Reading a marking left to right
Every element of an Ex marking answers a different question, and the equipment is only suitable if all of them are satisfied at once. A typical instrument nameplate reads something like Ex db IIC T4 Gb:
- Ex — certified explosion-protected equipment.
- db — how it is protected: a flameproof enclosure that contains an internal explosion.
- IIC — which gases it can handle, IIC being the most demanding.
- T4 — how hot its surfaces may get: 135 °C maximum.
- Gb — how reliable that protection is, which determines the zones allowed.
The three questions that decide suitability
- Is the protection level good enough for the zone? Ga is required for Zone 0, Gb for Zone 1, Gc for Zone 2 — and higher levels always cover lower zones. Ga equipment is fine in Zone 2; Gc equipment is not fine in Zone 1.
- Does the gas group cover the gases present? IIC covers everything below it, IIA covers only IIA. Hydrogen service demands IIC, and this is the element most often got wrong when equipment is substituted during maintenance.
- Is the temperature class low enough? The surface temperature limit must fall below the auto-ignition temperature of the gas. Counter-intuitively, a higher T number is less restrictive — T6 equipment is acceptable wherever T1 is required, but not the reverse.
Protection concepts at a glance
| Code | Concept | Principle | Typical EPL |
|---|---|---|---|
| ia | Intrinsic safety | Limits energy below ignition capability | Ga |
| ib | Intrinsic safety | As ia, single-fault tolerant | Gb |
| db | Flameproof | Contains an internal explosion | Gb |
| eb | Increased safety | Prevents arcs and hot surfaces arising | Gb |
| px | Pressurisation | Keeps the atmosphere out with clean air | Gb |
| mb | Encapsulation | Potting excludes the atmosphere | Gb |
| nA / ec | Non-sparking | No arcs in normal operation | Gc |
For instrumentation specifically, intrinsic safety is the workhorse: a 4-20 mA loop through a certified barrier or isolator carries so little energy that it cannot ignite the atmosphere even under fault. That makes live maintenance possible, which flameproof enclosures cannot offer — you cannot open a db enclosure in a live hazardous area.
Field notes
- The certificate governs, not the nameplate summary. Suffixes like X or U carry real meaning: X signals special conditions of use in the certificate, U marks a component not usable standalone.
- Intrinsic safety is a system property. The field device, the barrier and the cable parameters must be verified together — device Ci and Li against barrier Co and Lo. A certified transmitter on the wrong barrier is not an IS loop.
- ATEX and IECEx markings differ in prefix, not in the technical elements. ATEX adds the equipment group and category, such as II 2 G, ahead of the Ex marking.
- Dust has its own parallel scheme: groups IIIA to IIIC, protection levels Da/Db/Dc, and zones 20/21/22.
- Cable glands and enclosures need certification too. A certified transmitter fitted with an uncertified gland is not a compliant installation.
Frequently asked questions
What does Ex db IIC T4 Gb mean?
Ex marks it as explosion-protected equipment; db is flameproof enclosure protection; IIC is the most demanding gas group, covering hydrogen and acetylene; T4 limits the maximum surface temperature to 135 °C; and Gb is equipment protection level b for gas, suitable for Zone 1 and Zone 2.
What is the difference between gas groups IIA, IIB and IIC?
They rank gases by how easily they ignite and how readily a flame passes through a narrow gap. IIA covers propane and most hydrocarbons, IIB covers ethylene, and IIC covers hydrogen and acetylene. Equipment certified for IIC also satisfies IIB and IIA, but never the reverse.
What temperature class do I need?
The equipment surface temperature must stay below the auto-ignition temperature of the gas present. T4 permits 135 °C, T3 permits 200 °C, T6 permits just 85 °C. A lower T number is more restrictive, and T6 equipment satisfies every class below it.
Which protection concept suits Zone 0?
Zone 0 needs equipment protection level Ga — in practice intrinsic safety ia, or certain encapsulation and special protection types. Flameproof db enclosures are Gb and are not permitted in Zone 0.
Educational reference only. Hazardous area equipment selection and installation must follow the equipment certificate, the area classification drawing and IEC 60079 / local regulations, and be verified by a competent person. See our disclaimer.