Because their design significantly increases the power available over intrinsically safe fieldbus networks, the experimentally derived IEC 60079-27 FISCO, or “Fieldbus Intrinsic Safety Concept,” standard allows users to put more devices on a single fieldbus segment—yet still allow “live maintenance” on any part of the field
wiring as well as elimination of complex, mixed-protection techniques in field junction boxes. With other fieldbus techniques, such as high-energy trunk, live maintenance was limited to downstream of the fieldbus barriers, “since FISCO rules mandated one power supply at a time,” explains Ian Verhappen, MTL director of industrial networks. But with the introduction of redundant FISCO, “MTL has overcome that problem,” Verhappen adds. “Now the entire system is workable in the field while still having redundancy.”
MTL’s clever, patent-pending approach uses two FISCO power supplies—one in ‘active’ and the other in ‘standby’ mode—together with twin supply arbitration modules (SAMs) that negotiate and manage power supply changeover in the event of a power supply failure. The two modules’ continuous handshaking ensures that they can never both be “on” at the same time. “The SAM does the failover within one H1 packet; and since retransmission is accommodated within the Foundation protocol, no data is ‘lost,’” Verhappen says.
Power supply redundancy routinely is specified by knowledgeable end users and engineering companies for fieldbus installations in which failure could result in consequential damage or severe loss of production, adds Verhappen. With its redundant FISCO power supply offering, MTL believes that FISCO will be deployed across a wider range of fieldbus applications in the process industries, and in particular those where the live maintenance benefits of intrinsic safety are appealing. “Users will be able to confidently select FISCO for even the most demanding control environments, such as in oil and gas and refining,” Verhappen says.
The redundant FISCO power supply system is based on MTL’s proven 912x-IS DIN-rail power supplies. The SAMs are housed in individual plastic cases and are located adjacent to the FISCO power supplies being protected. A mechanical interlock prevents the removal of a power supply module without first removing its associated SAM. This ensures that power and communications are continuously maintained on the fieldbus segment during repair and maintenance services. Available output current will be approximately 20mA less than for existing simplex power supplies; output voltage will be approximately 0.2V lower.
Further, the systems will be supplied as carrier-mounted subsets comprising four segments of redundant power together with their associated SAMs. (Each carrier will, therefore, include eight power supplies and eight SAMs.) Versions with dedicated connections for integration into the major distributed control platforms will be supported, together with a “generic version” for use with hosts having screw terminal field connections. Onboard physical-layer diagnostic monitoring is planned for future release; meanwhile, the company’s FBT-6 portable tester is certified instrinsically safe for field wiring testing of redundant FISCO segments.
And while certainly an elegant approach to doing fully redundant, intrinsically safe fieldbus networks, justifying MTL’s approach will require a full life-cycle approach to system costs. Upfront, increased cost of the power supply system will increase the apparent cost to the distributed control system, as well as cabinet real estate requirements. Meanwhile, installed cost will be lower, since only Megablock wiring hubs will be required at the field junction boxes.
“It’s a tough sell because of project responsibilities for the control system itself, and for the junction boxes—which often falls to the engineering contractor,” Verhappen explains. “But the total costs will be lower.”