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FISCO
1
Fieldbus
Fieldbus
Safety analysis for conventional intrinsically safe instrument loop
using the “Entity Concept”
Hazardous area
Intrinsic Safety
Interface
IS certified device
Certified
Transmitter :
Safe area
Cc : 0.12 m F
Lc : 4.1mH
Intrinsically safe
IIC/Group A T4
Ui : 30V
Ii : 100mA
Ci : 0.01 m F
Li : 100 m H
 IEC 60079/14 :
 calculate cable parameters
 compare safety descriptions
 prepare System Descriptive
document
Centralised control
equipment
Certified IS interface :
Intrinsically safe
IIC/Group A
Uo : 28V
Io : 93mA
Co : 0.13 m F
Lo : 4.2mH
SYSTEM : IIC/Group A T4
Fieldbus
Safety analysis for conventional intrinsically safe instrument loop
using the Entity Concept
Hazardous area
IS certified device
Safe area
C:cable = 0.12 m F
L:cable = 4.1mH
Intrinsic Safety
Interface
Centralised control
equipment
Certified IS interface :
Intrinsically safe
IIC/Group A
Certificate
for
each
field
device
required
Uo : 28V
Io : 93mA
Co : 0.13 m F
Lo : 4.2mH
Fieldbus network
Re-calculate
if any
subsequent
modifications
Fieldbus
 FF-816 Physical Layer Profile Specification
 for hazardous area devices
 recommends minimum input
 voltage
 current &
 power levels
 maximum
 input capacitance &
 input inductance
FF-816
Ci
4
Ui
24 Volts
Ii
250 mA
Pi
1.2 watts
Ci
5 nF
Li
20 uH
 Bus interface output parameters
 Field device input parameters
Fieldbus
Fieldbus Intrinsically Safe Concept
(FISCO)

PTB, the German Test Authority, in collaboration with German
industry, examined the intrinsically safe needs of the
31.25 kbit/s physical layer of Fieldbus and developed an
approach which is applicable to IEC 61158-2
 the physical layer employed by both Profibus ‘PA’ &
Foundation Fieldbus ‘H1’ systems
 A PTB report dated August 1994 provided details of the
experimental evidence, from which the IEC standard has
emerged
 The resulting document covers all aspects of a low frequency
Fieldbus system
 allows a system to be built with only a minimum analysis of
the safety requirements
 allows very simple documentation
5
Fieldbus
The FISCO model
published in 1994
 IEC 60079-27 defines the FISCO model
“Fieldbus Intrinsically Safe Concept”
provides clear guidelines for the design
& installation of fieldbus networks in
explosion protected environments
Fieldbus network
Foundation Fieldbus H1
& Profibus PA
Host
interface
Tree
Spur
Daisy-chain
6
Fieldbus
Fieldbus Intrinsically Safe Concept
(FISCO)
 The experimental work covered a defined range of cable
parameters, circuit voltage & current;
 hence the permitted cable parameters in a FISCO IS circuit
are limited to :
Parameter
Loop resistance
Loop inductance
Capacitance
Max length of each
spur cable
Max length of each
trunk cable
15 ohm
0.4 mH
80 nF
Value
to 150 ohm / km
to
1 mH / km
to 200 nF / km
30 m in IIC & IIB
1 km in IIC & 5 km in IIB
Table 1 - FISCO cable parameters
7
 These limits are not difficult to achieve, since a typical Fieldbus
trunk cable has parameters of :
50 /km, 0,8 mH/km & 120 nF/km
 The length of the trunk is usually determined by the
operational requirements.
Fieldbus
Fieldbus Intrinsically Safe Concept
(FISCO)
 The original Foundation Fieldbus specification
(FF-816; IEC61158-2) was for a ‘ia IIC’ resistive power
supply, which suffers from very restrictive voltage &
current
18.4V
V
Resistive
I
MTL5053
non-FISCO
EEx II (1) GD
EEx [ia] IIC [-20 OC to +60 OC]
8
80 mA
Fieldbus
Fieldbus Intrinsically Safe Concept
(FISCO)
 The FISCO standard examined the ignition capability of constant
current power supplies
 it proposed acceptable levels of power for supplies with a
‘trapezoidal’ characteristic, as opposed to those with a
falling, resistor-limited characteristic.
 This permits a greater useable power for the Fieldbus trunk;
 particularly if the less sensitive gas group IIB (Ethylene)
is declared
 by use of a constant current power supply, its application
into Zone 0 is not allowed
17.5
Trapezoidal
V
Resistive
I
9
380
Fieldbus
FISCO in North America
 The use of active semiconductors to limit the current
 results in power supplies being restricted to ‘ib’ level of
protection in accordance with IEC & CENELEC
apparatus standards
 ie, system is not directly applicable to Zone 0
applications
 However, the design of MTL power supplies satisfies the
North American requirement for intrinsically safe
apparatus for Divisions 1 & 2
10
Fieldbus
The FISCO model
 Power Supply : constant current control :
 trapezoidal fall-off
 V max up to 17.5 volts

