KROHNE OPTISWITCH 5xx0C Transistor SIL EN - Manual (Page 5)

1 .2 Planning
The safety function of this measuring system is the identification and
signalling of the condition of the vibrating element.
A difference is made between the two conditions "covered" and
"
uncovered".
The safe state depends on the mode:
Overflow protection
(max. operation)
Dry run protection
(min. operation)
Vibrating element in safe
state covered uncovered
Output circuit in safe
state currentless currentless
The safe state of the measuring system is the switched-off status
(quiescent current principle):
l
C electronics: contactless electronic switch open
l
R electronics: relay output deenergised
l
T electronics: transistor output non-conductive
A safe failure exists when the measuring system switches to the
defined safe state or the fault mode without the process demanding it.
A dangerous undetected failure exists if the measuring system
switches neither to the defined safe condition nor to the failure mode
when the process requires it.
The processing unit must evaluate the output circuit of the measuring
system under the conditions of the quiescent current principle.
The processing unit must correspond to the SIL level of the
measurement chain.
If the demand rate is only once a year, then the measuring system can
be used as safety-relevant subsystem in "low demand mode"
(IEC 61508-4, 3.5.12).
If the ratio of the internal diagnostics test rate of the measuring system
to the demand rate exceeds the value 100, the measuring system can
be treated as if it is executing a safety function in the mode with low
demand rate (IEC 61508-2, 7.4.3.2.5).
An associated characteristic is the value PFD
avg
(average Probability
of dangerous Failure on Demand). It is dependent on the test interval
T
Proof
between the function tests of the protective function.
Number values see chapter "Safety-related characteristics".
Safety function
Safe state
Fault description
Configuration of the
processing unit
Low demand mode
OPTISWITCH series 5000 • - transistor (NPN/PNP) 5
1 Functional safety
32749
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EN
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