Connecting To Power Supply And Bus System; Preparing The Connection - Siemens SITRANS LG270 Operating Instructions Manual

Safety instructions
Voltage supply
Connection cable
Cable gland ½ NPT
Cable screening and
grounding
PBD-51041066
5 Connecting to power supply and bus
system
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
The instrument requires an operating voltage of 8 ... 30 V DC. Operat-
ing voltage and digital bus signal are carried on separate two-wire
connection cables.
The instrument is connected with standard two-wire, twisted cable
suitable for RS 485. If electromagnetic interference is expected which
is above the test values of EN 61326 for industrial areas, screened
cable should be used.
For instruments with housing and cable gland, use cable with round
cross-section. To ensure the seal effect of the cable gland (IP protec-
tion rating), you have to check for which cable outer diameter the
cable gland is suitable.
Use a cable gland fitting the cable diameter.
Make sure that the entire installation is carried out according to the
Fieldbus specification. In particular, make sure that the bus is termi-
nated with suitable terminating resistors.
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".
In systems with potential equalisation, connect the cable screen
directly to ground potential at the power supply unit, in the connection
box and at the sensor. The screen in the sensor must be connected
directly to the internal ground terminal. The ground terminal outside
on the housing must be connected to the potential equalisation (low
impedance).
In systems without potential equalisation, connect the cable screen
directly to ground potential at the power supply unit and at the sensor.
In the connection box or T-distributor, the screen of the short stub to
the sensor must not be connected to ground potential or to another
cable screen. The cable screens to the power supply unit and to the
next distributor must be connected to each other and also connected
to ground potential 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 interference signals
remains.
SITRANS LG270 - Operating Instructions 23