Vega VEGAFLEX 80, Руководство по безопасности

Страница: 5 / 20

5
3 Planning
VEGAFLEX 80 series • 4 … 20 mA/HART - two-wire
42960-EN-140924
3 Planning
3.1 Safety function
On its current output, the sensor generates a signal between 3.8 mA
and 20.5 mA corresponding to the level or the interface. This ana-
logue signal is fed to a connected processing system to monitor the
following conditions:
•
Exceeding a level or interface limit
•
Falling below a level or interface limit
•
Monitoring of a level or interface range
For the interpretation of the safety function, the following aspects
must be taken into account in respect to the tolerances:
•
Due to a dangerous undetected failure in the range between
3.8 mA and 20.5 mA, an incorrect output signal can be generated
which deviates from the real measured value by up to 2 %
•
Increased measurement deviations can occur at the boundaries of
the measuring range (see Technical Data in the operating instruc-
tions)
3.2 Safe state
The safe state of the current output depends on the safety function
and the characteristics set on the sensor.
Character-
istics
Monitoring upper limit val-
ue
Monitoring lower limit value
4 … 20 mA Output current ≥ Switching
point
Output current ≤ Switching
point
20 … 4 mA Output current ≤ Switching
point
Output current ≥ Switching
point
Possible fault currents:
•
≤ 3.6 mA ("fail low")
•
> 21 mA ("fail high")
3.3 Prerequisites for operation
•
The measuring system should be used appropriately taking pres-
sure, temperature, density and chemical properties of the medium
into account. The application-specific limits must be observed.
•
The specifications according to the operating instructions manual,
particularly the current load on the output circuits, must be kept
within the specified limits
•
Existing communication interfaces (e. g. HART, USB) are not used
for transmission of the safety-relevant measured value
•
The instructions in chapter " Safety-related characteristics ", para-
graph " Supplementary information " must be noted
•
All parts of the measuring chain must correspond to the planned
" Safety Integrity Level (SIL) "
Safety function
Safety tolerance
Safe state
Output signals in case of
malfunction
Instructions and restric-
tions