By John Rezabek, ISP
You’ll hear the acronym for the fieldbus non-incendive concept (FNICO) pronounced a few different ways, but will the concept itself be pronounced dead when the IEC revises its underlying standards? What exactly is FNICO, and should fieldbus practitioners—especially those who have used or are planning to use it—be concerned about the changes?
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. 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. By 2004, the German Federal Physical Technical Institute (PTB) developed the fieldbus intrinsically safe concept (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.
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. FNICO provides even greater power and flexibility, since the hazardous area requirements are not as strict. 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 and also is the means to carry segment power to all two-wire field devices. 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 and any valve on the segment goes to its fail position. 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, do we? 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.
While we never truly want live work on any part of the trunk in an operating plant, some plant operators would say the ability to live-work the trunk circuit is not really the issue. The goal is to eliminate finding a confusing mixture of circuits after they open the field junction box, which is the case with some alternative techniques. At least with FISCO and FNICO, the engineer doesn’t need to distinguish between circuits that are live-workable in the hazardous area and those that are not.
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. 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. While most of us would see to it that this fellow’s gate pass was revoked as well, we know many of our installations are in remote areas and emerging economies. Concerns about plant personnel with “emerging” skills and knowledge are driving many down the most conservative path.
Those applying FNICO have become a little concerned since it came to light that a new FISCO ic standard was to supercede and replace it. In a virtual conversation I had with Thomas Klatt, business development manager for Pepperl+Fuchs, on Foundation Fieldbus Forums, he says, “With the second edition of the FISCO standard (IEC 60079-27) FNICO was replaced by FISCO ic. All this trouble starts with the request from IEC to specify comparable explosion-protection methods in the same standard. Ex nL (non-incendive) is comparable to IS. So with the next revision of the non-incendive standard (IEC 60079-15) the Ex nL part will be completely deleted (probably in 2011).”
Some companies have FNICO implementations on the drawing board and are wondering what to do. These users and their consultants should take heart. It doesn’t appear you can buy any EX ic devices anytime soon. EX ic is the rating that’s to take the place of nL. “It is also recognized that FNICO and nL equipment will be used for many years and the second-edition FISCO standard details how these also can be used to create an ic system”, says Phil Saward, fieldbus product manager for MTL Instruments, in that same forum thread. “The comforting thought is that you can begin to think in terms of ic. However, if you do not like change, you can continue using FNICO or non-incendive equipment for the foreseeable future.”
FNICO remains a viable choice for the fieldbus physical layer. End users will benefit from exploring all the options offered with their favorite physical layer supplier.
John Rezabek is a process control specialist at ISP in Lima, Ohio
