01 February 2005
FISCO, FNICO plug in
Users, approvers tout the advantages of dual fieldbus safety concepts
By Ellen Fussell
It's a fieldbus approach versus a standard analog approach. It reduces users' barriers from entering into the fieldbus world. It reduces installation costs, hardware, and wire. Sound familiar? The fieldbus intrinsically safe concept (FISCO) has been on the tongues of fieldbus users in hazardous locations for some time. But lately, more users are turning to FISCO as well as the fieldbus nonincendive concept (FNICO), which allow them to plug more devices into their network in a safe way and save money at the same time.
The FISCO concept is an intrinsically safe (IS) apparatus for use on a bus-powered system. It's based on the concept of the Manchester-encoded, bus-powered system in accordance with IEC 61158-2, the fieldbus physical layer standard. "It provides a way of getting more power into the measurement system you're looking at for intrinsic safety," said Nick Ludlam, senior engineering specialist at Factory Mutual (FM) in Norwood, Mass. "For example, the equipment we've looked at could be a temporary 1 measurement device—process control equipment, U.S. for a Class 1, Zone 0 or Zone 1 and Class 1, Division (Div) 1. FNICO is the same except it's for electrical equipment for use in Class 1, Zone 2 or Class 1, Div 2—safe and normal operation. Manufacturers are asking approval companies such as FM to look at the FISCO and FNICO concepts because they can get more power into the system and it's easier for the end user to be able to connect into the systems. "They don't have to look at more of the parameters," Ludlam said. "They just connect it in."
Ludlam said the FISCO and FNICO apparatus are enhancements of the fieldbus entity concept manufacturers are using in the U.S. The entity concept is a method used to determine acceptable combinations of IS apparatus and connect associated apparatus, so far uninvestigated in such combination.
"If you take a piece of associated apparatus, such as a zener diode barrier, and connect it to a temperature transmitter, if the input parameters of the temperature transmitter are greater than the output parameters of the zener diode barrier, you can connect the two together," Ludlam said. "It means you don't have to go to an expert to analyze whether the system is safe or not. It saves money and time, and it makes it easier to install."
Physikalisch-Technische Bundesanstalt (PTB) a national metrology institute providing scientific and technical services did research "to prove that when you install something (a transmitter for example) in a certain way within certain cable parameters with certain voltage limits—and with a platform of a specific type—you wouldn't get any ignition in the system," Ludlam said. " A fieldbus system, whether FISCO or FNICO, will comprise a power supply and a number of transmitters with a special type of cable, spurs, and termination devices. And if they meet the requirements of the FISCO concept and the output parameters of the power supply, then you can connect it without looking into the equipment in more depth because PTB's research showed those limits were non-ignition capable."
Chemical plants or any plant making a product with a hazardous location can benefit from the FISCO and FNICO concepts, Ludlam said. "If you buy a power supply marked with FISCO or FNICO, and your transmitter is also marked, you can connect the two together—as long as the cabling has defined parameters—without any further consideration." Each of the transmitters or power supplies will be marked with FISCO or FNICO.
One published IEC standard related to just FISCO, Ludlam said. "And then in the U.S. we started trying to adopt the FISCO requirements into Zone 2 or Div 2 applications. We managed to get that done (although there was no specific standard) by individual approval agencies. And then the manufacturers bought into the concept, and they said they wanted a standard that covered both FISCO and FNICO. "Currently in the IEC, there's a standard at final draft international standard stage—IEC 600-27. That will be published sometime next year as a final draft international standard (FDIS).
However promising, the concept is still new, warned Ludlam, and hasn't really been used because it was only developed (and the results published) in Europe by PTB in 1994.
"Since then, it's been occasionally used but because there was no specific product standard the criteria couldn't be applied consistently," he said. The U.S. has listed the product for a few years based on the existing intrinsically safe standard and knowledge of what was happening in Europe. And the published IEC document—also a basis for IEC TS60079-27:2002—was a technical specification. "It's like a feeler to see if anyone is interested and has to be turned into an IEC standard in three years or dropped completely," Ludlam said. "It's only now that a full IEC standard is being finalized." Ludlam said the document is at FDIS, and the ballot is due for completion in March, "and all being well, we will have a U.S. version this time next year. In the mean time, the IEC document could still be used as a basis."
Views from the field
The whole purpose of the FISCO and FNICO concepts is to define the amount of energy that's in the segments to a device. In a hazardous environment, manufacturers need to make sure there is not enough electrical energy to spark or heat up and cause an explosion, said Dave Appleby, process marketing lead at Rockwell Automation in Cleveland, Ohio. "Bottom line, you can have more devices in a given area following the FISCO and FNICO model versus the standard intrinsically safe model. To the end user, it makes it easier to put devices into an intrinsically safe area. From the vendor's perspective, we need to make sure the devices comply with the amount of energy that's required in the IS equipment."
Todd Hubbell, technical support manager at Endress + Hauser Inc. in Indianapolis, Ind., agrees. "With the traditional wiring of instrumentation, when you went into a hazardous area, you had to size the barrier to the instrument—the electrical characteristics of instruments—and make sure the barrier matched. It's called the entity parameter concept. You had to look at entity parameters from the instrument itself and size the barrier so the installation was safe," he said. "With FISCO, that all goes away. So all you have to look at is how much power is available from the power source and how much power draw (current) there is. You size up how many instruments you can put on the network. With FNICO, because you're going into Div. 2 areas, you can get more power to start with, so you can put more instruments on the network—twice as much on FNICO installation as on a FISCO installation."
