Safety Function - Siemens SITRANS P Operating Instructions Manual

Functional Safety
10.1 Safety concept
See also
General functional safety (http://www.siemens.com/safety)
Functional safety in process instrumentation (http://www.siemens.com/SIL)
10.1.3

Safety function

The safety function of the device is the pressure measurement or the measurement of certain
process values which can be calculated from the pressure value.
The 4 to 20 mA analog output can be used as part of a safety instrumented function (SIF).
Ensure that you have only connected one device per channel and that the current output is
activated.
Besides the application-specific measuring errors under default reference conditions, an
additional safety accuracy of ± 2% of the maximum measuring range must be added:
Total tolerance (safety function) = ± [application-specific measurement error + 2 % safety
accuracy].
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. When 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
pressure transmitter and remote seal measurement errors.
Safety allowance
The diagnostics function reacts within 2 seconds after detection of an error.
Note
For use outside the standard reference conditions, contact Siemens to define an additional
safety accuracy.
See also
Remote seals and primary element for devices with functional safety (Page 41)
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SITRANS P320/P420 with 4 to 20 mA/HART
Operating Instructions, 11/2019, A5E44852162-AC