According to ONeill, It quickly became apparent to most prospective end users that intrinsic safety barriers were a very expensive solution for fieldbus. Because of the current-limiting resistor, most barriers could only provide 80mA to the fieldbus segment, meaning that a conventional IS segment could only support four fieldbus instruments, each taking a nominal 20mA. Foundation fieldbus protocol supports up to 32 devices on a segment, and most end users want segments capable of driving at least 12 to 16 instruments. With a barrier system only able to support four devices, choosing intrinsic safety as the site protection concept looked like a horrendous expense, requiring four times as many interface devices, four times as much wiring, plus extra cabinet space, etc., just to put fieldbus instruments in hazardous areas.
According to Bernd Schuessler, business development manager at Pepperl+Fuchs, Over the past several years, fieldbus technology has been rapidly adopted in many process industries. However, end users have been unsatisfied with the traditional solutions for fieldbus applications in hazardous locations applications because they could not enjoy the same benefits in terms of power, cable length and number of devices per segment in hazardous location applications compared to general-purpose applications due to energy limitations on the trunk.
These limitations have led MTL, Pepperl+Fuchs, Moore-Hawke and other major European and North American vendors in this space to come up with new IS approaches for fieldbus, each which overcomes the Entity limitations to a greater or lesser extent.
These include the fieldbus intrinsically safe concept (FISCO), the hybrid (or high-powered trunk) concept, a split-architecture approach and a new (and as of yet, unproven) dynamic arc recognition technology (DART) barrier approach.
Make a Big FISCO
The fieldbus intrinsically safe concept (FISCO) was developed by the Physikalisch-Technische (PTB) in Germany in the 1990s as a solution to providing more power over a fieldbus into a hazardous locations. The initial objective was to help to encourage the acceptance of Profibus in European refineries and chemical plants. The technology was also subsequently adopted for Foundation fieldbus. This was not difficult, since both share a common physical layer.
FISCO power supplies located in safe areas incorporate active current-limiting circuitry that allows more power to be delivered to fieldbus segments located in hazardous environments. Since the fieldbus segment is made intrinsically safe by placing the barrier at the boundary between safe and hazardous area, the trunk is live-workable.
This diagram illustrates the different fieldbus segments that can be created using the ENTITY, FISCO, Split-Architecture, and Hybrid (High-Power Trunk) approaches to Intrinsic Safety. (Diagram courtesy of Moore-Hawke)
A less restrictive FISCO approach―FNICO (Fieldbus Nonincendive Concept)―was also developed for use in nonincendive hazardous plant areas in which flammable gases or dust are not normally present. Here, less stringent, increased safety (as opposed to intrinsically safe) power restrictions increase both the number of fieldbus devices per segment and the allowable spur lengths.
While FISCO (both IS and nonincendive flavors) offers some clear advantages over the more conventional Entity approach, it also has its drawbacks.
According to Pepperl+Fuchs, Schosker, FISCO power supplies have more active circuitry than conventional IS barriers to allow more power to be put out into the hazardous area without impacting safety. However, FISCO increases the technical requirements in the field devices and reduces the flexibility on the network.
According to MooreHawkes ONeill, FISCO products are less reliable than conventional safety barriers simply because the FISCO power supply is such a complex design.
Pepperl+Fuchs Schuessler summarizes the limitations as follows, Although FISCO offers some additional power compared to the Entity approach, users still cannot enjoy the same benefits they get when using fieldbus in a general-purpose configuration. The overall cable length is theoretically limited to 1,000 meters, spurs are limited to 50 meters, and the current and voltage levels are still very low, which results in significantly shorter cable runs than the theoretical maximums FISCO power conditioners also do not offer redundancy, nor they offer any online physical layer diagnostics.
Nevertheless, FISCO has been gaining a degree of acceptance in fieldbus applications around the world.
At the Shell Petroleum Development Corporations Bonny Island Terminal Project in Nigeria, a new development comprising 24 oil storage tanks, MTLs FISCO power supplies integrate the fieldbus segments with a Yokogawa CENTUM DCS. MTL short-circuit-protected wiring hubs are used to create IS trunk and spurs that can be live-worked without gas clearance procedures. Non-FISCO, ENTITY-certified IS devices are also connected to the FISCO segments.
Carbowill Spoika has employed the FNICO approach in the Zone 2 hazardous area at the companys CO2 production plant in Wloclawek, Poland. Here, FNICO power supplies from MTL provide power to all 21 fieldbus segments. Since the entire network―both trunk and spurs―is energy-limited, technicians at the Carbowill CO2 plant can work on any part of the fieldbus segment in the hazardous area while energized, without having to obtain a gas clearance work permit. The fieldbus segments connect to fieldbus interface cards on Carbowills Emerson DeltaV DCS.