With FISCO, the concept is based on Class 1, Div. 1 approval—areas where you have hazardous gases or materials present all the time. FNICO is a Class 1, Div 2 approval. "So it's a step down from the FISCO concept as far as presence of hazardous gases," Appleby said. "In the nonincendive world, gases are only present for a short period of time or if something goes wrong."
Prior to the two concepts, manufacturers looked at everything the same in an IS area. The key thing with the two concepts is the Div 1 and 2 areas actually are different from one another and manufacturers can treat them differently. "Prior to this, you only had one standard you had to meet," Appleby said. "It didn't recognize the two divisions as being different. They're now saying the different zones can be treated differently."
More applications, less cost
The application for the FISCO concept would come into play anywhere there are solvents or fumes. A paint plant is a good Class 1, Div 1, example because "you can smell fumes all over the place," Appleby said. That means there are hazardous fumes present under normal circumstances. A bleach facility would be a Div 2 or FNICO facility because fumes are only present if something goes wrong.
FISCO and FNICO definitely have improved over the standard IS approach—per the physical layer specification, Hubbell said. "With the traditional approach, you can put three to four devices on a segment powered by the standard entity parameter power source. With FISCO, you could have up to 10 devices. With FNICO, you could do up to 20 devices per segment. In a nonhazardous area altogether, you could put up to 30 devices on a segment. But those are more Profibus numbers. With fieldbus, the host cards limit the amount of instruments." (See accompanying article on fieldbus installations.)
As far as Hubbell is concerned, the only down side to the FISCO/FNICO concept is a lack of understanding on how these concepts work. "They make the engineering, installation, and maintenance of the digital fieldbuses much easier and cost effective," he said. "More devices are available per segment reducing wiring costs and the need to power down the entire segment is eliminated for servicing devices on the network."
And you don't have to have different types of barriers either, nor do you have to pay attention to barrier versus instrument. Most process installations are Div 2, Appleby said. "Prior to FNICO, everything was designed around the higher standard. So we were more limited to devices we could put onto a segment, which means more hard wiring to the customers, and it's more costly," he said. "More hardware and wire (power supplies and barriers) meant we basically were forcing the customer to over engineer their hazardous areas."
You could compare that to what happens when you have a traditional IS installation using the entity approach to justify IS barriers per device. The worst case is to have an IS barrier for each device, whether Div 1 or 2, Hubbell said. "With the fieldbus networks on which you can hang the devices—Profibus and Foundation fieldbus—we could go back and re-evaluate the potential for explosion based on power supply and the FISCO model. Prior to having the two concepts, it put a burden on the end user; there was only one approach to solve the IS problem. And that led to him having to put in more hardware wiring. The cost was up."
The cost of instruments went up "because we were designing the instruments in some cases around Div 1 and 2—over engineering the safety on the instrument, so the instrument cost goes up as well," Hubbell said. The FISCO/FNICO concepts mean cost savings to the customer because people like Hubble can go into Div 2 areas instead of just a Div 1. The main difference is the power supply for a FNICO device can supply more power than one for a FISCO area. More power means more instruments on the network. And with FNICO, there's less explosive probability. Now manufacturers can put more devices into an area and look at fieldbus as a practical and real way to do installations. So, it opens the door more to going with any fieldbus as an approach. The beauty is in the specification, Appleby said. "In reality, you probably won't put all those devices on your network, but the theory is you can."
More on FISCO/FNICO installations
The Profibus PA and Foundation fieldbus H1 use the same physical layer, said Todd Hubbell, technical support manager at Endress + Hauser Inc. in Indianapolis, Ind. Within this physical layer, the number of instruments per segment is 32 devices per segment for a nonhazardous installation, up to 10 devices per segment for FISCO installations for hazardous areas, and up to 20 devices per segment in a nonincendive area for FNICO installations. Traditional wiring in a hazardous area requires three to four devices using the entity concept for hazardous areas.
"These are all theoretical numbers," Hubbell said. "The actual number of points per segment is based on the type of power supply hardware and the current draw from the devices themselves. With Foundation fieldbus (FF) the host card adds a further limitation. The host cards for most control systems enforce a limit of 15 to 16 devices per input channel. This is due to the amount of data transfer within a FF segment. It also helps ensure the timing for the communications does not get too long.
But the power draw from device to device should be relatively comparable given that both protocols use the same physical layer, said Rich Timoney, president of Foundation Fieldbus. "Most installations occurring today have a large percentage of the control loops using control in the device due to the higher system availability that single loop integrity provides," he said. "Both statistically and practically this is a fact. The real issue relating to the number of devices on a segment is the bandwidth required for the functions performed by the devices."
Hubbell said the limitations are different for FF systems for several reasons. "Most FF systems currently do not use a control-level network," Hubbell said. Instead, the H1 segment connects directly to an input channel on the host input card. Profibus PA, however, is currently always wired into a segment coupler which connects the PA segment to a Profibus DP segment. Profibus DP is capable of handling large amounts of data which help overcome the need to limit the number of devices on a segment.
Timoney said Foundation fieldbus is a control level network, "which makes it significantly different than PROFIBUS or HART," he said. "Most installations today are using control in the field. This provides for higher system availability—statistically and practically— due to the inherent single loop integrity."
Another reason for the limitations, Hubbell said, is Foundation fieldbus systems use peer-to-peer communications. "This allows these systems to have control functionality distributed between the devices on the network. In order to ensure the timing does not get too lengthy for control to be effective, there are limitations in place on the number of devices connected to the input channel."
"The regulating parameter here is bandwidth," Timoney said. "Foundation fieldbus prioritizes control messages to have absolute priority on the network. The more devices executing control on the segment, the more bandwidth is reserved for this aspect of communication. Regardless of the number of devices, control messages don't get slower."