Intrinsic Safety (IS) makes it possible to perform live maintenance at any point in the control loop because, by its nature, IS always keeps the amount of available energy on the wire pair below the ignition point for the gases/environment in which it's installed. There are two aspects to determining the hazardous area requirements for an installation: area classification (the type of gas present and the likelihood of it being present) and the temperature classification (maximum surface temperature of the device or apparatus).
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."
Fortunately, the majority of the hydrocarbon industry needs to meet only the T1 or T2 temperature limitations for the majority of its facilities, and this is often why the T-rating is overlooked when specifying and purchasing instruments. When we think about temperatures, we tend to be more concerned with the ambient temperature range in which the device can continue to operate.
Intrinsic safety is entity-based, meaning all the components need to be considered as a single entity. The devices in the loop also need to be treated as "simple apparatus" as defined in ISA–60079-11 (12.02.01)–2009 "Explosive Atmospheres — Part 11: Equipment Protection by Intrinsic Safety," which is summarized below:
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.