Intrinsically safe Foundation fieldbus H1 networks

Wiring in a Hazardous Area
Several installation and wiring methods are allowed in a hazardous area. Two of the most common methods are explosion-proof (Ex) and intrinsically safe (IS). Both methods limit the amount of energy that can be released or present in the environment.

Explosion-proof instruments are not immune to explosions. An explosion-proof instrument is designed so that if a vapor were to be ignited inside the instrument enclosure, the enclosure would contain the energy of ignition and disperse it into the classified area at a low enough level to prevent an explosion outside the instrument. Explosion-proof designs require special installation methods, and the instrument needs to be rated explosion-proof for the proper area classification (i.e., Class I, Division I, Groups C/D). 

An intrinsically safe (IS) design limits the electrical energy at the instrument to a level below the explosive limits of the area. The explosive limits and the amount of available power for an IS circuit is based on the ignition curves of a given gas or gas family and the related minimum ignition energy. Intriniscally safe designs have less stringent installation methods, such as the ability to use NEMA 4 instead of  NEMA 7/9 enclosures and standard wiring methods.

Intrinsic safety is defined by the ISA as a type of protection in which a portion of the electrical system contains only intrinsically safe equipment that is incapable of causing an ignition in the surrounding atmosphere.

An intrinsically safe circuit is defined by the NEC as 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 conditions.

An intriniscally safe circuit is defined by CENELEC as a circuit in which no spark or any thermal effect produced in the test conditions prescribed in this standard (which include normal operation and specified fault conditions) is capable of causing ignition of a given explosive atmosphere.

The two IS standards in use today are “ia” and “ib,” where safety is maintained with up to two faults or one fault, respectively. The “ia” standard is acceptable in Zone 0 classifications and “ib” standard is acceptable in Zone 1, Zone 2, Division I and Division II classifications.

An intrinsically safe design requires a systems approach to implementation  because each component can fail, and each component may contribute capacitance and/or inductance to the overall circuit. Some of the details to be mindful of during the design of an IS system are entity parameters for the safety apparatus and instrument; cable inductance and capacitance; grounding; vendor control drawings; and cabinet layout for IS wire routing. You may also need to contact your insurance carrier to discuss your design. In the past, Factory Mutual (FM) required that all IS devices be FM-approved. In most cases this is not a problem, but not all IS devices manufactured today carry FM approval.

Intrinsically Safe Foundation Fieldbus H1
The concept of intrinsically safe systems was developed in the early 1900s as a result of a mine explosion that killed 439 people. In the early days, IS systems were used mainly when there was no other method available. As time progressed, IS systems became an accepted standard in Europe. In 1965, the ISA published RP 12.2 “Intrinsically Safe and Non-Incendive Electrical Instruments.” In the years following, many standards and approval agencies around the world became more aligned to IS systems (ISA, IEC, NFPA, CENELC, UL, FM, OSHA and many more), allowing North America to take advantage of all of the products being used in Europe.

Intrinsically safe devices or apparatus are approved by either an entity or a system method. The entity method requires that devices be certified individually as an intrinsically safe device or apparatus. It becomes the responsibility of the user to select and integrate the individual components into a system using the device’s entity parameters and the vendor control drawing.