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

22 Capacitive electrodes EL - Profibus PA
Electrical connection
Qualified personnel
Instruments which are operated in Ex areas
must only be connected by qualified person-
nel. They have to note the installation regula-
tions and attached EC type approval
certificates and conformity certificates.
If the capacitive electrodes are to be
mounted on vessels 0.8 which must be pro-
tected against ignition by lightning strokes
(acc. to TRbF 100 no. 8, para. 1), they have
to be equipped with the external 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 conform-
ity 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
asymmetric
= 2 nF/km.
The max. cable length first of all depends on
the transmission speed:
up to 32 kbit/s: 1900 m Pr
up to 32 kbit/s: 1900 m Pr up to 32 kbit/s: 1900 m Pr up to 32 kbit/s: 1900 m Pr up to 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 cou-
pler 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 version
1000 m).
Ground terminal
The electronics housings of the sensors have
a protective insulation. The ground terminal in
the electronics housing is galvanically con-
nected 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 on both ends. To
avoid potential equalisation currents, a po-
tential equalisation system must be provided
in addition to the screening.
According to the specification, we recom-
mend the use of twisted and screened two-
wire cable, e.g.: SINEC 6XV1 830-5AH10
(Siemens), SINEC L26XV1 830-35H10 (Sie-
mens), 3079A (Belden).
Alternatively, when grounding at both ends in
non-Ex areas, the cable shielding can be
connected on one ground side (in the switch-
ing 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).