Which gas groups are explosion-proof telephones suitable for?

Wrong gas group selection can turn a “certified” device into a project delay, a rework cost, or a real ignition risk during commissioning.

Explosion-proof telephones are suitable only for the gas or dust group shown on their Ex marking and certificate. If the phone is certified for IIC, it can be used in IIB and IIA areas too, but not the other way around.

Explosion groups, zones, and what a telephone certification really covers

When a customer asks “Is your telephone explosion-proof?”, the real question is always three questions: where it will be installed (Zone), what can ignite (gas group or dust group), and how hot the device can get (temperature class or max surface temperature). A telephone housing may look heavy-duty, but the correct approval is proven only by the nameplate marking and the issued certificate.

Gas group is about flame propagation, not “how dangerous the plant feels”

Gas groups ¹ in the IEC/ATEX world (IIA, IIB, IIC) are based on how easily an explosion can pass through joints and gaps, and how aggressive the gas is to contain. The practical rule is simple:

• IIC is the most stringent. If a phone is certified IIC, it is also acceptable for IIB and IIA (within the same protection concept and EPL/Zone).

• IIB covers IIA, but not IIC.

• IIA is the least stringent.

Zone and EPL decide how often the hazard may be present

For gas:

• Zone 0 → EPL Ga (very high protection, continuous hazard)

• Zone 1 → EPL Gb

• Zone 2 → EPL Gc

For dust:

• Zone 20 → EPL Da

• Zone 21 → EPL Db

• Zone 22 → EPL Dc

Quick mapping table: gas groups and “typical” sites (use area classification as the final decision)

A phone that is Ex db IIC T6 Gb (example format) is telling you: flameproof protection (db), gas group IIC, temperature class T6, and EPL Gb (Zone 1 suitability). That is not marketing language. It is the installation boundary.

If a project involves unknown future products, mixed gases, or multiple sites, I often treat IIC as the default to reduce engineering risk, then verify temperature class and ambient range per location.

Now, the real selection questions start once we map those labels to real plants and real zones.

This is where most specification mistakes happen, because documents use short phrases like “terminal area” while the hazard is defined by the exact substance and its area classification.

How do IIA, IIB, and IIC classifications map to typical plants, tank farms, and terminals?

A “typical” site label can hide very different gas groups, so using plant type alone can cause under-spec or overspend.

IIA often covers many fuel storage and general hydrocarbon areas, IIB is common where ethylene is present, and IIC is required where hydrogen or acetylene can be present. The right answer comes from the site’s hazardous area classification and substance list.

Start with the substance, then confirm with the area classification file

A tank farm may store gasoline, diesel, ethanol blends, or specialty chemicals. A “terminal” could be fuel-only, or it could handle LPG and chemical feedstocks. In practice, selection becomes safe and fast when the workflow is consistent:

1) Identify the substances in the area (and credible releases).

2) Confirm the Zone (0/1/2) from the hazardous area classification ³ drawing.

3) Select gas group and temperature class to match the worst credible case.

4) Lock in EPL and ambient range based on the installation point.

Practical mapping by site type (what usually works, and what usually surprises teams)

Watch-outs that change the decision fast

• Mixed gas scenarios: if an area can contain both IIA and IIB gases, selection should follow the more demanding group (often IIB).

• Future-proofing: terminals often expand product types. Using IIC devices can avoid replacement later.

• “Same enclosure, different approval”: a phone model may look identical across approvals, but the approved variant is defined by the marking and certificate number.

For a purchasing team, the safest habit is to request the certificate and read the marking line-by-line. If the project spec says “Zone 1, IIB, T4”, then a “Zone 1, IIA, T4” phone is not equivalent, even if it is “explosion-proof” in general language.

Do hydrogen, acetylene, and ethylene zones require IIC devices with the correct temperature class?

A wrong temperature class can be as dangerous as a wrong gas group, because ignition can come from hot surfaces, not only sparks.

Hydrogen and acetylene zones typically require IIC-certified devices, while ethylene zones usually require at least IIB. In all cases, the device must also meet the required temperature class (T1–T6) or max surface temperature shown on the certificate and marking.

Gas group decision: hydrogen and acetylene are the “tightest” cases

Hydrogen and acetylene are commonly treated as IIC gases in hazardous area design. Ethylene is commonly treated as IIB. That means:

• For hydrogen and acetylene, using a device marked IIC is the normal requirement.

• For ethylene, a device marked IIB can be correct, and IIC is also acceptable.

If a procurement spec lists “IIC”, it is not asking for a nicer enclosure. It is asking for tighter flamepath control ⁴ and stricter test requirements.

Temperature class: match the device surface limit to the ignition risk

Temperature class ⁵ is about the maximum surface temperature the equipment can reach under fault conditions (depending on protection concept). The lower the number, the hotter it may run:

• T1 is the hottest allowance; T6 is the coolest allowance.

Selection is usually driven by the site specification (for example “T4”) based on the substance’s ignition properties and safety margin. The phone must meet both:

• the gas group (IIA/IIB/IIC), and

• the temperature class (T1–T6) or a dust max surface temperature (like T85°C).

