The safety of equipment used in explosive atmospheres is not achieved by a single protection method. Different Ex protection concepts are applied depending on the design of the equipment, its intended location of use and the potential ignition sources it may generate.
The common objective of all Ex protection methods is to prevent equipment from igniting the surrounding explosive atmosphere. However, the technical principle used to achieve this objective differs from one protection concept to another.
| Protection Type | Basic Protection Principle | Relevant Standard |
|---|---|---|
| Ex d | Contains an internal explosion within the enclosure. | EN IEC 60079-1 |
| Ex e | Prevents arcs, sparks and excessive temperatures. | EN IEC 60079-7 |
| Ex i | Limits electrical and thermal energy below ignition levels. | EN IEC 60079-11 |
| Ex p | Prevents the entry of an explosive atmosphere by maintaining protective pressure. | EN IEC 60079-2 |
| Ex m | Encapsulates components capable of causing ignition. | EN IEC 60079-18 |
| Ex t | Provides protection against combustible dust by enclosure and surface-temperature limitation. | EN IEC 60079-31 |
With Ex d protection, it is accepted that electrical components inside the equipment may produce sparks and that an explosion may even occur inside the enclosure.
The enclosure is therefore designed to withstand the pressure generated by an internal explosion. Special flamepaths at enclosure joints cool escaping hot gases and prevent the internal explosion from igniting the surrounding explosive atmosphere.
Standard: EN IEC 60079-1
Typical applications: Motors, contactors, switching devices, luminaires and other electrical equipment capable of producing arcs or sparks.
The basic principle of Ex e protection is not to contain an explosion after it occurs, but to prevent an ignition source from developing in the first place.
For equipment that does not normally produce arcs or sparks, additional measures are applied to electrical clearances, creepage distances, terminals, insulation and temperature limits. These measures reduce the possibility of sparks, overheating and other potential ignition sources.
Standard: EN IEC 60079-7
Typical applications: Terminal boxes, junction boxes, squirrel-cage motors, lighting equipment and electrical connection systems.
The purpose of Ex i protection is to keep the electrical and thermal energy present or stored in a circuit below the level capable of igniting an explosive atmosphere.
Any spark or temperature rise that may occur in the circuit must remain below the level required for ignition, including under the specified fault conditions for the relevant level of protection.
Intrinsic safety is often a system concept. The associated apparatus or barrier, interconnecting cable and field device may need to be evaluated together as a complete intrinsically safe circuit.
Standard: EN IEC 60079-11
Typical applications: Pressure and temperature transmitters, sensors, gas detectors, measurement devices and instrumentation circuits.
Ex p protection separates electrical equipment from the surrounding explosive atmosphere by maintaining a protective gas inside the enclosure.
Clean air or an inert gas is supplied to the enclosure so that the required protective pressure is maintained relative to the external atmosphere. This prevents the explosive atmosphere from entering the protected enclosure.
Depending on the protection arrangement, pressure monitoring, purging, alarms or automatic power disconnection may be required if the protective conditions are lost.
Standard: EN IEC 60079-2
Typical applications: Large electrical panels, control cabinets, analyser systems and equipment that is difficult to protect using another concept.
With Ex m protection, components capable of producing sparks or dangerous temperatures are enclosed within a suitable encapsulating compound.
The encapsulating material prevents the parts capable of causing ignition from coming into effective contact with the surrounding explosive atmosphere.
Standard: EN IEC 60079-18
Typical applications: Solenoid valve coils, relays, sensors, electronic modules and low-power electronic circuits.
Ex t is a protection concept specifically intended for electrical equipment used in combustible dust atmospheres.
The enclosure restricts the ingress of dust to a safe level, while the maximum surface temperature of the equipment is controlled so that surrounding dust clouds or deposited dust layers are not ignited.
Standard: EN IEC 60079-31
Typical applications: Motors, junction boxes, luminaires and control equipment used in grain, flour, sugar, coal, pharmaceutical and other combustible-dust processing facilities.
In addition to the main protection concepts above, other protection techniques may be used for specific types of equipment and applications.
Explosion protection is not limited to electrical equipment. Mechanical equipment such as fans, pumps, gearboxes, bearings and couplings may also generate ignition sources including frictional heating, hot surfaces, mechanically generated sparks and electrostatic discharges.
No Ex protection method is inherently better than all others. The correct concept depends on the equipment function, hazardous-area classification, Equipment Protection Level (EPL), gas or dust group, temperature requirements, environmental conditions and intended use.
For example, Ex d may be appropriate for a contactor that normally produces arcs or sparks, while Ex e may be more suitable for a junction box containing terminals that do not normally spark. For measurement and instrumentation circuits, Ex i is frequently used because the available electrical energy can be strictly controlled.
The IEC 60079 series defines technical requirements for equipment used in explosive atmospheres. IEC 60079-0 contains the general requirements, while the individual parts of the series define the requirements for the different Ex protection concepts.
However, the latest published IEC edition and the harmonised EN standard referenced under ATEX Directive 2014/34/EU do not necessarily become applicable on the same date.
For ATEX conformity assessment and preparation of the EU Declaration of Conformity, the applicable harmonised standard reference published in the Official Journal of the European Union (OJEU) should therefore be verified.
The applicable edition should also be checked against the product certificate, certificate conditions, technical documentation, manufacturer instructions and any relevant transition provisions before a new design, product modification or conformity assessment is finalised.
Important: This page provides an introductory overview of Ex protection concepts. The final selection of equipment and protection concept must take into account the hazardous-area classification, Equipment Protection Level (EPL), gas or dust group, temperature class or maximum surface temperature, environmental conditions and the manufacturer's instructions.
Where required by ATEX Directive 2014/34/EU, Notified Body involvement is determined by the equipment category and the applicable conformity assessment procedure. Certification and conformity assessment should be based on the current applicable standards and the conditions stated in the relevant certificate.
Ex d contains an explosion within the enclosure; Ex e prevents potential ignition sources from developing; Ex i limits electrical and thermal energy; Ex p keeps the explosive atmosphere away from the equipment; Ex m encapsulates components capable of causing ignition; and Ex t provides enclosure protection and surface-temperature control for combustible dust atmospheres.