What ATEX protection methods are available?

Overview of protection concepts

All ATEX protection concepts are defined in the IEC 60079 series of standards. Each concept is assigned a letter designation, and the protection level is indicated by a suffix letter (a, b, c) corresponding to the Equipment Protection Level (EPL).

Most electrical enclosures in hazardous area installations use one of four concepts: Ex d, Ex e, Ex i, or Ex t. The others apply to specific equipment types or are rarely used in general electrical enclosure work.

Ex d, flameproof enclosure

The flameproof concept accepts that ignition may occur inside the enclosure, but prevents it propagating outside. The enclosure is built to:

• Withstand the pressure of an internal explosion without rupture
• Cool hot combustion gases through narrow flame paths (machined joints) before they reach the outside atmosphere
• Maintain mechanical integrity through repeated explosion cycles

Flame paths are machined to tolerances defined by the gas group: IIC (hydrogen, acetylene) requires the narrowest gaps and longest paths; IIA (propane) is less demanding. The gas group marked on the certificate defines the worst-case gas for which the enclosure has been tested.

Cable entry glands into Ex d enclosures must be certified to Ex d, they form part of the flame path. Stopping boxes (adaptors for conduit entries) are also available.

Typical applications: Motor starters, contactors, switchgear, luminaires, junction boxes where sparking components must be housed.

Ex e, increased safety

The increased safety concept prevents ignition by ensuring that sparks and excessive temperatures simply cannot form inside the enclosure. No sparking component may be present.

Requirements include increased creepage and clearance distances, minimum IP54 ingress protection, certified terminals with defined tightening torques, and temperature rise limits. The level of protection depends on the EPL suffix:

• Ex eb (EPL Gb), one fault tolerance; Zone 1 and Zone 2
• Ex ec (EPL Gc), normal operation only; Zone 2 only

Typical applications: Terminal boxes, junction boxes, marshalling panels, control stations, luminaires (non-sparking lamp types), distribution boards.

Ex i, intrinsic safety

The intrinsic safety concept limits the electrical and thermal energy in the circuit to below the minimum ignition energy of the surrounding atmosphere. Unlike other concepts, Ex i applies to a system (barrier + cable + field device), not just a single enclosure.

• Ex ia (EPL Ga), two faults; Zone 0, 1, 2
• Ex ib (EPL Gb), one fault; Zone 1, 2
• Ex ic (EPL Gc), normal operation; Zone 2 only

Intrinsic safety cannot power high-energy equipment (motors, heaters). It is the dominant concept for instrumentation, sensors, and 4–20 mA loops.

Typical applications: Pressure transmitters, temperature sensors, flow meters, solenoid valves (small), gas detectors, HART devices.

Ex t, enclosure protection for dust

The enclosure concept for dust atmospheres prevents dangerous dust deposits from forming on hot surfaces inside, and prevents dust ignition sources from escaping. It is defined in IEC 60079-31 (equivalent to the older Ex tD method).

Requirements are based on ingress protection: Zone 20 requires IP6X (dust-tight); Zone 21 requires IP6X; Zone 22 requires IP5X (dust-protected).

The maximum surface temperature is marked on the certificate in degrees Celsius (T80°C, T135°C, etc.) rather than the T-class notation used for gas. This is because dust ignition temperatures vary more widely and the relevant temperature depends on both the cloud ignition temperature and the layer ignition temperature.

Typical applications: Grain mill junction boxes, flour silo controls, coal handling electrical equipment, pharmaceutical dust processing.

Ex p, pressurisation

The pressurisation concept purges the enclosure with clean air or an inert gas, then maintains a slight positive pressure to prevent flammable atmosphere from entering. It is the only practical method for enclosing large panels or analysers that cannot be reduced in size to suit Ex d.

Two types exist:

• Ex px, purge then pressurise; the enclosure is safe after purging and can contain non-Ex rated internal equipment
• Ex py, requires some internal equipment to be rated for the zone (for use when purge supply fails)

A purge and pressurisation controller is required, which monitors the supply pressure and initiates a power-off or alarm if pressure is lost. The supply of clean instrument air or nitrogen must be reliable.

Typical applications: Large analyser shelters, MCC rooms classified Zone 1, control rooms on FPSOs with limited safe area, large VSD (variable speed drive) enclosures.

Ex n

The non-sparking concept (now designated Ex ec in IEC 60079-7) applies only to Zone 2 equipment. It requires that under normal conditions no sparks or excessive temperatures occur, but provides no specific fault tolerance.

Ex ec is less stringent than Ex eb, it has lower creepage distances and no requirement for tightening torque documentation. For Zone 2 only applications it can offer a cost saving, but most manufacturers supply Ex eb equipment (which also covers Zone 2) rather than offering a separate Ex ec range.

Typical applications: Zone 2 lighting, Zone 2 socket outlets, Zone 2 control stations where Ex eb is over-specified.

Ex m, encapsulation

The encapsulation concept encloses the live parts in a compound (epoxy, silicone, or thermoplastic) that prevents the surrounding atmosphere from contacting any spark or ignition source. It is used for small, hermetically sealed components.

Ex m is common in intrinsically safe barriers, Zener diodes, and small electronic sub-assemblies. It is rarely used as the primary protection method for whole enclosures. The compound can crack over time with thermal cycling, so inspection is important in cycling environments.

Typical applications: IS barriers, solenoid valve coils, small switches, proximity sensors.

Choosing the right concept

The choice of protection concept depends on four factors: zone, equipment type, practical installation constraints, and cost.

Combined protection, using two concepts in the same assembly, separated by a certified barrier, is permitted and often practical. The most common combination is Ex db/eb (flameproof compartment for switching, increased safety compartment for terminals).