1 June 2005
Power to the bus
By Bernd Schuessler
End users throughout the fieldbus world are demanding more power to connect more instruments to their fieldbus segments in Class I, Division 1, and Class I, Division 2, hazardous location applications. Some are referring them to the fieldbus intrinsically safe concept (FISCO) and fieldbus nonincendive concept (FNICO) to realize their quest for more power on the fieldbus. A superior alternative exists for both that gives engineers all the power they need to fully load a fieldbus segment, even in a hazardous location. The high power trunk concept works for all fieldbus applications. It simplifies segment design, minimizes the amount of hardware used, supports a distributed architecture, allows for live maintenance, and offers state-of-the-art power redundancy and online physical layer diagnostics.
Typical FISCO power conditioners available today supply 12.8 V and 100mA for Class I, Div.1, groups A-D. In a real-world application, this can result in four to eight devices per segment. This is still far below the capabilities of a general purpose fieldbus installation where users can connect up to 16 instruments per segment.
If in a given application you have all devices certified in accordance with FISCO and fulfill the right conditions, you can reduce the proof for intrinsic safety to comparing the following parameters:
FNICO is similar to FISCO but for Class I, Div. 2, applications. For the FISCO model, the same basic rules apply for FNICO. The major difference is FISCO uses a higher safety factor for its electrical values. Due to the lower safety factor for FNICO, it's possible to supply more current into a Class I, Div.2, hazardous location. Typical values for FNICO are 12.3V at 215mA.
FISCO, FNICO limitations
Although FISCO/FNICO offers some additional power compared to the entity concept, 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 1000m, spurs are limited to 60m, and the current and voltage levels are still very low, which results in significantly shorter cable runs than the theoretical maximums.
For most applications, fieldbus users don't really need the energy limitation on the trunk because they don't perform live maintenance because of the high risk of losing an entire segment due to a single short.
Contrary to some general-purpose power conditioners, FISCO and FNICO power conditioners offer neither redundancy nor any online physical layer diagnostics.
End-users also end up with more hardware in their control room cabinets because several FISCO/FNICO power conditioners might have to be connected together to maximize the number of instruments on a single segment. This represents additional wiring cost and installation time and ultimately adds to the overall project cost. Having to install additional hardware in a centralized control room also goes against distributed fieldbus architecture. One of the benefits of using fieldbus technology is it allows users to distribute equipment into the field and away from the control room, thus saving expensive control room real estate.
The availability of FISCO/FNICO devices on the market is also limited. Although FISCO devices are more frequently available, FNICO instruments have not met the market and may never make it due to some new innovations such as the high power trunk concept.
High power trunk
The high power trunk concept is one new approach to solving hazardous area fieldbus applications. It contrasts to the FISCO/FNICO concepts because it does not limit the energy on the fieldbus trunk cable to intrinsically safe or nonincendive levels. Rather the energy on the spur connections limits itself to the instruments. This allows end users to get the maximum number of devices on a segment while also being able to achieve maximum cable lengths. Depending on the application, the protection (energy limitation) happens in the field inside the junction box.
Fieldbus users don't normally perform live maintenance on a fieldbus trunk cable because of the high risk of losing an entire segment due to a single short on the trunk cable. By not limiting the energy on the trunk, the high power trunk concept offers the same advantages you see in general-purpose applications in hazardous location applications. Typical values for a high power trunk solution are 30V at 500mA.
And products such as fieldbus barriers and fieldbus segment protectors make it easy to apply the high power trunk concept even in the most hazardous environments. You can use a fieldbus barrier in Class I, Division 1 applications. It provides intrinsically safe, short-circuit-protected spur connections for either entity- or FISCO-based instruments. You can galvanically isolate the high power trunk connections of the fieldbus barrier from the intrinsically safe (IS) spur connections.
You can also use fieldbus segment protectors in general-purpose as well as Class I, Division 2, applications. They provide short-circuit, energy-limited (nonincendive) outputs. When used in a Division 2/nonincendive application, they can connect to either a nonincendive field wiring apparatus or intrinsically safe rated devices, which enables the end user to connect/disconnect instruments under power.
Both fieldbus barriers and segment protectors are typically mounted in a Division 2 area where the high power trunk connections can be made following Division 2 wiring methods. Division 2 wiring methods include open cable tray with Power Limited Tray Cable/Instrument Tray Cable (PLTC/ITC), such as a standard type A Fieldbus cable, armored cable, or conduit. Many end-users are moving away from using conduit in Division 2 fieldbus installations due to cost and adopting open cable tray in combination with PLTC/ITC cable.
Fieldbus barriers and segment protectors move the energy limitation out of the control room cabinet and into the field by combining the features of short circuit protected junction boxes with a built-in barrier. This enables users to distribute their Fieldbus equipment around the plant, taking full advantage of Fieldbus technology. Another benefit of the high power trunk concept is it gives the end-user the freedom to choose any type of instrument entity, FISCO or FINCO.
In addition to fieldbus barriers and segment protectors, a new generation of fieldbus power conditioners with built-in redundancy and online physical layer diagnostics capabilities is becoming available. End users will be able to online monitor and detect noise levels on a segment, voltage and current levels, and ground faults.
The high power trunk concept allows fieldbus users to design segments free of any power restriction. In fact, the only limitations are the control system or the fieldbus specification. Finally, engineers can enjoy the same benefits in terms of power, cable length, and number of devices per segment in hazardous location applications as they do in general purpose applications.
Behind the byline
Bernd Schuessler is business development manager at Pepperl+Fuchs in Twinsburg, Ohio.
FISCO, FINCO: The early years
Even though several process industries have rapidly adopted fieldbus technology over the past several years, end users have been unsatisfied with the traditional solutions for hazardous location applications. They say they can't enjoy the same benefits in terms of power, cable length, and number of devices and segments in hazardous location applications compared to general-purpose applications. This is due to energy limitation on the trunk.
At the introduction of fieldbus technology, industry used the entity concept with cabinet mounted barriers and power supplies as the standard solution for hazardous area applications. This type of solution barely supplied enough power for three or four instruments per segment, and it was cumbersome to match entity parameters of the devices and the power source. End users voiced their concerns, and manufacturers responded with another solution for intrinsically safe (IS) segments. The result was fieldbus intrinsically safe concept (FISCO).
The developers of FISCO, Physikalisch-Technische Bundesanstalt (PTB) in Germany, based it on their experiments to find a solution to provide more power over a fieldbus into a hazardous location. In 2002, the published IEC 60079-27 standard described the FISCO model.
FISCO is based on the following conditions:
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