VEGA VEGAFLEX 52P (Profibus PA) - Manual (Page 23)

VEGAFLEX 51P and 52P 23
Electrical connection
3 Electrical connection
3.1 Connection – Connection cable
– Screening
Safety information – Qualified person-
nel
Instruments which are not operated with
protective low voltage or DC voltage must be
connected only by qualified personnel. This
also applies to the configuration of measuring
systems planned for Ex environment.
As a rule, do all connecting work in the com-
plete absence of line voltage. Always switch
off the power supply before you carry out
connecting work on the sensors. Protect
yourself and the instruments.
Connection cables and bus configura-
tion
Very often "Electromagnetic pollution" by
electronic actuators, energy lines and emit-
ting systems is so considerable that the two-
wire cable should be screened.
We recommend the use of screened cable.
This screening also guards against future
sources of interference. The screening
should be applied only to one sensor end
(fig. 3.1 a).
It is better if the screens are earthed on both
ends. However, it must be certain that no
earth compensation currents flow via the
sensor cable screens (fig. 3.1 b). Earth com-
pensation currents can be avoided by con-
necting the cable screen (in case of earthing
on both sides) on one earthed side (e.g. in
the switching cabinet) via a capacitor (e.g.
0.1 µF; 250 V AC) to earth potential. Use a
very low impedance earth connection (foun-
dation, plate or mains earth). In dust-Ex ap-
plications the screen should be on one end.
Potential losses can be caused by earthing
on both ends.
Note the Profibus specification. The connec-
tion cables must be specified for the ex-
pected operating temperatures in the plant
and must have an outer diameter of
6 … 12 mm, to ensure the seal effect of the
cable entry on the sensor.
For power supply and bus communication, a
two-wire cable acc. to the Profibus specifica-
tion (up to max. 2.5 mm
2 cross-section area
of conductor) is used. The electrical connec-
tion on the sensor is made by spring-loaded
terminals.
In a laboratory setup, a Profibus system will
also work with standard, unshielded two-wire
cable. In practice however, an automation
network and bus system can only be pro-
tected reliably against electromagnetic inter-
ference with screened cable. Acc. to the
Profibus specification (IEC 1158-2),
screened and twisted cables are prescribed.
All participants are connected in one line
(serially). At the beginning and end of the
bus segment, the bus is terminated by an
active bus termination. On the DP bus level,
most participants already have a bus termi-
nation implemented. With more than 32 par-
ticipants on the DP level, a so-called repeater
must be used to open and combine another
DP level with a max. of 32 additional partici-
pants. On the PA bus branch of the segment
coupler, the PA radar sensors work also with
max. 32 participants (Ex max. 10 partici-
pants).
A PA sensor can work only in conjunction with
a Profibus DP system, to which a Profibus PA
subsystem is connected. A PA Profibus par-
ticipant must consume min. 10 mA supply
current.