IIC
 Imax
183 mA
 Pmax
2.56 Watts
IIB
380 mA
5.32 Watts
 Safety Factor of 1.5 has been incorporated
17.5
Trapezoidal
V
Resistive
I
11
380
Fieldbus
Comparison of Safety Parameters
FOUNDATION
Fieldbus
FF-816
Ci
13
FISCO
Parameters
FISCO
IIC
FISCO
IIB
Ui
24 Volts
17.5 Volts
17.5 Volts
Ii
250 mA
183 mA
380 mA
Pi
1.2 watts
2.56 watts
5.32 watts
Ci
5 nF
5 nF
5 nF
Li
20 uH
10 uH
10 uH
Fieldbus
14
The FISCO model
 The FISCO model is based on the following mandate :
 Each segment has only one source of power
& no additional power is fed to the bus
 the field devices act as passive current sinks
 Passive line termination is in place at both ends of the
main bus line
 Line length limited to : 1 Km in IIC
: 1900 m in IIB (FF-816 limit)
 Spur length limited to : 30 metres (60 metres proposed)
 Each device : capacitance & inductance limited to
Ci < 5 nF, Li < 10 uH
 Linear, tree, & star networks are permitted
Fieldbus
Documentation & Inspection
 Major benefits of the FISCO system are :
 no detailed analysis is necessary to ensure the safety of
the system
 additions do not require reassessment of the whole
system
 the documentation is only a list of the connected
apparatus
 electronic format sufficient
 can simply cross-refer to the certification documents
of the apparatus
 the inspection procedure is that of ensuring the system
is unchanged & undamaged
 The reader is referred to Application Note AN9026 :
Section 1.7 : Documentation & Inspection
Section 8
: Certification & Safety;
Documentation of FISCO Systems
17
Fieldbus
FISCO - Detailed Design
 The calculation of how many field devices can be powered on a bus
is a simple application of ‘Ohms Law’
 take the current consumption of each field device,
 deduce the voltage drop in the trunk cable,
 remembering that the field devices require 9V to operate.
 If the assumption is :
 all field devices take 20mA
 all are located at the remote end of the IS trunk
 typically, the number of devices which can be powered from a
IIC power supply is 5 at the end of a 600m trunk
 The corresponding IIB power supply will supply
13 devices at the end of a 300m trunk
 demonstrating the desirability of a IIB gas classification where
possible
18
 In practice most instruments take less than 20 mA
thus permitting an increase in population
Fieldbus
Terminators
 FISCO power supplies typically contain a permanently connected
terminator which defines one end of the IS trunk
 The field end of the IS trunk also requires to be terminated
 Terminators are used to present a 100  impedance to each end
of the trunk
 to match the cable characteristic impedance
 to avoid reflections
 typical certification :
 ATEX Category II 1
 CENELEC EEx ia IIC T4
FBT1-IS
 Typically, the terminator is FISCO approved, and hence can be used
without any further consideration of the system safety
R
L
C
R
L
C
R
L
C
Terminator
100 
1 F
19
Fieldbus
Host
Typical application
L
N
E
DC
supply
T
T
Power
conditioner
T
Safe Area
Zone 1 / Div1
Hazardous Area
 The Fieldbus
 The incoming
power supply is
converted to an
intrinsically safe
supply to feed
the IS trunk
20
T
signal, which is
superimposed on the
IS voltage, is fed via
the interface device
to & from the field
devices
Fieldbus
Summary
 FISCO serves all gas groups & hazardous areas
 The number of field devices that can be connected to a
fieldbus segment will be limited only by :
 the power available
 the resistivity of the cable connection
 The segment can be extended without the need for new
calculations
(within the boundaries of the FISCO model )
 FISCO devices from different suppliers can be
interchanged without any re-calculation

22
No System certification required
Fieldbus
Summary
Entity
Approach
FISCO
Application
Line length limit all gases
: 1.9 Km (FF-816)
Max spur length : 120 m
Full calculation of safety parameters
Line length limit : 1 Km in IIC
: 1.9 Km in IIB (FF-816)
Max spur length : 30 m ( 60 m proposed )
No cable parameter considerations
required
Simple interchange of devices
Tree
Spur
Daisy-chain
23
31.25
kbit/s