MACHINE BUILDERS who install fieldbuses as part of their control systems, sometimes need more power on the wire to connect needed devices and instruments than usually is available in hazardous environments. To gain added power, they can choose solutions using Fieldbus Intrinsically Safe Concept (FISCO) or Fieldbus Non-Incendive Concept (FNICO).
At the introduction of fieldbus technology, the Entity concept with cabinet-mounted barriers and power supplies was the standard solution for hazardous area applications, says Bernd Schuessler, business development manager at Pepperl+Fuchs.
Many process industries embrace fieldbus technology, but, when it comes to hazardous environment applications, machine and skid builders who work in that space often need alternatives to those solutions. The power, cable length, and number of devices per fieldbus segment simply are lacking when compared to general-purpose applications.
FISCO provides more power over a fieldbus in a hazardous location. In 2002, the IEC60079-27 standard was published, describing the FISCO model," says Schuessler. "A FISCO fieldbus system must be based on the 'Manchester Bus-Powered' physical layer in accordance with IEC 61158-2. Only one active source is permitted per segment, and all other components act as passive current sinks.
"The one, real benefit of intrinsic safety is the ability to perform live maintenance, for which we all put up with difficult designs and additional costs."
FISCO power supplies can deliver 120 mA at 12.8 V in Groups A and B, which can support up to six devices with today's low-current fieldbus devices. More importantly, FISCO power-supply versions specifically designed for Groups C and D can provide 265 mA, which doesnt place any practical limit on the number of devices.
"FISCO often is selected by machine builders or their customers who have a preference for a fully IS technique because theyre familiar with a maintenance practice that allows live-working on any part of the installation, says Phil Saward, fieldbus product manager at MTL. FISCO avoids having to segregate live-workable and non-live-workable parts of the network in the field junction box.
FNICO power supplies for Class I, Div 2 applications were added to the 2005 release of IEC60079-27. In addition to higher output current, they can connect to any IS-certified fieldbus device including Entity or FISCO, as well as non-incendive and FNICO-certified devices. "FNICO doesnt solely rely on the availability of FNICO-certified devices, and that makes segment design easier," adds Saward.
Intrinsic safety (IS) grew out of the need to install electronic instrumentation in hazardous areas, where the device technology used in the 1970s and 80s required physical adjustments in those hazardous areas. "The one, real benefit of intrinsic safety is the ability to perform live maintenance, for which we all put up with difficult designs and additional costs," says Mike O'Neill, international sales director, MooreHawk Industries.
Non-incendive design is IS without consideration of internal faults. IS is based on a set of ignition curves (one per gas group) that plot short-circuit current versus open-circuit voltage. If a circuit plots above the curve, it will cause an explosion when exposed to that gas at optimum and defined concentrations. Non-incendive design uses the same ignition curves, but now the point is placed on the graph with reference to normal operation. It doesnt take account of any internal failures, so one clamping diode is enough and normal current is in accordance with manufacturers specifications.
IS causes problems for conventional instrument design and even more problems for intrinsically safe fieldbus, since there are more devices being connected. The only advantage that IS had was live maintenance, which is no longer necessary since fieldbus devices are communicating bi-directionally via the bus itself, adds ONeill. They can be maintained, reset, adjusted, or diagnosed from anywhere on the network. For machine builders who really want or need IS fieldbus, split-architecture-type packages are available. This moves the current-limiting part of the IS circuit into the field within an associated device coupler. The design puts the IS current-limiting resistor in series with each spur, but the voltage drop per spur is small since each spur current is small (15-20 mA).
An alternative approach for hazardous-area fieldbus applications is based on a high-power or high-energy trunk concept. In contrast to the FISCO/FNICO concepts, the high-power trunk concept doesnt limit energy on the fieldbus trunk cable to intrinsically safe or non-incendive levels, says Schuessler. Instead, the energy on the spur connections is limited to the individual instruments. This allows machine builders to get the maximum number of devices on a segment, and also achieve maximum cable lengths.
The high-power trunk concept essentially is a non-incendive (non-sparking) trunk, with non-incendive (energy-limited) spurs, which allows designers to install this system in Div 2 and connect to Foundation fieldbus devices. Devices can be non-incendive or IS, and can be added or disconnected live or without de-powering the trunk.
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