Beyond Fisco—for up to 16, intrinsically safe fieldbus-devices



Moore Industries-International, Inc. reports a new, intrinsically safe (IS) split-architecture system that supports up to 350mA per segment for hazardous locations.

Initially, fieldbus implementations used a conventional IS interface, applying 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 four devices somewhat frustrated early fieldbus justifications: plants had lots of cabling, and hardware costs went up.

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

Fisco’s capacity barrier has now been overcome by a split architecture design—approved by two certifying organizations—that has been field proven by the engineers of MooreHawke, a division of Moore Industries-International. The design employs a field-mounted device coupler and associated power supply with a safe-area interface. The total resistance required 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.

The design enables intrinsically safe fieldbus-segments to support up to 350mA

The split-architecture power supply avoids Fisco-circuit complexity by using a conventional wire-wound resistor that, in IS terms, is claimed to be infallible. Further augmenting overall system reliability, the design also incorporates full ac/dc power-conversion, simple linear power supply, and complete galvanic isolation, with built in redundant supplies. So fewer components should enhance reliability.

Richard Phelps , senior editor, Control Engineering

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