By Lee O'Toole
By Lee O'Toole
Many process plants have hazardous areas where there are explosive atmospheres. These can be ignited even by low levels of electrical or thermal energy, such as arcing or sparks from electrical circuits. It’s therefore essential that industrial displays are designed to prevent any such ignition opportunities in a hazardous environment — an environment where flammable vapours or gases, combustible dust, flammable liquids or ignitable fibres are present.
The solution is to build products that are intrinsically safe; that is, their energy consumption and storage comply with standards for safe level limits under specified fault conditions.
Industrial displays and potential hazards
Flat panel displays consume electrical energy and are connected to an external power source. Arcing or sparks arising from short circuits or failures are therefore possible. These could cause an explosive atmosphere to ignite. Such atmospheres exist within hazardous areas found in many process plants, including petrochemical, pharmaceutical, paint manufacture, pill coating and battery manufacture where there is a presence of flammable liquids, gases or vapours. Combustible dusts such as those in granaries and food processing are also potential hazards that can be ignited and cause an explosion.
One possible solution is to protect all hazardous area electronics by sealing it within a gastight, positively pressurized enclosure that isolates any possible source of electrical discharge from the immediate atmosphere. This approach is sometimes taken, but in Canada an intrinsically safe solution is used. To be classified as intrinsically safe within Canada, equipment must comply with standards set by the Canadian Standards Association. This defines the maximum energy that may be transferred or stored within the equipment under specified fault conditions. Equipment is considered intrinsically safe it if is incapable of generating sufficient electrical or thermal energy to cause an explosion in the atmosphere of its environment.
Intrinsically safe under all conditions
A product’s intrinsic safety within a hazardous environment depends not only on its own performance, but also that of any external power or signal I/O feeds. These must reach the product directly from a safety barrier located in a safe area. Such barriers usually include a series of diodes, resistors and fuses, arranged and sized so as to limit the energy provided to the device in the field. It is always important to remember that no piece of equipment can be considered intrinsically safe under all conditions. Its status depends on how it is connected, where it is located geographically, and the class of hazardous environment it is to operate in.
Some special considerations apply to electronic equipment destined for hazardous areas, in addition to reducing or eliminating internal sparking. Component temperatures should be controlled, and component spacing that could allow dust to establish short-circuits must be designed out. Safeguards, such as current limiting by resistors and fuses, must be used to ensure that under no circumstances can any component reach a temperature sufficient to cause autoignition of a combustible atmosphere. These issues have become increasingly critical as components, and the PCBs that hold them have become ever more compact and higher in power density. Additionally, single component failure modes can be catered for by using multiple series resistors and Zener diodes.
Importance of intrinsic safety
Intrinsic safety is important for ensuring the highest safety standards in the Canadian industrial arena. Displays must meet the governing standards set in place by the Canadian Standards Association according to the specific hazardous environment and the external system. If you need an intrinsically safe display, you must make sure that it is designed to comply with your target environment.
Lee O’Toole is with Anders DX, a U.K.-based expert in user interface solutions.