DART and the promise of high power with intrinsic safety
Dynamic Arc Recognition and Termination (DART) technology is set to become an IEC standard after first hitting the headlines in 2008, with the promise of making higher power possible in hazardous areas. Suzanne Gill, Control Engineering Europe, explains the latest.
Intrinsic safety (IS), the protection technique that ensures the safe operation of electrical equipment in hazardous areas by limiting the energy available for ignition, is well known as being the safest method for spark prevention in hazardous areas.
In 2008, at the Hannover Messe, Pepperl+Fuchs first introduced its alternative solution—the Dynamic Arc Recognition and Termination (DART) technology, which offered the promise of making more power available through dynamic spark protection and promising to lift this power limitation.
DART technology achieves higher usable power levels while continuing to provide Ex i (intrinsic safety) explosion protection for Zone 1 hazardous areas. It offers features such as simplified and maintenance-free installation technology and access to circuits without a hot work permit. Traditionally, these benefits have been possible only at extremely low effective power levels, resulting in intrinsic safety finding its most common application in instrumentation for process automation.
In 2008, the National Metrology Institute of Germany (PTB) and Pepperl+Fuchs brought together a consortium of 15 interested companies. Since its launch in 2008, a work group consisting of 13 companies and headed by the National Metrology Institute of Germany (PTB) has defined the interoperability and proof of explosion protection for DART. Part 39 of the IEC 60079 standard is currently being drawn up on this basis as a technical specification, and the final draft should be available in the spring of 2014 when it will be presented to the IEC for coordination. It is hoped that, with a standard to describe DART technology, manufacturers will be able to implement DART technology into their own products, making many applications in the hazardous area safer and more convenient or easier to handle.
Reliable spark prevention
To recap, DART technology is based on the idea that a spark transmits a very characteristic signal on the cable. This signal can be reliably detected and disabled at the source and at every consumer. Operating current and voltage can, therefore, be significantly higher. DART technology intervenes only at the exact moment when the electrical energy in a spark is converted into heat. The entire process takes just a few microseconds. However, the distance between the source and the spark is an important factor in timing as the spark signal spreads at a finite rate through the cable. The rated power, therefore, depends on the cable distance, while the second crucial variable is the load current that DART must interrupt.
The testing procedure devised by PTB to certify Dart Fieldbus was double-checked and confirmed by a second institution, and it is now certified in line with IEC 60079-11 intrinsic safety according to ATEX and IECEx rules.
As a result, the DART Power Hub and DART Segment Protector form part of the FieldConnex product line. These devices are components of the fieldbus infrastructure that form the connection between the control system and the field device via Foundation fieldbus H1 and Profibus PA protocols—but now with the addition of an intrinsically safe high-power trunk.
An important aim for DART Fieldbus components was to ensure that the technology could be handled easily in real-life situations. To this end, experts with prior experience of developing Fieldbus Intrinsically Safe Concept (FISCO) were on the teams at both PTB and Pepperl+Fuchs to ensure focus on user-friendliness.
With DART Fieldbus, the segment is planned and the explosion protection is validated in a single step. In addition to the planning requirements of the fieldbus standard, a few general conditions need to be met. These include:
- A topology featuring a trunk and spurs must be used.
- Fieldbus cable type “A” shall be used.
- The trunk may measure up to 1000 m in length.
Together with the DART Power Hub/Segment Couplers, up to four DART Segment Protectors can be connected to the trunk at any point along its length. Using the Segment Checker planning software, the planner can check the voltage levels with just a few clicks of the mouse. The correct functioning of the segment and explosion protection compatibility can therefore be guaranteed well in advance. A free download is of this software is available at www.segmentchecker.com.
Sparks are caused when a plug is disconnected, so a DART circuit needs to be able to deal with this event in such a way that the availability of the fieldbus segment is completely unaffected. The power supply is switched off for a mandatory few milliseconds to extinguish the spark before it is automatically switched back on again. An important function, considering that the majority of sparks are caused by the intentional establishing or breaking of connections to field devices. An energy reservoir in the DART Segment Protector fills the gap when the main power supply briefly disconnects, allowing field equipment to continue to function. An undesired restart of the instrumentation is prevented even after several short and successive breaks, as would be caused by contact bounce during maintenance work.
The fieldbus protocol continues to run by using its own repeat mechanisms to counteract the effect of these short breaks, just as with any other fieldbus installation. These events are not detected by the field instrumentation or control system.
More convenience in the control room cabinet
Installation work is also reduced compared with fieldbus in general. More important than the savings in capital expenditure, this reduces the risk of wiring errors that are tedious to find.
For the control room cabinet, Pepperl+Fuchs can offer cables with custom connections tailored to popular DCS systems. Cables with custom connectors can significantly reduce the number of manual wire connections in the cabinet. Having fewer connections both saves on wiring labor and reduces testing time and effort before and during the factory acceptance test.
In conclusion, we leave the final word to Andreas Hennecke, product marketing manager, process automation division at Pepperl+Fuchs. He said: “DART Fieldbus, in combination with effective and efficient solutions, can provide a value proposition to everyone in the value chain with best-in-class safety and possibly the lowest risk for errors and faults.”
- Suzanne Gill is editor of Control Engineering Europe. This article originally appeared at www.controlengeurope.com and was edited for the Control Engineering International pages in the January 2014 North American edition of Control Engineering. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, mhoske(at)cfemedia.com.
“DART and the promise of high power with intrinsic safety” appeared Dec. 17 at www.controlengeurope.com and was edited for use in for the Control Engineering International pages in the North American edition.
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