PR electronics PROFIBUS PA / FOUNDATION Fieldbus 6350 Manual De Instrucciones página 24

In each I.S. Fieldbus segment only one active source, normally the associated
apparatus, is allowed to provide the necessary power for the Fieldbus system. The
allowed voltage (Uo, Voc, Vt) of the associated apparatus used to supply the bus
must be limited to the range of 14V d.c. to 24V d.c. All other equipment connected
to the bus cable has to be passive, meaning that the apparatus is not allowed to
provide energy to the system, except to a leakage current of 50 µA for each
connected device. Separately powered equipment needs a galvanic isolation to insure
that the intrinsically safe Fieldbus circuit remains passive.
The cable used to interconnect the devices needs to comply with the following
parameters:
Loop resistance R': 15 ...150 Ω/KM
Inductance per unit length L': 0.4...1mH/km
Capacitance per unit length C': 80 ...200 nF/km
C' = C' line/line + 0.5 C' line/screen, if both lines are floating
or
C'= C' line/line + C' line/screen, if the screen is connected to one line
Length of spur Cable: max. 30m
Length of trunk cable: max. 1Km
Length of splice: max. 1m
Terminators
At each end of the trunk cable an approved line terminator with the following
parameters is suitable:
R = 90 ...100 Ω
C = 0 ...2.2 µF.
System evaluation
The number of passive devices like transmitters, actuators, connected to a single bus
segment is not limited due to I.S. reasons. Furthermore, if the above rules are
respected, the inductance and capacitance of the cable need not to be considered
and will not impair the intrinsic safety of the installation.
The sensor circuit is not infallibly galvanic isolated from the Fieldbus input circuit.
However, the galvanic isolation between the circuits is capable of withstanding a test
voltage of 500Vac during 1 minute.
6350QE02revAB.doc 2004-02-06 Rev. AB 5/7
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6350V103-ES