The protection method used in a specific hazardous location depends on the application, the technology available and the specific explosive atmosphere present.
In this article, we discuss the following methods of protection:
Table 1 provides the summary information for each protection method. The codes are used in the markings or labels for apparatus and enclosures. A letter is used to designate each of the protection methods e.g. ‘’m’’ for encapsulation, ‘’d’’ for flame, preceded by ‘’Ex’’ for explosion protection based on the IEC standard, ‘’AEx’’ for explosion protection based on the NEC standard, and ‘’EEx’’ for explosion protection based on the EN or EU-ATEX standard. The permitted use for each method and the protection principle upon which it is based are also shown in the table. For more details on classifications, you can read: Hazardous locations classifications.
Table 1 Protection Method, Code, Use and Principle
Contents
| Code | Permitted Use | Protection Principle |
| AEx m Ex ma Ex mb | Class I, Zone 1, 2 Zone 0 Zone 1 | Keep explosive atmospheres out |
The encapsulation protection method (Ex m) is based on the segregation concept. This method is predominantly used in Europe and is recognized by all certifications agencies. Electrical parts that can be a source ignition are encapsulated in an epoxy resin. Encapsulation prevents any electrical sparks from igniting the hazardous atmosphere.
The encapsulation method is normally used to protect small electrical components already mounted in an enclosure. Encapsulation is mainly used as a complement to other protection methods.
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Encapsulation in general, is used to protect small electrical components like magnetic valves, batteries, accumulators, optocoupler, sensors and assemblies already mounted on a board in an enclosure such as relays, transistors, and coils. Encapsulation is frequently used to complement other protection methods.
The main advantages of this method of protection are:
The encapsulation method of protection relies on embedding the circuits in a permanent resin to separate any source of ignition from an explosive atmosphere. Hence, any fault or breakdown in a circuit component requires removing the entire encapsulated unit. Usually the embedded circuit is replaced rather than replaced.
The major disadvantages of this method of protection are:
| Code | Permitted Use | Protection Principle | Standards |
| XP | Class I, Div 1, 2 | Contain explosion | FM, CSA, UL |
| AEx d Ex d | Class I, Zone 1, 2 Zone 1, 2 | Contain explosion Quench flame | IEC, CENELEC |
Explosion-proof and Flameproof are often used as interchangeable concepts and are associated with the use of enclosures to contain an explosion. Enclosures have been the primary methods of protection of equipment and instruments used in petroleum refineries and petrochemical industries for many years.
Explosion-proof protection and flame-proof methods are essentially the same. Both methods are based on the explosion containment concept. The explosion-proof method (XP) is primarily used in North America. The flameproof method (Ex d) is used in Europe and countries using IEC standards.
The explosion-proof/flameproof method places equipment into a special enclosure designed to perform the following functions:
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An explosion is allowed to take place, but it is confined within an enclosure built to resist the pressure created.
The Physics behind this method is that the explosive gases are initially cooled by heat conduction by contact with the enclosure and the gases are allowed to escape through specially designed openings in the enclosure. The escaping gases are cooled by expansion before reaching any external explosive atmosphere.
Containing an internal explosion is a function of the structural strength and internal volume of the enclosure. The enclosure walls must be thick enough to contain the explosive forces. The internal pressure created during explosion depends upon the gas/vapor that was ignited. For example, an explosion of hydrogen gas (Group B) creates a higher internal pressure than methane gas (Group D).
Cooling is a result of heat conduction through the walls of the enclosure and expansion of the internal explosive gas through precise openings in the enclosure. The temperature of the gas that escapes must be below the auto-ignition point of the surrounding hazardous atmosphere. Gas will escape and expand through any available path. Controlling the gas expansion and cooling is a function of the design of the enclosure such as the openings in the mating surfaces between the enclosure cover and base.
The design of an explosion-proof/flame proof enclosure must meet the standards of the country in which it is installed. Enclosures are evaluated in approved testing facilities before certification. In North America, prototypes of the enclosure are tested with a significant safety margin and additional tests on production models are not needed. In Europe, prototypes of the enclosure can be tested with a lower safety margin with additional tests required on production models, or higher level tests on prototypes with optional tests on production models.
The important elements in testing and labelling of enclosure include:
The explosions within an explosion-proof enclosure will go through the paths of least-resistance which are typically the conduit or cable entries within the enclosure. In order to ensure an effective explosion-proof system, seal fittings must be installed at various locations, through the conduit or cable system.
NEC (article 500-5) requires seal fittings to be installed in four major areas within the electrical system:
Explosion-proof enclosures require the use of rigid explosion proof conduit and seal fittings or cabling.
The material used to build an explosive-proof enclosure is usually metal (cast iron or aluminium). Plastic and non-metallic materials can be used for small enclosures.
