VEGA Capacitive electrodes EK Profibus PA - Manual (Page 22)

22 Capacitive electrodes EK - Profibus PA
Electrical connection
Qualified personnel
Instruments which are operated in Ex areas
must only be connected by qualified
personnel. they have to note the installation
regulations and attached EC type approval
certificates and conformity certificates.
If the capacitive electrodes are to be
mounted on vessels which must be
protected against ignition by lightning
strokes (acc. to TRbF 100 no. 8, para. 1),
they have to be equipped with the external
overvoltage arrester type B62-36G or the
internal overvoltage protection unit, type
B62-30W.
Connection cable and cable length
Connection cables must be in conformity with
the Profibus specification and the FISCO
model. The sensor cable must be in
conformity with the values of the reference
cable acc. to IEC 1158-2:
0.8 mm
2 ; R
DCmax.
= 44
Ω
/km;
Z
31.25kHz
= 80 … 120
Ω
; damping = 3 dB/km;
C
unsymmetrisch
= 2 nF/km.
The max. cable length first of all depends on
the transmission speed:
bis 32 kbit/s: 1900 m Pr
bis 32 kbit/s: 1900 m Pr bis 32 kbit/s: 1900 m Pr bis 32 kbit/s: 1900 m Pr bis 32 kbit/s: 1900 m Profibus P ofibus P ofibus P ofibus P ofibus PA A A A A
up to 94 kbit/s: 1200 m Profibus DP
up to 188 kbit/s: 1000 m Profibus DP
up to 500 kbit/s: 500 m Profibus DP
up to 1500 kbit/s: 200 m Profibus DP
up to 12000 kbit/s: 100 m Profibus DP
The distributed resistance of the cable, in
conjunction with the output voltage of the
segment coupler and the current requirement
or the voltage requirement of the sensors,
determines the greatest possible length of
the cable.
In the practical application of a PA bus
branch, the max. length of the cable is also
determined (beside the required supply
voltage and max. current consumption of all
participants on the PA bus branch) by the
bus structure and the type of segment
coupler used.
The cable length results from the sum of all
cable sections and the length of all stubs.
The length of the individual stubs must not
exceed the following lengths:
1 … 12 stubs
120 m (Ex: 30 m)
13 … 18 stubs 60 m (Ex: 30 m)
19 … 24 stubs 30 m (Ex: 30 m)
No more than 24 stubs are permitted,
whereby each branch longer than 1.2 m is
counted as a stub. The total length of the
cable must not exceed 1900 m (in Ex verison
1000 m).
Ground terminal
The electronics housings of the sensors have
a protective insulation. The ground terminal in
the electronics housing is galvanically
connected with the metallic process
connection. For sensors with a plastic thread
as process fitting, the sensor grounding
must be made by a ground connection to the
external ground terminal.
Screening
According to the Profibus specification,
screening should be done at both ends. To
avoid potential equalisation currents, a
potential equalisation system must be
provided in addition to the screening.
According to the specification, we
recommend the use of twisted and screened
two-wire cable, e.g.:
SINEC 6XV1 830-5AH10 (Siemens),
SINEC L26XV1 830-35H10 (Siemens),
3079A (Belden).
Alternatively, when grounding at both ends in
non-Ex areas, the cable shielding can be
connected on one ground side (in the
switching cabinet) via a capacitor (e.g.
0.1 µF; 250 V) to the ground potential. Make
sure that the ground connection has the
lowest possible resistance (foundation, plate
or mains earth).