Planning; Safety Function; Safe State; Prerequisites For Operation - Vega VEGAMET 381 Ex Safety Manual

Safety accuracy
Output signals in failure
mode
General instructions and
restrictions
VEGAMET 381 Ex • 4 ... 20 mA signal conditioning instruments

2 Planning

2.1 Safety function

The transmitter powered by the signal conditioning instrument
generates a signal between 3.8 and 20.5 mA which is proportional to
the process variable.
Safety function relay output
Dependent on this analogue signal and the adjusted switching points,
one or two relays for limit value monitoring will be switched.
Safety function current output
This analogue signal can be also fed to a connected processing unit
(e.g. safety-oriented PLC). The set switching points can be used for
limit value monitoring.
The safety accouracy of the output signals is ±2 % relating to the
complete value range of 16.7 mA (3.8 ... 20.5 mA).

2.2 Safe state

Safe condition relay output
The safe condition on the relay output is the opened closing contact.
For the sagety function only the closing contact (NO contact) must be
used (closed circuit principle).
Safe condition current output
The safe state of the current output depends on the mode and the
characteristics adjusted on the sensor.
Rising characteristics:
4 mA = 0 %;
20 mA = 100 %
Falling characteristics:
20 mA = 0 %;
4 mA = 100 %
Relay output
Closing contact is open
l
Current output
"fail low" ≤ 3.6 mA
l
"fail high" > 21 mA
l

2.3 Prerequisites for operation

The measuring system should suit the application. The applica-
l
tion-specific limits must be maintained
Monitoring upper limit
value
Output current > Switch-
ing point -334 µA
Output current < Switch-
ing point +334 µA
2 Planning
Monitoring lower limit
value
Output current < Switch-
ing point +334 µA
Output current > Switch-
ing point -334 µA
5