Foundation fieldbus in hazardous areas

Implementing Foundation fieldbus can seem like a daunting task, and when coupled with hazardous-area considerations, may approach information overload; however, it doesn't have to be an explosive concept.

By C. Bruce Bradley

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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.

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