Background
Combustion or fire is a chemical reaction in which a combustible material combines with an oxidant to release energy. Part of the energy release is used to sustain the explosion, by utilizing existing heat or creating a vacuum to consume more oxygen. Within a hazardous environment there are
three elements necessary for a fire or explosion:
- A combustible material also known as a fuel
- Ignition source - electrical arcing or general heat
- An oxidizer also known as oxygen
When a fuel, oxidizer and ignition source are present at the necessary levels, burning or explosion will occur. To reduce the possibility of fire or explosion, one of the key elements needs to be reduced or removed.
Another key factor in a fire or explosion are the explosive limits. These are the maximum and minimum concentration needed of a given combustible material to support a fire or explosion. To form an explosive mixture, the hazardous gas must have sufficient concentration levels. The minimum concentration of oxygen to combustible material is known as the
lower explosive limit or LEL. If the concentration is below the LEL, it will not be flammable or have enough flammable material present. The same condition would apply if gas concentration is too rich, meaning it will not ignite because it will not have sufficient oxygen to sustain the fire or explosion. This is known as the
upper explosive limit, or UEL. It should be noted that different gases will have different threshold limits.
In addition to the explosive limits, each hazardous material will have an
autoignition temperature and a
flashpoint. The flashpoint is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture. As the liquid is heated and reaches the flashpoint, autoignition could occur. An autoignition is the lowest temperature at which a combustible material will spontaneously ignite in a normal atmosphere. For example, if we have the heating of a combustible liquid, explosive vapors are created. As the material reaches the explosive limits creating the flashpoint. If the general atmosphere is increased, the autoignition temperature, fire, or explosion will occur.
To simplify, the specification of electrical devices into hazardous environments, IEC classification methods utilize autoignition, flashpoint, and explosive limits to place hazardous materials within groups.
The two predominant methods used for classification of hazardous environments are the NEC and IEC standards.
IEC and NEC standards both classify the level of risk into three main categories:
- Continuous
- Occasional
- Not normally present
All three categories provide the likelihood of an explosive atmospheric condition within the general environments. Within the IEC standard, the general risk categories are then classified as Zones. This reflects the physical material that could provide a potential fire or explosion. From liquids and gases, the area classifications are Zone 0, Zone 1, and Zone 2. By altering the physical material to dust or fibers, the area classification numerical value changes to Zone 20, Zone 21, and Zone 22.
Referring back to the necessary requirements for an explosive triangle, we know we need ignition source, oxygen, gas and or dust combustible material in the correct mixture within the atmosphere. In order to specify electrical devices and hazardous locations, we need to identify the general Zones within each area. When working with liquids or gas, we locate the most hazardous or flammable location directly above the material. Due to the possibility of the valve leaking, at the top of the containment structure we must classify the area as Zone 1. As we move further away from the possible explosive atmospheric condition the Zone classification would be reduced to Zone 2. By altering the combustible material properties to a solid form, the area classification above the material would be Zone 20. If the dust escapes the same failed valve, the external area around the same containment unit would be classified as Zone 21. As we continue to move away from the containment unit and with reduction of Category risk, the new classification for the general area would be Zone 22.
Now that we know combustible materials whether liquid, gas and or dust will have different upper and lower explosive limits, and we understand the general heat could be the ignition source, within the IEC standards the
T rating specifies the maximum surface temperature an electrical device will create. As the temperature increases, the associate rating will decrease, meaningless heat will have a higher T rating, while an increase in surface temperature will reduce the T number.
As a recap the IEC standards classify hazardous areas as environments that could contain explosive vapors, gases, and/or dust within the atmosphere. The primary categories provide the risk levels as constant, occasional, and not likely. Since elements have a variety of flammability and the physical properties could be different, IEC standards segment the types of combustible materials into Zone classifications. The most hazardous is Zone 0 or Zone 20. By reducing the risk level to have occasional explosive atmospheres the Zone classification is Zone 1 or Zone 21 and further reduction of Category risks would change the hazards classification to Zone 2 or Zone 22
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Flameproof enclosure for
wireless access point (Analynk) |
Methods of Isolation/Protection
Let's take a look at each method of protection and the general application. To ensure safety in a given situation, equipment is placed in protective level categories. As mentioned before Category 1 is the highest safety level, making Category 3 the lowest risk and safety level. The general 8 protective methods are:
- Flameproof enclosures
- Sand encapsulation
- Pressurization
- Oil encapsulation
- General encapsulation
- Increased safety
- Intrinsic safety
- Non-sparking
A
flameproof enclosure method is a type of protection where devices that are capable of igniting an explosive atmosphere are built inside an enclosure. This protective method prevents the transmission of the explosion to the external atmosphere surrounding the enclosure. This method of protection would be suitable for applications on:
- Power operated equipment
- Switchgear
- Motors
- Any equipment that produces ignition source during normal operation.
Sand encapsulation is a type of protection in which the enclosure of the electrical apparatus is filled with a finely granulated material. This ensures electrical arcs occurring within the electrical apparatus will not ignite the surrounding atmosphere.
Pressurization is a method of protection by which the entry of a surrounding combustible material is prevented by maintaining a protective gas within the enclosure. This is generally accomplished by creating a higher pressure within the enclosure than the surrounding atmosphere. This protective method would be used for any power operated equipment.
Oil encapsulation is a type of protection in which the electrical apparatus or part of the electrical apparatus are immersed within an oil-based fluid. The general application for this type of protection would be used for:
- Switchgear units
- Circuit breakers
- Transformers
Encapsulation is a type of protection in which the device that could ignite anexplosive atmosphere are enclosed within a resin. The material used would be resistant to environmental influences, heat and/or sparking from electrical components. The general application for this protective method would be used for:
- Electrical circuit boards
- Miniature motors
- Valves
Increased safety is a type of protection where measures are taken to prevent the possibility of high temperatures and the occurrence of ignition. This method includes the interior and external portions of the electrical apparatus. The general application for this protective method would include:
Connection and distribution boxes
Luminaires
Measuring instruments and devices that do not normally produce ignition within operation
Intrinsic safety is a protective method to ensure that the available electrical and thermal energy in the system is always low enough that the ignition of the hazardous atmosphere cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area as well as all electrical supply and signal wires are protected by safety barriers. The general application for intrinsic safety would be used for:
- Measuring and control engineering
- Data engineering
- Low electrical valves
Non-sparking equipment and wiring process is a protective method where apparatuses are designed with low power levels and low stored energy. This ensures that arc produced during normal functionality of the equipment, or as the result of equipment failure, has insufficient energy to ignite the hazardous atmosphere. The general application for non-sparking protective methods would be used for:
- Motors
- Lighting
- Junction boxes
- Electrical equipment
All equipment certified for use in hazardous areas must be labeled to show the type and level of protection applied.
In summary hazardous locations could exist in multiple industries. The geographical location will dictate the general method used for classification. The European Standard or IEC provides guidance of risk into three main categories. These risk levels are then divided into Zones and have numerical values that relate to the possibility of explosive gases or dusts present within the atmosphere. Because combustible material could have a variety of explosive limits, the method of protection will be important. Nevertheless all electrical devices placed within hazardous environments will follow the device markings to ensure fire or explosion does not occur.
