Explosion Protection Concepts
The practice of dividing potentially explosive atmospheres into zones was established to allow the degree of risk of an explosion occuring to be understood. This classification takes the different dangers from explosive atmospheres into account and allows explosion protection concepts for instrumentation, and other equipment, located in the zones to be established.
These Explosion Protection Concepts were conceived to prevent equipment and components from becoming ignition sources. There are three conditions which must co-exist in order to create an explosion, fuel, air and an ignition source. This is often referred to as the Ignition Triangle. The protection concepts are designed to break this triangle by excluding one or more of its components, e.g. excluding an ignition source, or segregating the fuel source from an oxygen supply.
Types of Protection Concepts
Intrinsically Safe - Ex ia, ib, ic
Intrinsically safe circuits are circuits in which a spark or thermal effect occurring is not able to ignite the explosive atmosphere. Voltage and current, including a safety margin, are kept permanently so low that no unacceptable temperatures can occur, and, in the event of open circuit or short-circuit, sparks and electric arcs possess so little energy that they are unable to ignite an explosive atmosphere.
Intrinsically safe circuits are very commonly used in control and instrumentation applications. You can expect to find them in measuring and monitoring instrumentation applications, and remote I/O technology based on several fieldbus applications.
Increased Safety - Ex e
Often used for instrument junction boxes and enclosures where the construction ensures reliable prevention of unacceptably high temperatures and sparks or electrical arcs.
Pressurized Enclosures - Ex p
The ingress of the surrounding atmosphere into the enclosure of electrical equipment is prevented by maintaining a protective gas (clean dry air, inert or a different suitable gas) inside it at a pressure above atmospheric pressure. Strength of the enclosure; the purged enclosure must withstand 1.5 times the overpressure experienced during normal operation.
Common areas where you may find this protection concept include switch cabinets, control cabinets and analyser cabinets housing analytical equipment.
Encapsulation - Ex ma, mb, mc
Parts that could ignite an explosive atmosphere by means of sparks or heat are potted so as to prevent ignition of the explosive atmosphere. This is achieved by encapsulating (molding) the components in a casting compound, resistant to physical - especially electrical, thermal and mechanical - and chemical influences.
Encapsulation techniques are often used in solenoid valves, relays and other control gear of limited power.
Liquid Immersion - Ex ob, oc
Parts which might ignite an explosive atmosphere are immersed in oil or other non-flammable, insulating liquid so that gases and vapours above the oil level and outside the enclosure cannot be ignited by electric arcs or sparks generated below the liquid level, or by hot residual gases from the switching process or by hot surfaces.
This concept is not often used for instrumentation, however it is not unusual to find electronic circuits completely imeersed in liquid inside gearboxes.
Powder Filling - Ex q
By filling the enclosure with a finely grained powder from for example quartz glass, an arc within the enclosure is unable to ignite the explosive atmosphere outside. There must be no risk of ignition by flames, nor by increased temperatures at the surface of the enclosure.
Flameproof Enclosures - Ex d
A type of protection in which the parts which could ignite an explosive atmosphere are located inside an enclosure which can withstand the pressure of an explosion of the explosive mixture inside, and prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure.
Technically unavoidable gaps are so long and narrow that hot gases jetting out will have lost their power to cause ignition by the time they reach the outside of the enclosure, or, alternatively, if the gaps are only required for the manufacturing process they might be sealed with adhesive.
Flameproof certified instrumentation is very common.
Standards for Explosion Protection
The following international standards are used for explosion protection:
- EN 1127-1:1997; Explosive atmosphere - Explosion prevention and protection. Part 1: Basic concepts and methodology
- EN 60079-10:2003; Electrical apparatus for explosive gas atmospheres. Part 10: Classification of potentially explosive atmospheres
- EN 50281-3:2002; Electrical apparatus for use in the presence of combustible dust. Part 3: Classification of areas where dusts are or may be present
- NFPA 70; National Electrical Code Chapter 5 Article 500.
It is worth noting that many - though by no means all - countries outside of Europe and North America use the IEC Standards as a basis for their own national standards.
The following pages on Control and Instrumentation.com give more detail on Hazardous Areas, ATEX and other relevant information:
Share this page with your colleagues
For those who want to delve further into Hazardous Areas and Explosion Prevention then the following books from Amazon will be of interest: