Connecting To Power Supply; Preparing The Connection - Siemens SITRANS LG250 Operating Instructions Manual

Safety instructions
Voltage supply
Cable gland ½ NPT
Cable screening and
grounding
PBD-51041388
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
•
If overvoltage surges are expected, overvoltage arresters should
be installed
Power supply and current signal are carried on the same two-wire
cable. The voltage supply range can differ depending on the instru-
ment 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.
Keep in mind the following additional factors that influence the operat-
ing voltage:
•
The output voltage of the power supply unit can be lower under
nominal load (with a sensor current of 20.5 mA resp. 22 mA in
case of fault message)
•
Influence of additional instruments in the circuit (see load values in
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 61326-1 for industrial areas, screened cable should
be used.
Use cable with round cross section for instruments with housing and
cable gland. To ensure the seal effect of the cable gland (IP protection
rating), find out which cable outer diameter the cable gland is suitable
for.
Use a cable gland fitting the cable diameter.
With plastic housing, the NPT cable gland or the Conduit steel tube
must be screwed without grease into the threaded insert.
Max. torque for all housings see chapter "Technical data"
If screened cable is required, 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 equalisa-
tion (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.
Warning:
Significant potential differences exist inside galvanization plants as
well as on vessels with cathodic corrosion protection. Considerable
SITRANS LG250 - Operating Instructions
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