Burkert 8137 Manuel D'instructions page 21

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Note safety instructions
Take note of
safety instruc-
tions for Ex ap-
plications
Select power supply
Selecting connection
cable
Cable screening and
grounding
LEVEL TRANSMITTER 8137 • 4 ... 20 mA/HART
5 Connecting to power supply
5.1 Preparing the connection
Always keep in mind the following safety instructions:
Connect only in the complete absence of line voltage
l
If overvoltage surges are expected, overvoltage arresters should
l
be installed
In hazardous areas you should take note of the appropriate
regulations, conformity and type approval certificates of the sensors
and power supply units.
Power supply and current signal are carried on the same two-wire
cable. The voltage supply range can differ depending on the
instrument version.
The data for power supply are specified in chapter "Technical data".
Provide a reliable separation between the supply circuit and the mains
circuits according to DIN VDE 0106 part 101.
Bear in mind the following factors regarding supply voltage:
Output voltage of the power supply unit can be lower under
l
nominal load (with a sensor current of 20.5 mA or 22 mA in case of
fault message)
Influence of additional instruments in the circuit (see load values in
l
chapter "Technical data")
The instrument is connected with standard two-wire cable without
screen. If electromagnetic interference is expected which is above the
test values of EN 61326 for industrial areas, screened cable should be
used.
Use cable with round cross-section. A cable outer diameter of 5 ... 9 mm
(0.2 ... 0.35 in) ensures the seal effect of the cable gland. If you are
using cable with a different diameter or cross-section, exchange the
seal or use a suitable cable gland.
If screened cable is necessary, connect the cable screen on both ends
to ground potential. In the sensor, the screen must be connected
directly to the internal ground terminal. The ground terminal on the
outside of the housing must be connected to the potential equalisation
(low impedance).
If potential equalisation currents are expected, the connection on the
processing side must be made via a ceramic capacitor (e. g. 1 nF,
1500 V). The low frequency potential equalisation currents are thus
suppressed, but the protective effect against high frequency interfer-
ence signals remains.
5 Connecting to power supply
21