ProSoft's “Phased Migration” Solutions - Control Design

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Hazardous area connection Hazardous area CLR: Current limiting resistor (restricts current) TX Zener diode (restricts voltage) Safe area Energy-limiting 300R 28V Barriers Isolators Simple and reliable More complex, statistically lower MTBF than barrier Extremely accurate in many Active devices: power and heat applications High-integrity bond required Flexibility in bonding practice Predictable response to earth faults Flexible response to earth faults Inexpensive Generally more expensive Applications are defined in terms of voltage and resistance Encapsulated design necessary Tight power supply limits (except ‘protected’/fused barriers Easier to fault find (earth reference) Barrier Schematic Fuse (restricts power) Isolator Schematic Hazardous area circuit � � � � � � � � � � � � � � � � � Protect regulate Floating supply Safe area circuit 4-20mA IS Earth Certified component Safe area connection Certified transformer Application-specific. Each barrier is defined in terms of the function that it is designed to perform. Replaceable supply fuse common Unregulated supply Power 250R load COMPARISON OF BARRIER AND ISOLATOR SCHEMATICS 18 INDUSTRIAL NETWORKING � Q3 � 2012 Wide power supply tolerance 0V Gas/Equipment Temperature Compatibility Gas ignition temperature °C Ammonia 630 Methane 595 Hydrogen 560 Propane 470 Ethylene 425 Butane 365 Cyclohexane 259 Diethyl Ether 170 Carbon Disulphide 100 700 600 500 400 300 200 100 Apparatus temperature classification DETERMINING T-RATINGS Figure 2. The type of gas present determines the T-rating. Because with entity systems you need to understand interaction between each component on a loop, I/O card, barrier or field device, the entity concept works well for loops with one I/O card and one field device. However, if you have multiple devices on a wire pair, as with fieldbus systems, the number of combinations that need to be verified quickly grows exponentially. This is one of the reasons most process fieldbus systems use FISCO as described in the June 2010 issue of <strong>Control</strong> (www.controlglobal.com/FISCO). Furthermore, intrinsically safe circuits need to be kept separate from non-intrinsically safe circuits with the following minimum requirements: �� from terminals for non-intrinsically safe circuits where intrinsic safety can be impaired by external wiring that, if disconnected from the terminal, can come into contact with conductors or components by distance or terminal location. �� between bare conducting parts of terminals shall be at least 50 mm, including ensuring that contact between circuits is unlikely if a wire becomes dislodged. �� intrinsically safe and non-intrinsically safe circuits in separate enclosures, or by use of either an insulating partition or an T1 T2 T3 T4 T6 T5
earthed metal partition between terminals with a common cover, the following applies: �� distance of 50 mm between the bare �� transformer feeding the control system and, �� therefore, has to be of low impedance to be �� WHY USE IS? �� the associated restrictions regarding its �� Field mounted instrument Interface cubicle Armor if used X Plant bond (through structural earthing routes) Isolated internal circuits �� opened or closed. �� electrically, not mechanically, protected, �� resistance. �� cables and fallible components. �� in the event of contact with bare wires. �� INTRINSIC SAFETY IS THE ONLY TECHNIQUE THAT PERMITS LIVE WORKING WITHOUT GAS CLEARANCE CERTIFICATES FOR ALL AREA CLASSIFICATIONS. Instrument Earthing (Grounding) Scheme X1 Instrument system DESIGN TYPICAL CONTROL ROOM GROUNDING PLAN Figure 3. The function of the IS ground is to provide a secure, high-integrity, lowimpedance path through which fault currents will fl ow while minimizing voltages seen in the hazardous area. L N E Neutral Mains Instrument panel Barrier cubicle Ex i Plant bond �� are separately powered, they do not present as large a load to the loop. �� diff erences between these two alternatives. �� Ian Verhappen P.Eng., is an ISA Fellow, ISA Certifi ed Automation Professional and recognized authority on Foundation fi eldbus and industrial communications technologies. When this article was fi rst published in <strong>Control</strong> in June 2011, Verhappen led global consultancy Industrial Automation Networks. 2012 � Q3 � INDUSTRIAL NETWORKING 19
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ProSoft's “Phased Migration” Solutions - Control Design

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