VEGA VEGAFLEX 55P (Profibus PA) - Manual (Page 17)

VEGAFLEX 55P (Profibus PA) 17
Electrical connection
3 Electrical connection
3.1 Connection – Connection cable
– Screening
Safety information – Qualified
personnel
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
"Electromagnetic pollution“ caused by elec-
tronic actuators, energy cables and transmit-
ting systems has become so pervasive that
shielding for normal two-wire bus cable is
usually a necessity.
We recommend the use of shielded cable.
Shielding is also a good preventative meas-
ure against future sources of interference.
Ground the shielding only on one sensor end
(fig. 3.1 a). It is advantageous to ground the
shielding on both ends. However, you must
make sure that no earth equalisation currents
flow through the cable shielding (fig. 3.1 b).
Earth equalisation currents (when grounding
at both ends) can be prevented by connect-
ing the cable shielding on one end (e.g. in
the switching cabinet) via a capacitor (e.g.
0.1 µF; 250 V AC) to the earth potential. Use
a very low-resistance earth connection (foun-
dation, plate or mains earth). In Ex applica-
tions, the shielding should be grounded only
at one end. Potential transfer can occur if
both ends are grounded.
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 in order to ensure the sealing
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. For this reason,
screened and twisted cables are prescribed
acc. to Profibus specification (IEC 1158-2).
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, there can also be a maximum of 32
participants, i.e. PA sensors (Ex max. 10
participants).
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.