Parameterization and programming
ELX3351
49
Version: 1.3.1
Typification of the 2/3/4-wire connection of current sensors
Current transducers/sensors/field devices (referred to in the following simply as ‘sensor’) with the industrial
0/4-20 mA interface typically have internal transformation electronics for the physical measured variable
(temperature, current, etc.) at the current control output. These internal electronics must be supplied with
energy (voltage, current). The type of cable for this supply thus separates the sensors into
self-supplied
or
externally supplied
sensors:
Self-supplied sensors
• The sensor draws the energy for its own operation via the sensor/signal cable + and -.
So that enough energy is always available for the sensor’s own operation and open-circuit detection is
possible, a lower limit of 4 mA has been specified for the 4-20 mA interface; i.e. the sensor allows a
minimum current of 4 mA and a maximum current of 20 mA to pass.
• 2-wire connection see Fig.
2-wire connection
, cf. IEC60381-1
• Such current transducers generally represent a current sink and thus like to sit b and – as a
‘variable load’. Refer also to the sensor manufacturer’s information.
Fig. 39: 2-wire connection
Therefore, they are to be connected according to the Beckhoff terminology as follows:
preferably to
‘single-ended’ inputs
if the +Supply connections of the terminal/ box are also to be used -
connect to +Supply and Signal
they can, however, also be connected to
‘differential’ inputs
, if the termination to GND is then
manufactured on the application side – to be connected with the right polarity to +Signal and –Signal
It is important to refer to the information page
Configuration of 0/4..20 mA differential inputs
(see
documentation for the EL30xx terminals, for example)!
Externally supplied sensors
WARNING
An external supply of sensors / actuators, which are connected to signal ter-
minals of the ELX series is not permitted!
In terms of intrinsic safety, all signal terminals of the ELX series are energy-supplying, as-
sociated equipment. For this reason, connected sensors or actuators are supplied exclu-
sively via the respective channel of the terminal and must not be externally supplied in any
form (e.g. via an additional, external supply voltage).
This limitation is also independent of whether the additional, external supply is energy lim-
ited in the sense of IEC 60079-11.
Connecting any externally powered, intrinsically safe circuits to a ELX signal terminal con-
tradicts the intended use and the specified
technical data for explosion protection [
.
The explosion protection provided by the specified type of protection thus automatically ex-
pires.
