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FISCO

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CertifiedTransmitter :

Intrinsically safeIIC/Group A T4

Ui : 30V Ii : 100mA

Ci : 0.01 FLi : 100 H

Cc : 0.12 F

Lc : 4.1mH

SYSTEM : IIC/Group A T4

Certified IS interface :

Intrinsically safe IIC/Group A

Uo : 28V

Io : 93mA

Co : 0.13 F

Lo : 4.2mH

Safety analysis for conventional intrinsically safe instrument loop using the “Entity Concept”

Safety analysis for conventional intrinsically safe instrument loop using the “Entity Concept”

Intrinsic SafetyInterface

Centralised controlequipmentIS certified device

Hazardous area Safe area

IEC 60079/14 : calculate cable parameters compare safety descriptions prepare System Descriptive

document

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Fieldbus network

Safety analysis for conventional intrinsically safe instrument loop using the Entity Concept

Safety analysis for conventional intrinsically safe instrument loop using the Entity Concept

Intrinsic SafetyInterface

Centralised controlequipmentIS certified device

Hazardous area Safe area

Certificatefor

eachfield

devicerequired

C:cable = 0.12 FL:cable = 4.1mH

Certified IS interface :

Intrinsically safe IIC/Group A

Uo : 28VIo : 93mA

Co : 0.13 FLo : 4.2mH

Re-calculateif any

subsequentmodifications

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FF-816

Ui 24 Volts

Ii 250 mA

Pi 1.2 watts

Ci 5 nF

Li 20 uHCi

FF-816 Physical Layer Profile Specification for hazardous area devices

recommends minimum input voltage current & power levels

maximum input capacitance & input inductance

Bus interface output parameters Field device input parameters

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us Fieldbus Intrinsically Safe Concept

(FISCO)

Fieldbus Intrinsically Safe Concept (FISCO)

PTB, the German Test Authority, in collaboration with Germanindustry, examined the intrinsically safe needs of the 31.25 kbit/s physical layer of Fieldbus and developed anapproach 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 theexperimental evidence, from which the IEC standard hasemerged

The resulting document covers all aspects of a low frequencyFieldbus system

allows a system to be built with only a minimum analysis ofthe safety requirements

allows very simple documentation

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us The FISCO model

published in 1994

The FISCO modelpublished 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

Foundation Fieldbus H1& Profibus PA

Fieldbus network

Tree

Daisy-chain

Spur

Hostinterface

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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 :

These limits are not difficult to achieve, since a typical Fieldbustrunk 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.

Parameter ValueLoop resistance 15 ohm to 150 ohm / kmLoop inductance 0.4 mH to 1 mH / kmCapacitance 80 nF to 200 nF / kmMax length of each

spur cable 30 m in IIC & IIBMax length of each

trunk cable 1 km in IIC & 5 km in IIB

Table 1 - FISCO cable parameters

Fieldbus Intrinsically Safe Concept (FISCO)

Fieldbus Intrinsically Safe Concept (FISCO)

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ResistiveV

I

The original Foundation Fieldbus specification (FF-816; IEC61158-2) was for a ‘ia IIC’ resistive powersupply, which suffers from very restrictive voltage &current

Fieldbus Intrinsically Safe Concept (FISCO)

Fieldbus Intrinsically Safe Concept (FISCO)

MTL5053 non-FISCO

EEx II (1) GDEEx [ia] IIC [-20 OC to +60 OC]

80 mA

18.4V

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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

Fieldbus Intrinsically Safe Concept (FISCO)

Fieldbus Intrinsically Safe Concept (FISCO)

Trapezoidal

ResistiveV

I 380

17.5

by use of a constant current power supply, its applicationinto Zone 0 is not allowed

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us FISCO in North AmericaFISCO 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 & CENELECapparatus 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

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us The FISCO modelThe FISCO model

Power Supply : constant current control : trapezoidal fall-off

V max up to 17.5 volts IIC IIB Imax 183 mA 380 mA Pmax 2.56 Watts 5.32 Watts

Safety Factor of 1.5 has been incorporated

Trapezoidal

ResistiveV

I 380

17.5

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FF-816 FISCOIIC

FISCOIIB

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 uHCi

Comparison of Safety ParametersComparison of Safety Parameters

FOUNDATIONFieldbus

FISCOParameters

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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

The FISCO modelThe FISCO model

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Major benefits of the FISCO system are :

Documentation & InspectionDocumentation & Inspection

no detailed analysis is necessary to ensure the safety ofthe system

additions do not require reassessment of the wholesystem

the documentation is only a list of the connectedapparatus

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 & InspectionSection 8 : Certification & Safety;

Documentation of FISCO Systems

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us FISCO - Detailed DesignFISCO - Detailed Design

In practice most instruments take less than 20 mAthus permitting an increase in population

The calculation of how many field devices can be powered on a busis 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 wherepossible

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us TerminatorsTerminators

R L C R L C R L CTerminator100 1 F

FISCO power supplies typically contain a permanently connectedterminator which defines one end of the IS trunk

The field end of the IS trunk also requires to be terminated

Typically, the terminator is FISCO approved, and hence can be used without any further consideration of the system safety

Terminators are used to present a 100 impedance to each endof the trunk

to match the cable characteristic impedance to avoid reflections typical certification :

ATEX Category II 1 CENELEC EEx ia IIC T4

FBT1-IS

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Zone 1 / Div1Hazardous Area

Host

TDCsupply

L

N

E

T

T

Safe Area

T

The incoming power supply is converted to an intrinsically safe supply to feed the IS trunk

The Fieldbus signal, which is superimposed on the IS voltage, is fed via the interface device to & from the field devices

Powerconditioner

Typical applicationTypical application

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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

No System certification required

SummarySummary

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31.25kbit/s

Tree

Daisy-chain

Spur

SummarySummary

FISCOApplication

FISCOApplication

EntityApproach

EntityApproach

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

requiredSimple interchange of devices

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

requiredSimple interchange of devices

Line length limit all gases : 1.9 Km (FF-816)

Max spur length : 120 mFull calculation of safety parameters

Line length limit all gases : 1.9 Km (FF-816)

Max spur length : 120 mFull calculation of safety parameters