The main advantages of this method of protection are:
The enclosure must be properly machined and the surface of the packing face must be protected from corrosion. The flameproof surfaces of the enclosure cannot be painted and no plastic parts are allowed. Periodic inspections are required to ensure mechanical integrity of the enclosure. Special higher cost materials may be required for installations in corrosive atmospheres; therefore the initial costs may be higher than other methods of protection.
The primary disadvantages of this method of protection are:
| Code | Permitted Use | Protection Principle |
| AEx e Ex e | Class I, Zone 1, 2 Zone 1, 2 | No arcs, sparks or hot surfaces |
This method of protection is based on the prevention concept. It is mainly used in Europe and is recognized by NEC/CEC for use in North America in Zone classification.
Increased-safety apparatus are designed to minimize heat and eliminate arcs or sparks inside or outside the enclosure. Various features are incorporated to reduce the probability of excessive temperatures and the occurrence of arcs or sparks in the interior and the external parts of the equipment in the normal service.
A close review of the insulation and fastening of the field wire terminal blocks must be periodically performed to ensure the integrity of the field wiring and to prevent the wiring from breaking-off and causing an arc or spark.
Increased safety is often used in combination with flameproof protection method. Increased safety has significant cost savings compared with flameproof because no special cables or cable glands are needed.
The main advantages of this method of protection are:
This method of protection has the following disadvantages:
| Code | Permitted Use | Protection Principle | Standards |
| IS | Class I, Div 1,2 | Limit energy of sparks and surface temperature | FM, CSA, UL |
| AEx ia AEx ib Ex ia Ex ib | Class I, Zone o, 1, 2 Class I, Zone 1, 2 Zone 0, 1, 2 Zone 1, 2 | Limit energy of sparks and surface temperature | IEC, CENELEC |
This method of protection is based on prevention concept. This method is used both in North America and Europe. In North America, intrinsically safe systems allows for two independent faults. Europe has two categories of intrinsically safety:
Intrinsic safety works by ensuring the amount of energy available in a circuit is too low to ignite the most ignitable mixture of gas and air. Safety is inherent and unaffected by failure of mechanical enclosures, air pressure, interlocks, etc. Intrinsic safety protects both the equipment and wiring.
An intrinsic safe system typically consists of one or more devices located in hazardous area connected to a controller and /or power supply located in a non-hazardous area. An Intrinsically safe barrier is the interface between the hazardous and non-hazardous area.
Intrinsic safety method of protection prevents instruments and other low-voltage circuits from releasing sufficient energy to ignite an explosive atmosphere, to achieve this; an energy-limiting circuit is connected between the hazardous and non-hazardous locations. This energy-limiting circuit can be built into instrumentation or added as a separate unit known as safety barrier. The safe barrier lets measurement or control signals through, but in the event of an electrical fault, it limits the voltage and current entering the hazardous location.
Intrinsically safe barriers consist of 3 components:
Common terms used when describing intrinsically safe systems:
Intrinsically Safe Circuit or Device
A circuit or device in which any spark or thermal effect is incapable of causing ignition of flammable or combustible material under normal or prescribed fault conditions.
Simple Apparatus or Device
Any apparatus/equipment in an intrinsically safe system that is incapable of generating or storing more than 1.5 V, 100 mA, 20 mJ or 25 mW.
Associated Apparatus or Device
This is any apparatus or device which contains a circuit that is not intrinsically safe, but has an output that is intrinsically safe e.g. the safe barrier.
Intrinsic safety provides the highest level of safety. It is often used in place of flameproof method of protection to provide significant cost savings because no special flameproof cables or cable glands are needed.
The major advantages of this method of protection are:
| Code | Permitted Use | Protection Principle | Standards |
| NI | Class I, Div 2 | No arcs, sparks or hot surfaces | NEC |
| Ex nA Ex nC EEx nC | Zone 2 Zone 2 Zone 2 | No arcs, sparks or hot surfaces | IEC |
Non-incendive and non-sparking methods of protection are based on the prevention concept.
The non-incendive method (NI) is mainly used in North America for Division 2 applications. The European equivalent of non-incendive is the non-sparking method of protection.
Non-incendive apparatus and circuits are incapable of igniting an explosive atmosphere. Each apparatus is tested under normal operating conditions in the laboratory. The apparatus is not evaluated for safety under fault conditions.
Arcing or sparking contacts are not allowed or contained within an enclosure that passes a sealed device test. Sealing methods include welding, soldering, or brazing metal or glass. This method of protection s usually used in control stations in Division 2 locations.
Non-incendive installations provide cost savings when compared with explosion-proof or flameproof methods of protection because special enclosures and conduit are not required.
Normal field wiring is used and no explosion-proof conduit or sealing is needed.
The main Advantages of this method of protection are:
You can also read: Functional Safety in Instrumentation Systems (SIS, SIF & SIL)
Non-incendive installations are used only when low level of protection is required.
The main disadvantages of this method of protection are:
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