Intrinsically safe fieldbus applications

This article takes a look at ways companies get around limited power and other hazardous environment barriers to connect more fieldbus devices using a technology that goes beyond FISCO.

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By Mike O'Neill, C.Eng.

AS ONE OF the more successful communication protocols for process control and industrial automation, fieldbus has proven its merit in bringing projects on-line earlier and more efficiently, allowing advanced digital feedback and control to processing plants previously stuck in the analog age. This allowed far more exacting process control, greater autonomy of control loops, accurate trending, greater centralized monitoring, and lower installation costs through easier wiring and faster commissioning. However, when it came to the hazardous environments often encountered in many chemical, pharmaceutical, plastics and petroleum plants, fieldbus initially fell short when intrinsically safe (IS) techniques were required.

Users of intrinsically safe devices in conventional control schemes have been used to the flexibility and ease of use afforded by the Entity Concept since the late 1980s. FM in the U.S. led the way in simplifying the process of confirming the safety of intrinsically safe loops and the Entity Concept now governs how every non-fieldbus IS loop is designed and documented.

Initially, fieldbus implementations went the same way: use a conventional IS interface, apply the industry-standard Entity Concept and the loop (now a segment) would be safe. The problem was conventional IS interfaces under the Entity Concept allowed only 80mA or so, barely enough to drive four devices at an average draw of 20mA per device. Fieldbus segments with only 4 devices somewhat defeated the point of early fieldbus justifications; plants still had lots of cable, and hardware costs went up.

Technology stepped up to the challenge in the form of FISCO (Fieldbus Intrinsically Safe Concept) which was developed in the late 1990s. Work in Germany had established that if cables and device parameters were defined by boundary values, modern electronic current-limiting designs could allow more current and still remain intrinsically safe. By taking advantage of this new technology, FISCO succeed in making more current available in hazardous locations-a full 115mA in worst-case (hydrogen) areas, enough to comfortably drive five devices, rather than the 80mA (4 devices) allowed by the Entity Concept.

Improvement, indeed. Yet FISCO makes this incremental gain at the expense of operational limitations because these units are complex pieces of electronics, generally based on switch-mode power supplies with duplicated current-limiting networks. This complex circuitry creates more heat and reduces unit reliability (complexity = more components = lower MTBF). Furthermore, a primary requirement of FISCO design is that the maximum allowable trunk and spur lengths fall from 1900m to 1000m, and from 120m to 30m, respectively. In addition, all devices and cable must be FISCO-compatible, further limiting choices in installing fieldbus networks.

For some time then, many I&C engineers have been searching for a new solution that would allow them to maximize intrinsically-safe segment capacity and operational ease within hazardous applications to the same level currently enjoyed within non-hazardous implementations.

   
 

A unique I.S. split-architecture system supports up to 350mA per segment to provide safe and reliable installation and operational advantages in hazardous locations.

The Split Architecture Solution
At last, technology has come through once again with a solution. The capacity barrier of FISCO has now been significantly surpassed by a novel split architecture design that has already proven itself in the field.

Engineers at MooreHawke, a division of Moore Industries-International in North Hills, California, developed this new technology by reexamining traditional approaches to pushing the capacity limits of intrinsically safe segments.

It quickly became evident that the primary cause of low segment power in fieldbus applications was the placement of the main current-limiting resistor at the point of highest current: the IS interface between the safe area and the hazardous area.

In response, MooreHawke developed a split architecture approach using a field-mounted device coupler, and an associated power supply with a safe-area interface. Here, the total resistance requirement is obtained via a split resistance; a small resistor is used in the IS interface and a larger resistor is placed in the field device coupler. The small (trunk) resistor 'sees' a large current (sum of all devices), but only generates a small voltage drop. The larger (spur) resistor 'sees' a small current (single device) and so only generates a small voltage drop.

Subsequently approved by the FM (US) and SIRA (ATEX) certifying organizations, this design enables intrinsically safe fieldbus segments to support up to 350mA, enough to power 16 devices at 500m, while still being intrinsically safe for hydrogen at the individual spur connection. This allows fieldbus designs to be just as cost-effective and efficient in hazardous locations as it has been in non-hazardous applications.

In terms of reliability, the split-architecture power supply steps around the complexity associated with FISCO circuits by the use of a conventional wire-wound resistor which, in IS terms, is deemed to be infallible. To further augment the overall systems reliability, the MooreHawke design also incorporates full AC/DC power conversion, simple linear power supply, and full galvanic isolation, with built in redundant supplies. Here again, fewer components translate into greater reliability. MTBFs (AKA MTTF) rise.

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