KROHNE OPTISWITCH 5xx0C NAMUR SIL EN - Manual (Page 8)

With high demand rate, a recurring function test is not requested in
IEC
61508. The function of the measuring system is demonstrated by
the frequent use of the system. In double channel architectures it is a
good idea to verify the redundancy through recurring function tests at
appropriate intervals.
The test must be carried out in a way that verifies the flawless
operation of the safety functions in conjunction with all system
components.
This is ensured by a controlled reaching of the response height during
filling. If filling up to the response height is not possible, then a
response of the measuring system must be triggered by a suitable
simulation of the level or the physical measuring effect.
The methods and procedures used during the tests must be stated
and their suitability must be specified. The tests must be documented.
If the function test proves negative, the entire measuring system must
be switched out of service and the process held in a safe state by
means of other measures.
In the double channel architecture (1oo2D) this applies separately to
both channels.
The function of connected instruments can be checked in the "max."
and "min." modes with the "Simulation key". The test procedure is
described in detail in the operating instructions manual.
For this test, take note of the condition of the vibrating element:
l Mode "max.": Vibrating element "uncovered"
l
Mode "min.": Vibrating element "covered"
1 .7 Safety-related characteristics
The failure rates of the electronics, the mechanical parts of the
transmitter as well as the process fitting are determined by an FMEDA
according to IEC 61508. The calculations are based on component
failure rates according to SN 29500. All values refer to an average
ambient temperature during the operating time of 40 °C (104 °F).
For a higher average temperature of 60 °C (140 °F), the failure rates
should be multiplied by a factor of 2.5. A similar factor applies if
frequent temperature fluctuations are expected.
The calculations are also based on the specifications stated in chapter
"P
lanning".
After 8 to 12 years, the failure rates of the electronic components will
increase, whereby the derived PFD and PFH values will deteriorate
(IEC 61508-2, 7.4.7.4, note 3).
Function test of con-
nected instruments
Basics
Service life
8 OPTISWITCH series 5000 • - NAMUR
1 Functional safety
32747
-EN
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