Fig. 24 2-Wire Connection - Beckhoff ELX3312 Operating Manual

Basic function principles
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 between + and – as a
'variable load'. Refer also to the sensor manufacturer's information.
Fig. 24: 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
An external supply of sensors / actuators, which are connected to signal terminals of the
ELX series is not permitted!
In terms of intrinsic safety, all signal terminals of the ELX series are energy-supplying, associated equip-
ment. For this reason, connected sensors or actuators are supplied exclusively via the respective channel
of the terminal and must not be externally supplied in any form (e.g. via an additional, external supply volt-
age).
This limitation is also independent of whether the additional, external supply is energy limited in the sense
of IEC 60079-11.
Connecting any externally powered, intrinsically safe circuits to a ELX signal terminal contradicts the in-
tended use and the specified technical data for explosion protection. The explosion protection provided by
the specified type of protection thus automatically expires.
36
WARNING
Version: 1.3.0 ELX3312 and ELX3314