KROHNE OPTISWITCH 3x00C Relay SIL EN - Manual (Page 5)

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 is present when the measuring system switches
to the defined safe state or the fault mode without the process
demanding it.
If the internal diagnosis system detects a failure, the measuring
system goes into fault mode.
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.
Safe state
Fault description
Configuration of the
processing unit
Low demand mode
OPTISWITCH series 3000 • - Relay (DPDT) 5
Functional safety
32743
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EN
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080414