VEGA FIELDBARRIER - Manual (Page 15)

FieldBarrier
Subject to reasonable modifications due to technical advances. Copyright Pepperl+Fuchs, Printed in Germany
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08/
2007 122329
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the advantage that fieldbus stations can be disconnected from or connected to the
transmission line during ongoing operation.
If a fieldbus segment is designed in Intrinsic Safety ignition protection type, proof of
intrinsic safety must be provided. To make this proof of Intrinsic Safety as simple as
possible, there are 2 different models:
• the FISCO model
• the Entity model
Both models are explained briefly below.
5.2.1 The FISCO model
The FISCO model was developed by the German Federal Physical Technical Institute
(PTB), which published information on it in Report PTB-W-53 "Examination of intrinsic
safety for fieldbus systems“. FISCO stands for Fieldbus Intrinsically Safe COncept
and is standardized in the IEC 60079-27. This model is based on the following pre-
requisites:
1. To transmit electrical power and data, the bus system uses the "Manchester Cod-
ing Bus Powered“ physical layout in accordance with IEC 61158-2. This is the case
for both the FOUNDATION fieldbus and the PROFIBUS MBP.
2. Only one active source is permitted on a bus segment (the power repeater/the seg-
ment coupler/the FieldBarrier). All other components work as passive current sinks.
3. The basic current consumption of a bus station is at least 10 mA.
4. For each bus device it must be ensured that:
V
i
> V
o
of the Segment coupler/power repeater/FieldBarrier
I
i
> I
o
of the Segment coupler/power repeater/FieldBarrier
P
i
> P
o
of the Segment coupler/power repeater/FieldBarrier
.
5. Each bus station must fulfill the following requirement:
C
i
< 5 nF
L
i
< 10 µH
6. The permissible line length for EEx ia IIC applications is 1000 m.
7. The permissible spur length for Ex applications is 60 m per spur line. The definition
of the spur must be observed in this connection.
8. The transmission line that is used must conform to the following cable parameters:
Resistor coating: 15
Ω/km < R’ < 150 Ω/km
Inductance coating: 0.4 mH/Km < L’ < 1 mH/km
Capacitance coating: 45 nF/km < C’ < 200 nF/km (including the shield)
Taking the shield into consideration, the capacitance coating is calculated as fol-
lows:
C’ = C’
conductor/conductor
+ 0.5 * C’
conductor/shield
if the bus line is potential free or
Which of the two following models should be used is stipulated by
national requirements and/or laws.