The automation system program must monitor the measured values of both SITRANS
P devices. As soon as the measured values differ by e.g. 2% or more, the system must be
brought into the safe state and the fault must be located.
Two SITRANS P devices are required for multi-channel operation for SIL 3. Operation with
one device is not permitted.
Note
Switching-off of system at high monitoring accuracy
The two transmitters are connected to the process at different positions. Actual differences in
pressure ≥ the total tolerance (safety function) can occur when the process is started up or if
there are other pressure variations. A difference in pressure ≥ the total tolerance (safety
function) will shut down the system.
● Match the monitoring accuracy of the automation system to the process.
● Mount the two transmitters exposed to equal conditions.
10.1.3.1
Device statuses
The following table provides a definition of the states of the device:
Device status
Description
Error class
Normal mode (4-20 mA)
The safe current output outputs
the measured value within the
defined safety accuracy.
-
Detected failure (safe state)
The safe current output is
≤ 3.6 mA or ≥ 21 mA (specified
as failure signal).
(λ
DD
) Rate of dangerous detec‐
ted errors
Dangerous state
A dangerous state exists when a
current output is in the range
4-20 mA and deviates from the
correct process value by more
than the specified safety accura‐
seconds.
(λ
DU
) Rate of dangerous undetec‐
ted errors
10.1.3.2
Safety parameters
You can find the safety parameters in the certificate for functional safety in accordance with
IEC 61508.
Note
Useful lifetime
Constant failure rates are assumed for the calculation of PFD/SFF. This assumption is valid
for environmental conditions that are typical for an industrial environment, corresponding to
IEC 60654-1 class C (weatherproof locations) with an average long-term temperature of 40°C.
After 15 years, however, the failure rates may increase.
Functional Safety
10.1 Safety concept
SITRANS P320/P420 (mA/HART)
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Operating Instructions, 06/2018, A5E44852162-AA