Specifying switches for hazardous environments
September 27, 2016
In specifying switches as part of a wider device or system, design engineers have much to consider. Such choices are important regardless of the targe...
In specifying switches as part of a wider device or system, design engineers have much to consider. Such choices are important regardless of the targeted use, but in hazardous environments, choosing the right switch becomes critical and can, quite literally, mean the difference between life and death.
Explosions can occur to varying degrees of enormity: all that is required is a fuel, oxygen, and an ignition source. So what factors does a design engineer need to consider in choosing a switch? Which product will remove the spark potential to prevent risk of an explosion from occurring? What European or international standards must the products adhere to? Where might such switches be used? And what types of switches are available?
In choosing a switch, several considerations will come into play. First, the designer needs to consider the country where the system will ultimately be deployed. Standards, rules and regulations that have to be met in Brazil may be very different from those in Europe and vice versa. Understanding those requirements, and ensuring your choice of switch matches the relevant approvals, is essential.
Size is perhaps the second consideration. Design engineers often have a limited space envelope in which the switch needs to be incorporated, and can have a significant impact on the range of switches from which he can choose. Further consideration of the application can come from determining which electrical ratings apply and which style of actuator is required to capture the mechanical movement to convert the action into an electrical signal.
The most important consideration, perhaps, is the environment in which the switch is going to be used. Many hazardous locations areas can be outdoors, so functionality over a wide operating temperature is crucial. Using oil production and distribution as an example, the same functional performance must be expected if the switches are being used in the Russian peninsulas to the deserts of Saudi Arabia. The level to which the switch must prevent the ingress of water or other liquids (i.e. its sealing) also needs to be assessed and to this extent, the range of hazardous environments and the range of applications are considerable.
C2000 MCU has examples using vector control of motors, incorporating torque, speed and position; multiple current sense topologies; analog and digital position sensor interfaces; flexible real-time connectivity; and series of platform releases.
The term “hazardous environment” immediately conjures up images of oil rigs, petrochemical plants, and drilling platforms where the damage caused by risk of explosion is potentially catastrophic. One only has to remember the Buncefield oil depot explosion in the UK ten years ago, believed to be the biggest of its kind in peacetime Europe and measuring 2.4 on the Richter scale.
Indeed, switches can be found in all manner of applications, such as in blowout prevention systems used to seal, control, and monitor erratic pressures and uncontrolled flow that occur during oil and gas drilling. They are vital for the safety of the crew, rig, environment, and to maintain the oil or gas well. If a disruption occurs, an emergency system can disconnect the rig from the well, automatically triggering a switch that closes the blowout preventer and closes choke valves.
Switches can also be found in mud pumps, used in drilling to pump mud around the cutting area to keep that area wet and lubricated, and to prevent a build up of pressure. Volumes are controlled by measuring the frequency of the pumping actions, which can be measured by a switch that detects the electrical signal generated by each operation.
A further use is to detect the stability of a drilling rig, to detect lateral drift, and to cut the power to the drill if too much lateral movement presents a danger. Switches are also used to indicate the position of pipeline “pigs” used to clear pipe of any debris to eliminate potentially damaging blockages.
Switches are also incorporated into decontamination showers for people working with hazardous chemicals, and again need to be proven not to create a flash in the presence of flammable liquids.
Not every hazardous environment where switches are required is as immediately obvious. Grain, for example, presents little or no hazard on its own, yet moving thousands of tons of grain by conveyor into silos can create highly flammable dust, which the slightest spark can ignite. A similarly hazardous dust is created during the production of wood pellets as the wood is ground and compressed, so any electrical control system must be guaranteed not to create a spark that could create an ignition scenario. Media such as sawdust and cement also pose a combustible threat, especially when dust is being extracted as part of an industrial process.
The challenge for the designer is that different environments have different hazardous designations and often require different agency approvals. So what approvals will the designer typically experience?
In choosing a switch for a particular application, the designer needs to be aware of the necessary Standards to which the product must adhere. Detailing the plethora of standards is an article in its own right, but in summary, the principal Standard in Europe is ATEX. ATEX is in fact the name commonly given to the legislation within the CE guidelines for controlling explosive atmospheres and the suitability of equipment and protective systems used in them.
The International Electrotechnical Commission System for Certification to Standards Relating to Equipment for Use in Explosive Atmospheres (IECEx System) is an attempt to create an international standard to which all product must adhere, though its profile is still comparatively limited. Its aim is ultimately to eliminate the need for multiple national certifications while preserving an appropriate level of safety.
Certification for equipment specifically used in North America (the U.S. and Canada) falls under the auspices of the UL and CSA approvals bodies, whereas in Russia and the former Soviet states, equipment must meet the GOST standard now superseded by the EAC standard.
The challenge to designers is in understanding the nuances of each of these standards, and while all are intended to achieve the safe specification, installation, and operation of electrical equipment in hazardous areas, they are subtly different in principles, classification, and approach.
So what kinds of switches are available? In choosing a switch, and assuming the product meets the relevant local standard, a designer may look for a device with particular features.
For example, the switch should be made from non-sparking materials, which might sound obvious. Gold contacts options are better able to control low energy electrical loads. Switches that have an all-metal drive train offer extended life and consistent operating characteristics, even at high temperatures, and a dual bearing design on side rotary construction increases resistance to side loading and similarly extends product life.
Choosing a switch with sintered bronze bearings gives greater resistance in corrosive environments and switches that comprise a rugged, corrosion resistant head and body that are either 316L stainless steel or phosphate treated and epoxy finished also ensure longer life. Operating temperatures can be extended (down to -40 °C) through the use of optional fluorosilicon seals.
Whatever switch you choose, and whatever the environment, pick a manufacture that you can trust with your life. Because in the world of hazardous environments, that is precisely what you may be doing.