Explosion

An explosion is defined as a sudden reaction involving rapid physical or chemical decay accompanied by an increase in temperature or pressure or both. The most common types of reaction are those between flammable gases, vapors or dust with oxygen contained in the surrounding air. Production lines or workplaces may be explosion hazard areas it these conditions are fulfilled. Hazardous areas typically exist in chemical plants, refineries, paint shops, cleaning facilities, mills and flour silos, tanks and loading facilities for flammable gases, liquids and solids.

By inhibiting ignition of a potentially explosive atmosphere, explosion-proof electrical equipment can eliminate danger at the source. Structural measures are taken in buildings used for domestic purposes to ensure that potentially explosive atmospheres cannot be formed. Deliberate removal of such safety measures may result in an explosion hazard.

Source of ignition

Source of ignition in industrial electrical equipment are as follows:

Hot surfaces - Surfaces heated by coils, resistors or lamps, brakes or hot bearings.

Electrical sparks - When circuits are broken or static discharge takes place.

Friction and impact sparks - When casings or enclosures are struck.

The design of explosion - proof electrical equipment eliminates these sources of ignition and this is confirmed by testing and certification.

Notwithstanding this, other sources of ignition may occur such as the following:

• flames and hot gas
• chemical reactions or biological processes which occur spontaneously at certain oxygen levels or temperatures
• lightning
• intense electromagnetic radiation
• ionizing radiation
• adiabatic compression and shock waves
  

Protection Types

For all protection types the rule applies that parts to which the potentially explosive atmosphere has unhindered access must not attain unacceptable temperatures. The temperatures must fall within the temperature class that applies to the particular potentially explosive atmosphere.

ICE 79-1 - Flameproof enclosure:
Marking "EEx d" in accordance with EN 50 014

Principle

Type of protection in which components that could ignite a potentially explosive atmosphere are fitted in an enclosure that will contain the pressure of an explosion, preventing ignition of inflammable gas outside the enclosure.

Technically unavoidable gaps in the enclosure are so small and their length's are restricted so that any hot gas released through them will have lost its power to effect ignition. If such gaps are only required as part of the production process
they may be sealed by the manufacturer with adhesive or gasket.

Applications: Equipment whose operation normally involves sparks or arcing and/or hot surfaces such as Switch gear,slip rings, collectors, potentiometers, fuses, lamps or heating cartridges.

ICE 79-7 - Increased safety:
Marking "EEx e" in accordance with EN 50 014

Principle

Additional measures are taken to achieve a higher degree of safety. This ensures reliable prevention of unacceptably high temperatures and sparks or arcing both on the inside and outside parts of electrical equipment whose normal operation does not involve unacceptably high temperatures, sparks or arcing.

Applications: Installation material such as Marshalling and junction boxes, terminal compartments for heating systems, batteries, transformers, ballast and squirrel cage motors.

ICE 79-18 - Encapsulation:
Marking "EEx m" in accordance with EN 50 014

Principle

Parts that could ignite a potentially explosive atmosphere by means of sparks or heat are potted so as to prevent ignition. This is done by encapsulating the components in a compound which is protection against physical - especially
electrical, thermal and mechanical - and chemical influences.

Applications: Static coils in ballast, solenoid valves or motors, relays and other control gear of limited power and complete PCB´s with electronic circuits.

ICE 79-2 - Pressurized enclosure:
Marking "EEx p" in accordance with EN 50 014

Principle

The enclosure is filled with a pressurized gas (air, inert gas or other suitable gas) in order to prevent the ingress of a surrounding atmosphere. The pressure within the enclosure is maintained above atmospheric pressure with or without constant flushing of protective gas.

Applications: Electrical equipment whose normal operation involves sparks, arcing or hot components and complex industrial standard equipment (controllers) whose protection type enables them to be operated in hazardous areas. Large machines, slip ring or collector motors, Switchgear and control gear cabinets and analysis devices.

ICE 79-11 - Intrinsically safe:
Marking "EEx i" in accordance with EN 50 014

Principle

Intrinsically safe electrical equipment contains only circuits that meet the requirements of intrinsically safe circuits. Intrinsically safe circuits are circuits in which no spark of thermal effect occurring under the test conditions laid down in the standard can ignite the potentially explosive atmosphere of subgroup IIA, IIB or IIC. The test conditions cover normal operation and certain fault conditions stipulated in the standard.

Applications: Instrumentation and control, Sensors working on physical, chemical or mechanical principles. Actuators working on optical, acoustic and, to a certain extent, mechanical principles.

Ignition temperature - temperature class

Ignition temperature is influenced by various factors such as the size, shape, type and composition of a surface.

In IEC 79-4, IEC, CENELEC and other standards the authorities have agreed on a procedure for the determination of
ignition temperature" which comes close to giving the lowest practical value. This involves dividing the gases and vapors
into temperature classes. In accordance with these temperature classes, electrical and other technological equipment is
rated for surface temperature in such a way that the possibility of automatic ignition is excluded. The standards state in
each case the extent to which the standard values may be exceeded and the requisite safety margins.



Examples of the categorization of gases and vapors in temperature classes and explosion protection subgroups:



Marking
The rules for marking the electrical equipment are uniformly laid down in the standards relating to general technical
requirements.

Principle
The manufacturer who has put the item of electrical equipment on the market and who must be able to identify it.

The type or types of protection the item of electrical equipment conforms to:

• The temperature class for which it is suitable
• The explosion hazard group or subgroup applicable to the item of electrical equipment
• The test center issuing the test certificate
• Any special conditions that have to be observed
• The standard or revision of the standard applicable to the item of electrical equipment