According to IEC 61511-1, paragraph 11.4.4, the failure
tolerance HFT can be reduced by one for proven-in-use
subsystems if the following conditions are met:
l The instrument is proven in use
l Only process-relevant parameters can be modified on the
instrument (e.g. measuring range, current output in case of
failure …)
l The
modification of these process-relevant parameters is
protected (e.g. password, …)
l The
safety function requires less than SIL4
The assessment of the modification management was part of
the proof for "Proven in use".
1.2 Planning
The measuring system generates on the current output a
signal between 3.8 mA and 20.5 mA corresponding to the
level.
This analogue signal is transmitted to a connected processing
unit to monitor the following conditions:
l Exceeding
a preset level
l Falling below a preset level
When the switching point set on the processing unit is
reached, a signal is outputted.
The safe state depends on the mode:
Monitoring upper level Monitoring lower
level
Safe state Exceeding the switching
point
Falling below the
switching point
Output current in safe
state
> switching point (-1 %) < switching point (+1 %)
Failure current "fail low" <3.6 mA <3.6 mA
Failure current "fail high" >21.5 mA >21.5 mA
The current tolerance ±1 % refers to the full measuring range
of 16 mA.
A safe failure is present when the measuring system switches
to the defined safe state or the fault mode without the process
demanding it.
Proven in use
Safety function
Safe state
Fault description
VEGAPULS series 60 - 4 … 20 mA/HART 5
Functional safety
31338-EN-070510
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