By C. Bruce Bradley, PE

Some portion, if not the majority of most chemical plants today are electrically classified as a hazardous area. This classification adds a layer of complexity to the design and implementation of the control system.  Implementing control networks such as Foundation fieldbus into the system may make the task seem overwhelming.

The National Electrical Code (NEC) defines classified or hazardous locations as those areas “where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings.” The NEC recognizes class and zone as two methods for electrically classifying an area as hazardous. The class method is the primary method use in the U.S.

The Class System
The hazardous area classification has three components. The class separates fuels into families. The three classes are 1, 2 and 3, which signify an environment of flammable gases, vapor or liquids, combustible dusts, or ignitable fibers and flyings, respectively.

The division term separates the area into two parts based on the probability that an explosive fuel and air mixture will be present. In Division 1 areas, ignitable concentrations of fuel can exist all or some of the time under normal operating conditions. In Division 2 areas, under normal operating conditions, ignitable concentrations of fuels are not likely to exist.

The group term categorizes materials with similar explosive properties. The group categories are A, B, C, D, E, F and G, which signify acetylene, hydrogen, ethylene, propane, metal dust, coal dust and grain dust, respectively.

A chemical plant with a classification of Class 1, Division 2, Group C/D would have an environment of flammable gases, vapor or liquids with similar explosive properties to ethylene and propane in ignitable concentrations not likely to exist under normal operating conditions. For more information regarding hazardous area classifications reference NEC Chapter 5, “Special Occupancies” or “Intrinsically Safe Foundation Fieldbus H1 Networks” in the January 2007 issue of Control.

ISS Fieldbus Implementation Options
	Explosion-Proof	Intrinsically Safe (Entity)	Intrinsically Safe (FISCO)	Intrinsically Safe (Routemaster)	Nonincendive (FNICO)	Hybrid (HPT)
Control Drawing	Not Required	Required	List of devices only	Required	List of devices only	Required
Entity Calculations	Not Required	Required	Not required. Cable meets FISCO specs	Required, but only done once for the worst-case scenario of the longest spur and the type of cable specified	Not required. Cable meets FNICO specs	Required, but per spur only, since each spur is a separate circuit.
Maximum Current	None	80 mA	120mA IIC 265 maIIB	350 mA	180mA IIC 320 mA IIB	>500 mA
Allowable Area Classification Implementation	Division 1 and 2	Division 1 and 2	Division 1 and 2	Division 1 and 2	Division 2	Division 2 for the trunk and Division 1 or 2 for the spurs
Hot Work Permit Required to Maintain/Troubleshoot While Energized	Yes	No	No	No	No	Yes for the trunk, no for the spurs

Engineering and Installation
Several accepted engineering and installation methods — explosion-proof, purging, oil immersion, encapsulation, intrinsically safe and nonincendive — can reduce the risk of an explosion. The basic concept for each is to eliminate at least one of the three parts of the combustion triangle (fuel, oxygen and heat). We will focus on the three designs that apply to fieldbus: explosion-proof, intrinsically safe and nonincendive. All three methods reduce the risk of ignition by limiting the amount of energy that can be released or present in the environment, but they each accomplish this differently.

Contrary to what the name sounds like, explosion-proof (Ex) designs do not mean an explosion or ignition is impossible. An explosion-proof design and installation requires that if a fuel were ignited inside the device enclosure, the enclosure would contain the energy of ignition and disperse it into the classified area at a level low enough to prevent a secondary ignition from occurring outside the enclosure. Explosion-proof designs require special installation methods and NEMA 7/9 for the proper area classification rating of the electrical devices and enclosures. Such systems cannot be worked on while energized without a gas clearance certificate or “hot-work” permit.

Intrinsically safe (IS) circuit designs limit the electrical energy at the device to a level below the explosive limits of the environment and remain safe in the event of a component failure. An intrinsically safe circuit, as defined by the NEC, is “a circuit in which any spark or any thermal effect is incapable of causing ignition of a mixture of flammable or combustible material in air under prescribed test conditions.” An IS circuit uses a safety device, such as a safety barrier, to limit the power in the hazardous environment based on the ignition curves of a given gas family and its related minimum ignition energy. Intrinsically safe designs have less stringent installation methods and allow more standard (NEMA 4) enclosures. These circuits can be worked on while energized without a hot-work permit.

Nonincendive (NI) circuit designs are similar to IS designs. The NEC defines a nonincendive circuit as “one  other than field wiring, in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under specified test conditions, of igniting the flammable gas-air, vapor-air or dust-air mixture.” Nonincendive circuit designs do not take component failure into consideration, and therefore have a reduced safety level compared to IS circuit design. They can be worked on while energized without a hot-work permit.

