Explosion Protection Concepts
The protection concepts behind an Ex marking: how each one prevents ignition, where it may be used, and what it is typically applied to.
Flameproof enclosure
The enclosure may let gas in and contain an internal explosion; flame paths cool escaping gas so it cannot ignite the outside atmosphere.
Levels and zones. db (Gb) — Zones 1 and 2. Closest US counterpart: explosionproof (Division 1).
Motors, switches, junction boxes, light fittings where arcing or hot parts cannot be avoided.
Increased safety
No arcs or sparks in normal service; extra measures on creepage, clearance, terminals and temperature rise prevent ignition.
Levels and zones. eb (Gb) Zones 1 and 2; ec (Gc) Zone 2 only — ec replaces the old nA.
Terminal boxes, squirrel-cage motors, luminaires.
Intrinsic safety
Energy in the circuit, including stored energy, is limited so that neither a spark nor a hot surface can ignite the atmosphere, even with faults.
Levels and zones. ia (Ga) Zone 0; ib (Gb) Zone 1; ic (Gc) Zone 2. US counterpart: intrinsically safe (Division 1).
Instruments, transmitters, sensors; needs barriers or isolators and an entity or system check.
Pressurized enclosure
Clean air or inert gas keeps the enclosure at a higher pressure than outside, so the atmosphere cannot enter; power is interlocked with purge and pressure.
Levels and zones. pxb, pyb (Gb); pzc (Gc). US counterpart: purged and pressurized, Types X, Y, Z (NFPA 496).
Analyzer houses, large motors, control panels.
Encapsulation
Parts that could ignite are embedded in compound so the atmosphere cannot reach them.
Levels and zones. ma (Ga), mb (Gb), mc (Gc).
Solenoid coils, small electronics, sensors.
Liquid immersion
Ignition sources are submerged in a protective liquid.
Levels and zones. ob (Gb), oc (Gc).
Rare today; some transformers and switchgear.
Powder filling
The enclosure is filled with quartz or glass powder so an internal arc cannot ignite the outside atmosphere.
Levels and zones. qb (Gb).
Capacitors, electronic ballasts, small components.
Type of protection "n"
Zone 2 protection family: nC (enclosed break or sealed devices), nR (restricted breathing). Former nA moved to ec and nL to ic.
Levels and zones. Gc — Zone 2 only. US counterpart: nonincendive (Division 2).
Zone 2 luminaires, motors, relays.
Dust protection by enclosure
The enclosure keeps dust out and limits surface temperature.
Levels and zones. ta (Da) Zone 20; tb (Db) Zone 21; tc (Dc) Zone 22. US counterpart: dust-ignitionproof or dusttight.
Equipment in dust zones.
Optical radiation
Limits the power of light sources such as lasers and fiber optics so the beam cannot ignite the atmosphere.
Levels and zones. op is (inherently safe), op pr (protected), op sh (interlocked) — EPL depends on the variant.
Optical sensors, fiber-optic links.
Selecting a concept
- Pick the protection concept from the EPL the zone requires, then check the gas or dust group and the temperature class against the materials present.
- Several concepts are often combined in one product, for example a motor with an Ex db terminal box and Ex eb stator (marked Ex db eb).
- Installation rules (cable glands, sealing fittings, conduit seals) belong to the concept and the code: NEC Articles 501 to 506 and IEC 60079-14.
Sources: IEC 60079-0 and the concept parts (-1, -2, -5, -6, -7, -11, -15, -18, -28, -31), NEC 2023 Articles 500–506 — described in our own words. © RokBench.