the "Validity" and "Quality" status information.
The diagnostics function will respond within 60 seconds in the worst-case scenario.
The safety accuracy together with the application-specific measurement error allows the
system operator to include a backup for process monitoring. Even if a random individual
error occurs that is within the safety accuracy, the system can still be safely shut down.
As there is also always a possibility of dangerous faults occurring, these are categorized and
listed in the manufacturer declaration for the device (SIL declaration of conformity, functional
safety pursuant to IEC 61508 and IEC 61511).
Example:
A silo is to be securely monitored to check that the level does not exceed 10 meters.
Application-specific measurement error: 0.1%
Safety accuracy: 2.0%
Total tolerance: 2.1%
2.1% of 10 meters is 21 centimeters. If process monitoring is set to 9.79 meters, safe
shutdown is guaranteed even in the event of a random individual error within the safety
accuracy.
Note
Use of remote seals
If remote seals are used, the application-specific measurement error is the product of the
transmitter and remote seal measurement errors.
SITRANS P DS III with PROFIBUS PA
Operating Instructions, 06/2013, A5E00053276-07
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