VEGA VEGAFLEX series 60 Recurring function test - Manual (Page 3)

1 About this document
The recurring function test serves to check the safety function and
reveal possible undetected, dangerous failures (λ
du
). The functional
capability of the measuring system has to be tested in adequate time
intervals. It is the user's responsibility to select an appropriate type of
testing. The time intervals are subject to the PFD
avg
-value according to
the specifications in the Safety Manual (SIL).
With high demand rate, a recurring function test is not requested in
IEC 61508. The functional efficiency 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 effect of the redundancy
through recurring function tests at appropriate intervals.
With this supplementary information you can carry out a recurring
function test of level sensors VEGAFLEX 61, 62, 63, 65 and 66 without
dismounting the instrument or moving the level of the medium to the
switching point.
With this procedure, 88 % or 92 % of all dangerous undetected
instrument failures (λ
du
) are detected (DC
Proof
).
The described function test fulfils the requirements according to SIL.
If you have already created a sensor documentation during setup, you
can check the sensor at a detection rate of 92 % of all dangerous
undetected failures.
The remaining dangerous undetected failures are 11 FIT.
FIT = Failure In Time (1 FIT = 1 failure/10 9
h)
Make sure that also a later sensor documentation is possible. This
sensor documentation of a recurring function test must be at least 6
months old.
If you created no sensor documentation during setup, you can check
the sensor only at a detection rate of 88 % of all dangerous undetected
failures.
The remaining dangerous undetected failures are in this case 16 FIT.
FIT = Failure In Time (1 FIT = 1 failure/10 9 h)
Why a function test?
Fault detection rate λ
du
92 %
Fault detection rate λ
du
88 %
Recurring function test • for VEGAFLEX series 60 3
1 About this document
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EN
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