Interested in linking to "The I.S. Alternative"?
You may use the Headline, Deck, Byline and URL of this article on your Web site. To link to this article, select and copy the HTML code below and paste it on your own Web site.
|About the Author
Ian Verhappen is an ISA Fellow and can be reached at firstname.lastname@example.org. He has 25+ years experience in instrumentation, controls and automation. Check out his Google+ profile.
Intrinsic Safety (IS) makes it possible to perform live maintenance at any point in the control loop because, by its nature, it 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 its being present) and the temperature classification (maximum surface temperature of the device or apparatus).
The following figure 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, while Division 0 assumes the gas is always present (Figure 1).
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 only needs to meet 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:
In addition to the above, the following (taken from the ISA standard) also applies to simple apparatus installations:
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 (www.controlglobal.com/articles/2010/FISCO1006.html).
Furthermore, intrinsically safe (IS) circuits need to be kept separate from non-IS circuits with the following minimum requirements:
The final critical element in any IS circuit is the important issue of what to do with any "extra"energy that might result in the event of a fault in the loop, such as a short circuit. Normally the answer is to run this current to ground or earth. Figure 3 shows a typical control room/interface room earthing/ground scheme.
The function of the IS ground is to provide a secure, high-integrity, low-impedance path through which fault currents will flow, while minimizing voltages seen in the hazardous area. The most likely source of high-voltage invasion is from the local distribution transformer feeding the control system and, practically speaking, the IS ground is there to shunt fault current from such an invasion back to the neutral of this transformer. It, therefore, has to be of low impedance to be the preferred path for the fault current (while the barrier fuse blows).
Now that we know the basics of the various components of an IS circuit and the associated restrictions regarding its installation, what are some of the arguments for using intrinsic safety that explain why it is so widely used in other parts of the world?
IS "works" by reducing the power going to the field. The simpler alternative which uses diodes/resistors, is called a passive zener barrier. If a barrier introduces too much voltage drop due to too high a resistance being added to the network by the barrier, resulting in insufficient power to drive the output full scale, then you may need to use an isolator. Since isolators are separately powered, they do not present as large a load to the loop.
This happens on occasion for analog-output devices such as valves. The figure on the left shows the differences between these two alternatives.
Is intrinsic safety part of your future? Only you can decide. However, as you can see, there are a number of reasons that you should consider it when designing a new installation. With our increasing focus on safety, intrinsic safety installations reduce the overall risk of explosion through human error.
Ian Verhappen P.Eng. is an ISA Fellow, ISA Certified Automation Professional and a recognized authority on Foundation fieldbus and industrial communications technologies.Verhappen leads global consultancy Industrial
Automation Networks Inc. Check out his Google+ profile.