Chapter 3 Functional Safety
Mini-Switch LB 471
3.7 Recurrent Performance Test
The recurrent performance test is used to check the safety func-
tion to uncover possibly undetected dangerous failure. The opera-
tional capability of the measuring system has to be checked in ad-
equate intervals.
It is in the responsibility of the operator to select the type of test
and the proof test interval. The intervals are dependent on the
PFD
avg
value defined in the table and chart in the section “Safety-
Technical Data” (see also FMEDA Report).
The test has to be carried out such that the proper safety function
will be proven through interaction of all components.
This is the case when the level is controlled within the scope of a
filling. If a filling is not feasible, the measuring system has to be
triggered to respond by suitable simulation of the level or of the
physical measurement effect.
The methods and procedures used in the tests have to be named
and their degree of suitability has to be specified. The tests have
to be documented.
If the function check is negative, you have to take the entire
measuring system out of service and keep the process in a safe
state through other measures.
3.8 Safety-Technical Data
Failure rates of the electronics were determined through FMEDA
according to IEC 61508. Calculations are based on the component
failure rates according to SN 29500. All numerical values refer to
an average ambient temperature of +40°C (104°F) during the op-
erating period. Calculations are further based on the information
provided in chapter 3.4 Project Planning.
safe detected failure (1 FIT = failure/10
9
h)
dangerous detected failure
dangerous undetected failure
T
Reaction
Failure reaction time
max. service life of the measuring system for the
safety function.
To Next Page
Loading...