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The Fieldbus Intrinsically Safe COncept (FISCO) specification considers IS fieldbus as a system that allows the end user to specify FISCO-certified devices and integrate them without the engineering requirements of the Entity approach. The available bus current is increased to 120 mA in the IIC (A/B) gas group and 265 mA in the IIB (C/D) gas group. Using 20 mA devices, that would theoretically equate to six and 13 devices respectively. Engineering requirements to create safety documents are reduced, and the only safety documentation required is a listing of the devices utilized on the network. Reduced engineering and an increased number of devices are two obvious advantages of FISCO.
The Fieldbus NonIncendive COncept (FNICO), a derivative of FISCO, is a specification that considers nonincendive fieldbus as a system. It is intended for division 2 classifications and takes advantage of the less stringent requirements of a nonincendive design. End users specify FNICO-certified devices and integrate them without the engineering requirements of the Entity approach.
The available bus current is also increased to 180 mA in a IIC (A/B) gas group and 320 mA in IIB (C/D) gas group. Using the 20 mA device, that would theoretically equate to 9 devices and 16 devices respectively. For the same reasons, FNICO is an obvious advantage over Entity. FNICO has a bigger advantage over Entity in a division 2 area, but because of their reduced safety factor, they are only allowed in division 2 areas.
The High Power Trunk (HPT) is a hybrid approach, where the fieldbus trunk is installed nonincendive (non-sparking and not FNICO), and the individual device spurs are installed as intrinsically safe spurs. The trunk and safety barrier is installed in either a safe or division 2 area, and the spurs can be wired to devices located in either a division 1 or 2 area. The devices can be Entity, FISCO, FNICO or a combination. In the HPT design, the trunk cannot be worked on while energized without a hot-work permit; however, the spurs can be.
In summary, implementing Foundation fieldbus can seem like a daunting task, and when coupled with hazardous-area considerations, may approach information overload; however, it does not have to be an explosive concept. Believe it or not, with today’s technology and product offerings, fieldbus is simpler than anytime before to implement.
Key questions to ask when considering implementation:
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