By Mike Bacidore, Managing Editor
Many fieldbus practitioners have wondered if the IEC’s revision of its field intrinsically safe concept (FISCO) standard would mean the death of fieldbus non-incendive concept (FNICO). "Prior to 2004, users and designers of Foundation fieldbus H1 and Profibus-PA were concerned that conforming to the entity-concept intrinsic safety requirements would seriously limit the usefulness of fieldbus," says John Rezabek, process control specialist at ISP (www.ispcorp.com) in Lima, Ohio. Strict adherence to the standard could result in a limit of five or fewer devices per segment, as power supplies and devices were designed to limit the total energy at any point in the network, he explains. "By 2004, the German Federal Physical Technical Institute (PTB) developed FISCO, which provides much-improved flexibility for those installing fieldbus segments and devices in and through Zone 1 hazardous areas where explosive vapors can be present 10-1,000 hours/year," says Rezabek.
Intrinsic safety (IS) and nonincendive energy limitation (Ex nL) keep energy down to a level that’s not able to ignite the surrounding atmosphere during normal operation and under fault conditions for Zone 1 and Zone 0 applications, explains Thomas Klatt, business development manager, fieldbus systems, at Pepperl+Fuchs (www.pepperl-fuchs.com). The IEC decided Ex nL, which was standardized in IEC 60079-15 (FNICO), had to be deleted, and the IEC decided that IS standard IEC 60079-11 (FISCO) had to be extended by Ex ic, because Ex nL only was allowed to be used in a Zone 2 environment and this type of application was not really covered in the IS standard, says Klatt. The IS standard described the requirements for Ex ia for zones 0, 1 and 2 and Ex ib for zones 1 and 2, so in July 2006, the 5th Edition of IEC 60079-11 was published containing the explosion protection Ex ic for Zone 2 only, he explains. For IS in conventional wiring, a proof must be done for each loop, comparing safety parameters and energy-storing components, but the point-to-point connections makes the proof easy to do, explains Klatt.
"When it comes to fieldbus applications, one always has a multipoint connection: one fieldbus power supply, a couple of instruments and the topology in between—the trunk, a couple of spurs, some wiring components in between," says Klatt. "Under such circumstances, a proof can become really complex. This was the reason the German Physikalisch Technische Bundesanstalt (PTB) developed the FISCO model in 1993. In 2002, it became a technical specification (TS IEC 60079-27) and, in 2005, a real IEC standard (IEC 60079-27). With the 1st Edition of IEC 60079-27, FNICO also was published."
FNICO is FISCO’s sibling for installations where the devices are restricted to Zone 2, and where live working—opening enclosures, lifting terminals, connecting test equipment—is performed without a gas test, explains Rezabek. "FNICO provides even greater power and flexibility, since the hazardous area requirements are not as strict," he says. "FNICO also is more flexible, in that it doesn’t require a wholly separate raceway or conduit system for its circuits or networks. A distinction of both FISCO and FNICO is that they allow live work on the trunk as well as spurs and devices. The trunk refers to the homerun of the network that carries all signals back to the house. Users planning live work on the trunk, while the process is online or capable of creating explosive or hazardous vapor clouds, should ponder this practice’s basis in reality. Consider that when one lifts even one conductor of the trunk, all power and communication on the segment ceases. Any craftsman or tech who tried this in most of the plants I visit, might find their gate pass doesn’t function the next day. We really never want anyone doing live work on the trunk. Even if we are just connecting a tester, this can be done on a spur and might well need a gas test to use it anyway."
As Rezabek says, the standards overlap for Zone 2 applications. "Due to the request of the IEC, it was required to adapt the FISCO/FNICO standard," explains Klatt. The new edition specified FISCO ia for Zones 0, 1 and 2; FISCO ib for Zones 1 and 2; and FNICO for Zone 2. FNICO was based on nonincendive and had to be replaced by FISCO ic in the 2nd Edition of IEC 60079-27, which says that apparatus designed and approved to the FNICO requirements of the first edition of this standard may be used in an ic FISCO system, says Klatt. "This means one is allowed to use FNICO-approved components in applications today and also after the new edition of IEC 60079-15 is published, probably in 2011," he says.
"The anticipated replacement of Ex nL by Ex ic has led to the recent removal of Ex nL from the IEC 60079-15 construction standard," explains Phil Saward, technical manager, industrial networks marketing, at MTL Instruments (www.mtl-inst.com). "This is assumed to mean that apparatus certified under the IEC Ex scheme, which mandates the use of the relevant IEC Ex standards, can no longer be approved as Ex nL. However, the equivalent EN 60079-15 version, which is still used for ATEX approvals in European countries, still permits the nL technique." Given the normal grace period for changes to EN standards, it seems likely that new approvals to nL will still be permitted for at least a year, says Saward.
"From the installer's perspective, the important point is that Ex nL still remains in the Code of Practice for installations in hazardous areas and is therefore permitted until further notice," explains Saward. "The Code of Practice for installations at IEC level is dated December 2007 and therefore is unlikely to be replaced for several years." For FISCO networks, the second edition of IEC60079-27 remains current and still allows nL apparatus, including FNICO-certified power supplies and field instruments, to be used in FISCO Ex ic installations in Zone 2, he says.
"While FISCO allows live work on the trunk, it also is obliged to ensure that a second fault does not create enough energy to ignite a passing cloud of flammable gases," explains Rezabek. "So, for example, when a fork-truck driver spears a junction box and takes out the trunk, the resulting spark would not cause an explosion. We know many of our installations are in emerging economies, and concerns about plant personnel with emerging skills and knowledge are driving many down the most conservative path."