Do not confuse nonincendive equipment with nonincendive circuits. Nonincendive equipment, as defined by the NEC, is “equipment having electrical/electronic circuitry that is incapable, under normal operating conditions, of causing ignition of a specified flammable gas-air, vapor-air, or dust-air mixture due to arcing or thermal means.”

The power rating of a nonincendive device may require that the energy level in the interconnecting wiring exceed the ignition curves of the rated area; therefore, this design cannot be worked on while energized without a hot-work permit. The device cover cannot be removed without a tool. Nonincendive requirements and specifications can be ambiguous to the system designer, so intrinsically safe circuit designs are often chosen over nonincendive designs.

Since intrinsically safe and nonincendive circuit designs keep the energy level in the classified area below ignition points, a number of details must be considered during the engineering process. The overall system design of these circuits must include Entity parameter evaluation for the safety apparatus and devices, interconnecting cable/wire inductance and capacitance, safety grounding, vendor-provided control drawings, device-testing agency approvals, device ratings and markings appropriate for the area and cabinet layout for wire routing. (Per NEC requirements, IS and nonincendive wiring must be separated from standard wiring).

Designers may want to discuss their plans with their insurance carriers because some carriers, such as Factory Mutual (FM), may require that the devices used be FM-approved. Not all devices manufactured today carry approvals from all of the testing agencies. The user is responsible for  selecting, integrating and implementing the components into a safe system per the vendor-provided control drawing. This type of design is referred to as an Entity-based system.

Foundation Fieldbus Implementation
A fieldbus network consists of a power supply, power conditioner, optional repeater(s), cables, junction box(es), terminators and devices. The control loops configured on the network use function blocks that can be executed in the instrument, host device or a combination of both. The Foundation fieldbus specification covers the protocol, cabling, topology and other details. For more information, visit the Fieldbus Foundation website.

Like traditional 4-20 mA circuit designs, Foundation fieldbus can be implemented in hazardous locations using explosion-proof, intrinsically safe and nonincendive designs.

Explosion-proof designs are not practical for fieldbus implementations due to the cost, bulky enclosure size and inability to work on the network while energized without a hot-work permit.

Traditional Entity-based fieldbus designs use safety barriers and limit the available bus current to about 80 mA. A typical fieldbus device might draw 20 mA of current, meaning that, in theory, up to four devices on a segment could be powered up. Allowing only four devices does not realize the possible economic benefit of a network design when compared to traditional 4-20 mA, Entity-based implementations.

In an effort to reduce end-user engineering and increase the bus current available, two standards were created specifically for Foundation fieldbus implementation in hazardous areas.

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.

Conclusion
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.

Determing the Right ISS Fieldbus

Key questions to ask when considering implementation:

• What’s the area classification? Nonincendive is allowed only in division 2 areas; intrinsically safe designs are allowed in division 1 and 2 areas.
• What are the size and scalability requirements? How many devices do you plan to implement? What are your future requirements for expansion? How long does the trunk need to be? 
• What are the technology/product characteristics? Does the product provide short-circuit protection for the trunk and the spurs? If during maintenance you accidentally short a spur, will the trunk be protected? Are the system components hot-swappable?
• What about risk? What level of safety or risk are you willing to accept? Intrinsically safe designs take into account component failures and allow maintenance while energized without a hot-work permit. Nonincendive designs do not take into account component failures and may not allow maintenance while energized without a hot-work permit.
• What about maintenance and downtime? Can your process be down (fieldbus de-energized) in order to troubleshoot or expand the network? Does your facility have a hot-work permit philosophy allowing live work on energized equipment? 
• Who’s doing the engineering? Will the engineering be done in-house? If you are using an engineering firm, interview the proposed staff. Try to use a firm that has experience in hazardous-area fieldbus design because this implementation is a hybrid approach for many A&E firms. The design requires knowledge of hazardous area classification, which is primarily an electrical engineering function, as well as control systems design experience, primarily a control-system engineering function. In many cases, these resources may exist in two separate departments (electrical and control systems). 
• What about the control system? What type of control system will be used? Contact your control system vendor and inquire about technologies and products that have been tested and proven on your control system. Are there any software limitations or hidden costs that punish you when using fieldbus on your control system? 
• What about technician expertise? What is the skill set of the maintenance technician(s) that will support this technology? Will training be required?
• How will the devices be calibrated? Until recently, calibrating fieldbus devices in-house was difficult. In the past couple of years a small number of fieldbus calibration devices have appeared on the market.

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About the Author

C. Bruce Bradley, PE, works for Boehringer Ingelheim Chemicals, Petersburg, Va. Email him at cbradley@bichemicals.com.