Protecting apparatus from potential ignition sources

In the last article, We discussed the general principles of working in explosive atmospheres and classifying hazardous locations, This article is discussing methods for protecting apparatus from potential ignition sources, the advantages and disadvantages of each method, and the uses permitted by international code for each method.

This article describes the following methods of protection:

? Encapsulation

? Explosion-Proof and Flameproof

? Increased Safety

? Intrinsic Safety

? Non-Incendive and Non-Sparking

? Pressurized and Purged

The following table provides 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 method (e.g., "m" for encapsulation, "d" for flameproof), 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 presented in the table.

1- Encapsulation

The encapsulation protection method (Ex m) lies under the segregation concept. Encapsulation is a protection method where the electrical components are sealed to prevent contact with the explosive atmosphere. Encapsulation essentially creates a protective “shell” around the components by fully enclosing them in a compound or another non-metallic enclosure with adhesion such as resin.??Encapsulation prevents ignition?of the explosive gases or vapors due to potential sparking, arcing or excessive heat.

Encapsulation is generally used to protect small electrical, components such as batteries, magnetic valves, accumulators, optocouplers, and sensors and assemblies already mounted on a board in an enclosure such as relays, transistors, and coils.

Advantages:

• Provides good mechanical protection
• Effective in preventing contact with an explosive atmosphere
• Used to protect circuits that do not contain moving parts
• Relatively low initial cost and low maintenance cost
• Little or no customer installation requirements

Disadvantages:

• Maintaining or repairing the apparatus is difficult
• Routine tests of the dielectric resin are necessary to ensure integrity
• Resin must not break down in the presence of hazardous substances
• Applicable only for small apparatus or devices
• The encapsulation method relies on embedding the circuits in a permanent resin to separate any source of ignition from an explosive atmosphere. Therefore, any fault or breakdown in a circuit component requires removing the entire encapsulated unit. Usually the embedded circuit is replaced rather than repaired.

2- Explosion-Proof and Flameproof

The explosion-proof protection method and the flameproof protection method 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 allows the explosion to happen but contaminate the explosion from existing outside the apparatus, 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.

https://www.youtube.com/watch?v=MTP1FGZFPKU

In the above link you can see Exd explosion test

The material used to build an explosive-proof enclosure is usually metal (cast iron or aluminum). Plastic or non-metallic materials can be used for small enclosures. The design and manufacture of an explosion-proof enclosure must meet the standards of the country in which the enclosure is to be installed.

Advantages:

• Provides good mechanical protection
• Well-suited for electrical apparatus where high levels of power are required (power levels above those use with intrinsic safety installations)
• No specific requirement for the materials inside the enclosure
• No barriers are needed; full power can be supplied to the apparatus

Disadvantages:

• Enclosures can be heavy and create special mechanical and structural
• Enclosures require special conduit or cabling which increases installation and costs
• Condensation inside the enclosure or conduit pipe may occur in humid atmosphere
• Equipment maintenance and calibration is complicated because the enclosure cannot be opened during operation without exposing the internals to the explosive atmosphere
• 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 needed to ensure the 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.

3- Increased Safety

The increased safety method of protection is based on the prevention concept.

Increased-safety apparatus are designed to minimize heat and eliminate arcs or sparks inside or outside the enclosure.

Field wiring is a critical link in an increased safety installation. 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.

Advantages:

• Can be used in Zone 0 installations when combined with flameproof methods of protection
• Non-metallic, anti-static enclosures can be used

Disadvantages:

• Cannot be used independently in Zone 0 installations
• Generally used in conjunction with another approved method of protection
• Many routine tests are required to ensure integrity
• Not suitable for apparatus or devices producing high temperatures or arcs

4- Intrinsic Safety

The intrinsic safety protection method is based on the prevention concept.

The use of the intrinsic safety protection method is increasing when measurement and control operations are carried out in hazardous locations.

Intrinsic safety is a technique for ensuring that 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 design protects both the apparatus and its wiring.

An intrinsically safe system typically consists of one or more devices located in the 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 areas.

Intrinsic safety provides the highest level of safety (fully tolerant). It is often used in place of a flameproof method of protection to provide significant cost savings because no special flameproof cables or cable glands are required.

Advantages:

• Inherently the safest method, provides the highest level of safety
• Installation easier than flameproof method
• Only method that can be used in Zone 0 (ATEX)
• Apparatus can be opened during operation
• No armored cable or EEx d cable glands are needed

Disadvantages:

• Increased installation costs because an intrinsic safety barrier is required
• Current, voltage, and power of the circuits are limited, and the total capacitance and inductance of apparatus and wiring must be under certain limits.

5- Non-Incendive and Non-Sparking

The non-incendive and non-sparking methods of protection are based on the prevention concept.

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 to glass. This protection technique is commonly 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 required.

Advantages:

• Easier, less expensive installation
• No special cabling or barriers are needed
• Apparatus can be opened during operation

Disadvantages:

• Non-incendive installations are used only when low level of protection is needed.
• Not suitable for environments containing dust
• Only applicable for use in Division 2 or Zone 2 installations

6- Pressurized and Purged

The pressurized protection method is based on the segregation concept.

The hazardous atmosphere in the apparatus enclosure is replaced with dry air or inert gas. The internal atmosphere is kept at a slightly higher pressure to prevent the external, hazardous atmosphere from entering the enclosure which contains O2 which allows ignition.

The primary advantage of this method of protection over other methods (e.g., explosionproof) is its use with large electrical components and suitable for large enclosures. For example, control panels or entire rooms can be protected using the pressurized method. The standards applied to pressurized method of protection are among the strictest.

Advantages:

• Independent of the hazardous location classification
• Several choices of inert gas are available for pressurization
• Suitable for all apparatus or devices than can be mounted in an enclosure

Disadvantages:

• To avoid pressure loss, the protective gas supply must be able to compensate, during operation, for enclosure leakage. Because it is possible for the dangerous mixture to remain inside the enclosure after the pressurization system has been turned off, it is necessary to expel the remaining gas by circulating a certain quantity of protective gas before restarting the electrical equipment.
• Additional safety devices are required
• Enclosures can be expensive and difficult to maintain
• Any leakage of inert gas must be replaced to maintain system integrity

7- Other protection methods

In addition to the above protection methods, there are other methods that is not commonly used need to be recognized. They can be summarized in the above Table.

Summary

Prevention Types and Approved Zones Locations (IEC / ATEX):

Prevention Types and Approved Zones Locations (North American):

References:

1. https://www.eaton.com/us/en-us/products/low-voltage-power-distribution-control-systems/encapsulation.html
2. https://www.youtube.com/watch?v=MTP1FGZFPKU
3. https://www.automation.com/en-us/articles/2013-1/methods-of-protection-in-hazardous-explosion-risk
4. https://r-stahl.com/en/uk/services-and-seminars/explosion-protection-knowledge/intrinsic-safety-type-of-protection/
5. A session conducted by Engineer Yasser Yasseen (Engineering Manager at ANOPC)