Beckhoff EL3773 Documentation page 128

Commissioning
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. 145: 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
• 3- and 4-wire connection see Fig. Connection of externally supplied sensors, cf. IEC60381-1
• the sensor draws the energy/operating voltage for its own operation from 2 supply cables of its own.
One or two further sensor cables are used for the signal transmission of the current loop:
◦ 1 sensor cable: according to the Beckhoff terminology such sensors are to be connected to
'single-ended' inputs in 3 cables with +/-/Signal lines and if necessary FE/shield
◦ 2 sensor cables: for sensors with 4-wire connection based on +supply/-supply/+signal/-signal,
check whether +signal can be connected to +supply or –signal to –supply.
- Yes: then you can connect accordingly to a Beckhoff 'single-ended' input.
- No: the Beckhoff 'differential' input for +Signal and –Signal is to be selected; +Supply and –
Supply are to be connected via additional cables.
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)!
Note: expert organizations such as NAMUR demand a usable measuring range <4 mA/>20 mA for error
detection and adjustment, see also NAMUR NE043.
The Beckhoff device documentation must be consulted in order to see whether the respective device
supports such an extended signal range.
Usually there is an internal diode existing within unipolar terminals/ boxes (and related product groups), in
this case the polarity/direction of current have to be observed.
128 Version: 2.5 EL3773