Methods of Protecting Instruments from Explosive Atmospheres

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:

• Encapsulation
• Explosion-proof and Flameproof
• Increased safety
• Intrinsic safety
• Non-incendive and Non-sparking

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

Encapsulation

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.

Advantages of Encapsulation

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:

• It provides good mechanical protection.
• It is effective in preventing contact with an explosive atmosphere.
• Employed to protect circuits that do not contain moving parts.
• It has low relatively low initial cost and low maintenance costs.
• Little or no customer installation requirements.

Disadvantages

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:

• Maintaining or repairing the equipment is difficult.
• Encapsulation is not recognizable by all standards.
• Routine tests of the dielectric resin are required to ensure integrity.
• Resin must not breakdown in the presence of hazardous substances.
• It is only applicable for small equipment.

Explosion-proof and Flameproof

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:

• Contain an internal explosion
• Prevent flame propagation to the external atmosphere

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 Enclosure Terminology

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:

• Enclosure Pressure – the gas-air or vapor-air mixture in the enclosure is adjusted to the value that will give the maximum pressure. The strength of this enclosure is measured and labelled.
• Maximum Experimental Safe Gap (MESG) – this is the opening between the machined joints of the enclosure through which an explosion is allowed to propagate. The safe gap is the maximum distance allowed. If the gap is larger, the released explosive gas will not cool below the auto-ignition temperature of the explosive atmosphere in which the enclosure will be used.
• Time to peak (TTP) – during an explosion pressure test, the TPP is the time required for the pressure to reach its maximum.
• Flame path (Flame joint) – the pathway flames or hot gases take when exiting an enclosure, such as the surfaces where two parts of an enclosure  come together or the conjunction of enclosures. The purpose of the flame path or flameproof joint is to prevent the transmission of internal explosion to the explosive atmosphere surrounding the enclosure.

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:

1. At the entrance to an approved enclosure housing arcing or sparking devices within 18 inches of the enclosure.
2. In conduit systems, when exiting the classified hazardous location.
3. At each entrance to an approved enclosure housing terminals, splices or taps with conduit openings of 2 inches (51 mm) or greater.
4. In cable systems where the cables can transmit gases or vapors through the cable core and when exiting the classified location.

Explosion-proof enclosures require the use of rigid explosion proof conduit and seal fittings or cabling.

• Conduit – this is commonly used in North America and consists of steel tubing protecting separate lead wires, if explosive gas enters the tubing, it could travel to another device. Hence, it is recommended to install a conduit seal at particular locations. Such as entry into an explosion-proof enclosure. A conduit seal has one or two sealing openings into which a special sealing compound is poured. When the compound hardens it forms a solid barrier that prevents passage of gases or flames from one portion of the electrical installation to another. The seal also prevents pre-compression or pressure piling in the conduit.
• Cable – cabling commonly used in Europe. It consists of an armoured or unarmoured sheath protecting separate lead wires. Cables provide more flexibility and lower costs compared to conduits. Cable glands are used as seals for terminating cable in explosion-proof enclosure with direct or indirect entries.

Advantages of Explosion-proof & Flameproof Protection Methods

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:

• It provides good mechanical protection.
• Well-suited for electrical apparatus where a high level of power are required (power levels above those used with intrinsic safety installations).
• No specific requirement for the materials inside the enclosure.
• No barriers are required, full power can be supplied to the equipment.

Disadvantages

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:

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

Increased Safety

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.

Advantages of Increased Safety

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:

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

Disadvantages

This method of protection has the following disadvantages:

• Cannot be used independently in Zone 0 installations.
• Generally used in conjunction with another approved methods of protection.
• Many routine tests needed to ensure integrity.
• Not suitable for equipment or devices producing high temperature or arcs.

Intrinsic Safety

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:

• Two independent faults (ia) for use in Zone 0
• One fault (ib) for use in Zone 1 and 2

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:

• Resistor to limit the maximum current entering the hazardous location.
• Shunt clamping device e.g. Zener diodes to limit voltage between ground and signal line.
• Fuse to limit the maximum current that can flow through the diodes.

The Intrinsic Safety Terminology

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.

Advantages of Intrinsic Safety

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:

• The installation is easier than the flameproof method.
• Inherently the safest method, it provides the highest level of protection.
• Equipment can be opened during operation.
• No armoured cable or EEx d cable glands required.

Disadvantages

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

Non-incendive and Non-sparking

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.

Advantages of Non-incendive protection method

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:

• No special cabling or barriers are needed.
• It is easier, less expensive installation.
• Apparatus can be opened during operation.

You can also read: Functional Safety in Instrumentation Systems (SIS, SIF & SIL)

Disadvantages

Non-incendive installations are used only when low level of protection is required.

The main disadvantages of this method of protection are:

• It is not suitable for environments containing dust.
• Only applicable for use in Div 1 or Zone 2 installations.

Related articles:

• Hazardous Locations Classifications
• What is the Safety Instrumented Function (SIF)?
• Demand Mode vs. Continuous Mode Safety Function
• Safety Function in a Machine Control System
• What is Safety Related Control System (SRCS)?
• How HAZOP Technique is used to identify Hazards in a Process
• How Ground Fault Interrupter (GFI) is used as a Safety Device