38477BA2B
1 – 66 25.5.09
9 Functional Safety Volume 1
• In the operating mode with high demand rate the reaction time
of the LB 490 to dangerous apparent faults is max. 1 day.
If the demand rate is not more than once a year, the measuring
device may be operated as a safety-relevant sub-system in the
operating mode with low demand rate (IEC 61508-4, 3.5.12).
If the ratio of the internal diagnostic test rate of the measuring sys-
tem to the demand rate exceeds the value 100, the measuring sys-
tem can be treated as if were executing a safety function in the
operating mode with low demand rate (IEC 61508-2, 7.4.3.2.5).
Diagnostic rate: once every 5 minutes + 15 minutes = 20 minutes
The associated parameter is the value PFD
avg
(average Probability
of dangerous Failure on Demand). The value is dependent on the
test interval T
Proof
between the function checks of the PLT protec-
tive function.
Numerical values see section "9.9 Safety-Technical Data".
Safe state The intrinsically safe state is reached when the current output indi-
cates the following values:
< 3.6mA
> 21.5mA.
A safe failure is defined as a failure that causes the measuring sys-
tem to go to the defined intrinsically safe state without a demand
from the process.
A dangerous undetected failure is present if the measuring system,
following a demand from the process, does not go to the defined
intrinsically safe state.
To Next Page
