Machine builders designing systems for installation in hazardous areas must carefully consider the safety of the machine's operator interface as well as hardware components that will be exposed to the extreme environment. Components to consider might include operator interfaces such as PC-based HMIs, LCDs, HMI panels, panel PCs and thin clients, along with hardware such as controllers and distributed I/O and auxiliary devices like warning lights.
"Most of the controllers and I/O we deal with, including those from Allen-Bradley, are rated for Class I, Div. 2, hazardous areas and can be used right out of the box in those types of classified areas,” says Jeff White, director of technical services at Interstates, a system integrator that provides turnkey electrical engineering, construction, instrumentation, and control systems solutions, headquartered in Sioux Center, Iowa. “In this sense, they do not differ from traditional devices."
That said, even though the hardware can be installed out of the box, you must be aware of what you need to do to make the whole installation safe, adds Jamie Schmidt, lead project engineer at Interstates. "Although the I/O components might be rated for a specific area such as Class I, Div. 1, or Class I, Div. 2, you must follow a certain set of rules to ensure safety in putting the components out there,” she warns. “For example, one of the systems we use is Allen-Bradley's Flex Ex I/O. Rules mandate that you must use specific power supplies to supply the intrinsic power to the I/O. There are also rules for the communications going through the modules to provide an intrinsically safe solution, for putting the components together for the infrastructure and for connecting your field devices to the components. Basically, the field devices have an intrinsically safe rating that needs to be matched to the I/O point to maintain the intrinsic rating."
Additionally, intrinsically safe components may or may not be required, depending on the area's class rating. "For hazardous area installations, you must also consider whether it is a Class I or a Class II area," continues White. "Class I areas involve a hazardous or explosive gas, and Class II involves hazardous dust. For Div. 1 installations for gas areas, the common options are explosion-proof installations, purge and pressurizing and intrinsically safe (Figure 1). When you talk about a Class II, Div. 1, instead of explosion-proof, you're looking for dust-ignition-proof installations."
Interstates designs systems for installations in plants that are based on many factors, says Schmidt. "For example, in deciding whether to use an explosion-proof approach as opposed to an intrinsically safe one, we base our decision on the customer's application and try to follow that approach for the whole plant," she says. "It's also important to account for a company's maintenance practices. For example, after we install equipment in an explosion-proof box, maintenance personnel can't just open up the box if hazardous gases are present. Personnel need to use devices that detect the presence of any hazardous gases. When a hazardous gas is present, operators must shut down the equipment before opening the box. In contrast, an intrinsically safe approach typically involves barriers installed in a nonhazardous-rated area that limit energy to the field device. In this case, personnel can work on the devices live because there will not be enough energy to cause an explosion. The intrinsically safe approach thereby allows more flexibility in some maintenance practices."
Additionally, warns Schmidt, you must ensure you use tools approved for a hazardous area when working on a live system. "It is still a hazardous area, so meters and testing equipment must be approved for that area," she says. "Therefore, maintenance must be aware of the safe working practices—it's not just a free-for-all, because there are still considerations to keep the system safe."