FISCO 1 Fieldbus Fieldbus Safety analysis for conventional

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. 1 m. H Intrinsically safe IIC/Group A T 4 Ui : 30 V Ii : 100 m. A Ci : 0. 01 m F Li : 100 m H n IEC 60079/14 : n calculate cable parameters n compare safety descriptions n prepare System Descriptive document Centralised control equipment Certified IS interface : Intrinsically safe IIC/Group A Uo : 28 V Io : 93 m. A Co : 0. 13 m F Lo : 4. 2 m. H SYSTEM : IIC/Group A T 4

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. 1 m. H Intrinsic Safety Interface Centralised control equipment Certified IS interface : Certificate for each field device required Intrinsically safe IIC/Group A Uo : 28 V Io : 93 m. A Co : 0. 13 m F Lo : 4. 2 m. H Fieldbus network Re-calculate if any subsequent modifications

Fieldbus l FF-816 Physical Layer Profile Specification l for hazardous area devices l recommends minimum input l voltage l current & l power levels l maximum l input capacitance & l input inductance l Bus interface output parameters l Field device input parameters 4

Fieldbus Intrinsically Safe Concept (FISCO) l 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 l the physical layer employed by both Profibus ‘PA’ & Foundation Fieldbus ‘H 1’ systems l A PTB report dated August 1994 provided details of the experimental evidence, from which the IEC standard has emerged l The resulting document covers all aspects of a low frequency Fieldbus system l allows a system to be built with only a minimum analysis of the safety requirements l allows very simple documentation 5

Fieldbus The FISCO model published in 1994 n 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 H 1 & Profibus PA Host interface Tree Spur Daisy-chain 6

Fieldbus Intrinsically Safe Concept (FISCO) l The experimental work covered a defined range of cable parameters, circuit voltage & current; l 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 m. H 80 n. F Value to 150 ohm / km to 1 m. H / km to 200 n. F / km 30 m in IIC & IIB 1 km in IIC & 5 km in IIB Table 1 - FISCO cable parameters 7 l These limits are not difficult to achieve, since a typical Fieldbus trunk cable has parameters of : 50 /km, 0, 8 m. H/km & 120 n. F/km l The length of the trunk is usually determined by the operational requirements.

Fieldbus Intrinsically Safe Concept (FISCO) l The original Foundation Fieldbus specification (FF-816; IEC 61158 -2) was for a ‘ia IIC’ resistive power supply, which suffers from very restrictive voltage & current 18. 4 V V Resistive I MTL 5053 8 non-FISCO EEx II (1) GD EEx [ia] IIC [-20 OC to +60 OC] 80 m. A

Fieldbus Intrinsically Safe Concept (FISCO) l The FISCO standard examined the ignition capability of constant current power supplies l it proposed acceptable levels of power for supplies with a ‘trapezoidal’ characteristic, as opposed to those with a falling, resistor-limited characteristic. l This permits a greater useable power for the Fieldbus trunk; l particularly if the less sensitive gas group IIB (Ethylene) is declared l 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 l The use of active semiconductors to limit the current l results in power supplies being restricted to ‘ib’ level of protection in accordance with IEC & CENELEC apparatus standards l ie, system is not directly applicable to Zone 0 applications l 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 l Power Supply : constant current control : l trapezoidal fall-off l V max up to 17. 5 volts l IIC l Imax 183 m. A l Pmax 2. 56 Watts IIB 380 m. A 5. 32 Watts l Safety Factor of 1. 5 has been incorporated 17. 5 Trapezoidal V Resistive I 11 380

Fieldbus 13 Comparison of Safety Parameters FOUNDATION Fieldbus FISCO Parameters

Fieldbus 14 The FISCO model l The FISCO model is based on the following mandate : l Each segment has only one source of power & no additional power is fed to the bus l the field devices act as passive current sinks l Passive line termination is in place at both ends of the main bus line l Line length limited to : 1 Km in IIC : 1900 m in IIB (FF-816 limit) l Spur length limited to : 30 metres (60 metres proposed) l Each device : capacitance & inductance limited to Ci < 5 n. F, Li < 10 u. H l Linear, tree, & star networks are permitted

Fieldbus 17 Documentation & Inspection l Major benefits of the FISCO system are : l no detailed analysis is necessary to ensure the safety of the system l additions do not require reassessment of the whole system l the documentation is only a list of the connected apparatus l electronic format sufficient l can simply cross-refer to the certification documents of the apparatus l the inspection procedure is that of ensuring the system is unchanged & undamaged l The reader is referred to Application Note AN 9026 : Section 1. 7 : Documentation & Inspection Section 8 : Certification & Safety; Documentation of FISCO Systems

Fieldbus FISCO - Detailed Design l The calculation of how many field devices can be powered on a bus is a simple application of ‘Ohms Law’ l take the current consumption of each field device, l deduce the voltage drop in the trunk cable, l remembering that the fielddevices require 9 V to operate. l If the assumption is : l all field devices take 20 m. A l all are located at the remote end of the IS trunk l typically, the number of devices which can be powered from a IIC power supply is 5 at the end of a 600 m trunk l The corresponding IIB power supply will supply 13 devices at the end of a 300 m trunk l demonstrating the desirability of a IIB gas classification where possible 18 l In practice most instruments take less than 20 m. A thus permitting an increase in population

Fieldbus Terminators l FISCO power supplies typically contain a permanently connected terminator which defines one end of the IS trunk l The field end of the IS trunk also requires to be terminated l Terminators are used to present a 100 impedance to each end of the trunk l to match the cable characteristic impedance l to avoid reflections l typical certification : l ATEX Category II 1 l CENELEC EEx ia IIC T 4 FBT 1 -IS l Typically, the terminator is FISCO approved, and hence can be used without any further consideration of the system safety 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 / Div 1 Hazardous Area l The Fieldbus l The incoming 20 power supply is converted to an intrinsically safe supply to feed the IS trunk T signal, which is superimposed on the IS voltage, is fed via the interface device to & from the field devices

Fieldbus Summary è FISCO serves all gas groups & hazardous areas è The number of field devices that can be connected to a fieldbus segment will be limited only by : è the power available è the resistivity of the cable connection è The segment can be extended without the need for new calculations (within the boundaries of the FISCO model ) è FISCO devices from different suppliers can be interchanged without any re-calculation è No System certification required 22

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