VEGA VEGAPULS 56 Profibus PA - Manual (Page 37)

VEGAPULS 56 Profibus PA 37
Electrical connection
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. length of the cable.
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 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)
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 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 outer ground termi-
nal.
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 screen should be grounded
on both ends. To avoid potential equalisation
currents, a potential equalisation system
must be provided in addition to the screen-
ing.
According to specification, we recommend
the use of twisted and screened two-wire
cable, e.g.: SINEC 6XV1 830-5AH10 (Sie-
mens), 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 switch-
ing cabinet) via an Y
C
-capacitor (e.g. 10 nF,
1500 V) to the ground potential. Make sure
that the ground connection has the lowest
possible resistance (foundation, plate or
mains earth).
Profibus PA in Ex environment
When used in Ex area, a PA bus with all con-
nected instruments must be carried out in
intrinsically safe protection class "i“. Four-wire
instruments requiring separate supply must
at least have an intrinsically safe PA connec-
tion. VEGA sensors for PA-Ex environment
are generally "ia” two-wire instruments.