Common selection table for these gases (project-spec approach)

Two mistakes that keep showing up in real tenders

• Mistake 1: “IIC only matters for gas, not for terminals.” Gas group is not about the site label. It is about the gas behavior. If hydrogen ⁶ can be present, IIC is not optional.

• Mistake 2: “T-class is the same as ambient.” A phone may be rated T4 at Ta = -20°C to +40°C, but at Ta = +60°C the allowed T-class or power limits can change. Always read the Ta range and any special conditions (“X” conditions) on the certificate.

In my quoting process, the fastest way to avoid site arguments is to ask for the hazardous area classification sheet and the required marking line. Once the marking is clear, the product choice becomes straightforward.

How do dust groups IIIA/IIIB/IIIC change selection—Ex tb for flour, coal, and aluminum powder?

Dust hazards are often underestimated, and a gas-certified device is not automatically dust-approved for the same location.

Dust group selection changes the enclosure and ingress requirements: IIIA for fibers, IIIB for non-conductive dust, and IIIC for conductive dust. Flour and coal dust commonly fall under IIIB, while aluminum powder is conductive and typically requires IIIC approval with Ex tb (or other certified dust protection) and a defined max surface temperature.

Dust is its own world: group + zone + max surface temperature

For dust, the marking typically uses:

• III (dust),

• IIIA / IIIB / IIIC (fiber / non-conductive / conductive),

• Ex t protection (often Ex tb for Zone 21, Ex tc for Zone 22),

• and a max surface temperature like T85°C instead of T1–T6.

So a dust-certified telephone might be marked like Ex tb IIIC T85°C Db (example format), which indicates suitability for conductive dust and an EPL for Zone 21 (Db).

How flour, coal, and aluminum powder change the selection

Why Ex tb is commonly the “workhorse” choice

Ex t (protection by enclosure) focuses on preventing dust ingress and controlling surface temperature. For telephones in dusty industrial areas, Ex t is common because:

• A phone is a surface-mounted device exposed to dust layers.

• Cable entries and keypad gaps are real ingress points.

• The enclosure rating and gasket design become part of safety.

Practical checks beyond the group letters

• IP rating and IP6X needs: dust-tight performance matters, not only “IP65 marketing”.

• Surface temperature limits: dust layers can trap heat. The marking must state max surface temperature suitable for the dust ignition risk and the site’s layer thickness assumptions.

• Cleaning and maintenance reality: in flour or coal handling, devices get coated. Selection should consider how often the device will be cleaned and whether seals will be damaged.

For many customers, the simplest safe rule is: if the area is dust-classified, request a device with dust marking (Ex t) plus the right IIIA/IIIB/IIIC group, rather than assuming a gas-certified phone is enough.

Which certificates verify gas group approval—ATEX/IECEx marking, EPL level, and ambient temperature range?

A nameplate without a real certificate is a risk for compliance, insurance, and commissioning sign-off.

Gas group approval is verified by matching the device nameplate marking to an official ATEX and/or IECEx certificate. The certificate should confirm the protection concept (Ex d/Ex e/Ex i/Ex t), the gas/dust group, the EPL (Ga/Gb/Gc or Da/Db/Dc), the temperature class or max surface temperature, and the permitted ambient temperature range (Ta).

Read the marking like a checklist, not like a logo

A correct Ex marking line usually encodes everything needed for installation approval. For gas equipment, it often includes:

• Protection concept(s): Ex db, Ex eb, Ex ia, etc.

• Gas group: IIA/IIB/IIC

• Temperature class: T1–T6

• EPL: Ga/Gb/Gc

• Ambient range: Ta = -40°C to +60°C (example format)

For dust:

• Ex tb / Ex tc

• Dust group: IIIA/IIIB/IIIC

• Max surface temperature: T85°C (example)

• EPL: Da/Db/Dc

• Ambient range

ATEX vs IECEx: what each proves in procurement terms

EPL and zone: prove the match, don’t assume it

Even with the right gas group, a phone can still be wrong if the EPL is insufficient:

• Zone 1 needs Gb (or higher).

• Zone 2 needs Gc (or higher).

• Zone 0 needs Ga (rare for standard telephones unless intrinsically safe design is used).

Ambient temperature range (Ta) is not a “nice-to-have”

Many outdoor terminals and tank farms face extreme heat, sunload, or winter cold. A phone certified only for Ta -20°C to +40°C may fail compliance in a +55°C location, even if the gas group and T-class look correct. The correct step is to:

• check the certificate Ta range, and

• confirm the marking shows that range (or that the ordered variant is rated for it).

In my documentation pack for B2B customers, the most helpful page is usually a one-line required marking ¹⁰ statement. Once the project team agrees on that marking, the certificate check becomes a simple match exercise instead of a debate.

Conclusion

Gas group, dust group, zone/EPL, temperature class, and Ta range must all match the site classification; a correct Ex certificate and nameplate marking are the only reliable proof.

Footnotes