VEGA VEGAPULS 43 Profibus PA - Manual (Page 29)

VEGAPULS 43 – Profibus PA 29
Electrical connection
The distributed resistance of the cable, in
conjunction with the output voltage of the
segment coupler and the current requirement
(VEGAPULS 10 mA) or the voltage require-
ment (VEGAPULS 9 V) of the sensors, deter-
mines the max. cable length.
In a practical application of a PA bus branch,
the max. length of the cable is also deter-
mined (beside the required supply voltage
and max. current consumption of all partici-
pants on the PA bus branch) by the 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)
More than 24 stubs are not 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 must be grounded through
a ground connection to the exterior ground
terminal.
Screening
„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. According to the Profibus
specification, the screening should be
grounded on both ends. To avoid potential
equalisation currents, a potential 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 an Y capacitor
1) to ground
potential. Make sure that the ground connec-
tion has the lowest possible resistance (foun-
dation, plate or mains earth).
Profibus PA in Ex environment
When used in Ex area, the PA bus (including
all connected instruments) must be
configured as intrinsically safe protection
class „i“. Four-wire instruments requiring
separate supply must at least have an intrin-
sically safe PA connection. VEGA sensors for
PA Ex environment are generally „ia two-wire
instruments“.
1) max. 10 nF, e.g. voltage resistance 1500 V,
ceramic