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Figure 1 shows how area classifications are determined for North America using the class-and-division principle. The division is based on the likelihood of a specific type of gas being present at any point in time. As a rule of thumb, Division 2 assumes the potentially explosive gas is present one hour/year, and Division 0 assumes the gas is always present.
The second aspect of hazardous area control is the temperature rating. Figure 2 shows how the type of gas present determines the required "T-rating."
Simple apparatus are defined as those devices in the following three categories:
1. Passive components, including items such as switches, junction boxes, resistors and simple semiconductor devices that neither store nor generate energy. Sensors that use catalytic reaction or other electrochemical mechanisms are not normally simple apparatus.
2. Stored energy sources consisting of single components in simple circuits with well-defined parameters; for example, capacitors or inductors, whose energy storing values should be considered when determining the overall safety of the system.
3. Generated energy sources; that is, thermocouples and photocells that do not generate more than 1.5 V, 100 mA and 25 mW.
Comparison of Barrier and Isolator Schematics
|Simple and reliable||More complex, statistically lower MTBF than barrier|
|Extremely accurate in many
|Active devices: power and heat
Flexibility in bonding practice
|High-integrity bond required
Predictable response to earth faults
|Flexible response to earth faults|
|Inexpensive||Generally more expensive|
|Applications are defined in terms of voltage and resistance||Application-specific. Each barrier is defined in terms of the function that it is designed to perform.|
|Encapsulated design necessary
Tight power supply limits (except ‘protected’/fused barriers
|Replaceable supply fuse
|Easier to fault find (earth
|Wide power supply tolerance|
In addition to the above, the following (taken from the ISA standard) also applies to simple apparatus installations:
• Simple apparatus shall not achieve safety by the inclusion of voltage and/or current-limiting and/or suppression devices.
• Simple apparatus shall not contain any means of increasing the available voltage or current, for example dc-dc converters.
• Simple apparatus located in the explosive gas atmosphere shall be temperature-classified.
• Where simple apparatus forms part of an apparatus containing other electrical circuits, the whole shall be assessed according to the requirements of ISA–60079-11 (12.02.01)–2009.
Because with entity systems you need to understand interaction between each component on a loop, I/O card, barrier or field device, the entity concept works well for loops with one I/O card and one field device. However, if you have multiple devices on a wire pair, as with fieldbus systems, the number of combinations that need to be verified quickly grows exponentially. This is one of the reasons most process fieldbus systems use FISCO as described in the June 2010 issue of Control.
Furthermore, intrinsically safe circuits need to be kept separate from non-intrinsically safe circuits with the following minimum requirements:
• All terminals for intrinsically safe circuits shall be separated from terminals for non-intrinsically safe circuits where intrinsic safety can be impaired by external wiring that, if disconnected from the terminal, can come into contact with conductors or components by distance or terminal location.
• When separation is accomplished by distance, the clearance between bare conducting parts of terminals shall be at least 50 mm, including ensuring that contact between circuits is unlikely if a wire becomes dislodged.