What are Ex t requirements for audible/visual warnings on explosion-proof telephones?

Many sites add a beacon or sounder to a hazardous-area phone and assume “same enclosure, same safety.” That shortcut often fails in inspection.

Ex t ¹ is dust protection by enclosure. A phone-plus-beacon setup only stays compliant when each device is correctly certified for the zone/dust group, the surface temperature is marked in °C, and every cable entry preserves the required IP level.

The practical Ex t baseline for warning devices on Ex telephones

What Ex t really protects, and what it does not

Ex t is built around two controls: dust ingress control (IP) and surface temperature limitation. It does not automatically control arcs, sparks, or energy in wiring. That matters because many warning add-ons are powered devices with drivers, LEDs, and sounder electronics. So the core question is always: is the beacon/sounder a certified Ex device on its own, or is it covered as part of the telephone assembly certificate?

When the beacon is outside the phone enclosure, it is normally treated as a separate Ex product. It needs its own marking and certificate for the same dust zone where it is installed. If it is inside the enclosure and the product is sold as an option, it must be inside the certified bill of materials and the certificate scope for that exact configuration.

Ex t marking must use Tmax (°C), not gas T-class

For dust, the marking uses a maximum surface temperature value in °C. That is what inspectors look for. Gas T-class ² (T1–T6) belongs to the gas marking line. Mixing these two temperature systems is one of the fastest ways to get rejected.

Ex t also assumes IP is kept in the real installation

A test sample can be IP66 in a lab, but your installed system can drop to “unknown” if:

• a gland is substituted,

• an unused hole is closed with a non-certified plug,

• a gasket is pinched,

• a cable is out of the gland’s OD range.

Ex t lives or dies at the cable entry.

A simple “system view” table

Part of the system	What must be true under Ex t	Typical mistake
Telephone enclosure	Marked Ex t (if dust rated) with Tmax and EPL	Treating IP as “optional”
Beacon/sounder	Certified for the same dust zone, or included in phone certificate	Using a general industrial beacon
Cable entry	Glands and plugs keep required IP	Thread mismatch or wrong seal
Documentation	Certificates match nameplates and configuration	Only a datasheet is provided

If these basics are set, the rest of the work becomes clean procurement and clean installation. Next is the most common buyer question: do the beacon and sounder need to “match” the phone’s Ex t and IP rating?

A clear answer prevents costly over-spec and avoids under-spec that fails commissioning.

Must beacons and sounders share the same Ex t and IP rating?

Small differences in marking can create big confusion. Installers see “Ex” and assume all parts are equal.

Beacons and sounders do not need to have identical markings to the telephone, but each device installed in the dust zone must have an Ex rating that is suitable for that exact zone, dust group, and temperature limit, and must meet the IP requirement linked to its Ex t level.

“Same rating” is not the same as “same suitability”

A telephone could be rated:

• Ex tb IIIC T80°C Db IP66

while a beacon could be rated:

• Ex tc IIIC T90°C Dc IP66

These are not identical, but both can be suitable in different dust zones. The real rule is the zone requirement at the mounting point:

• Zone 21 ³ needs at least Db (or higher like Da).

• Zone 22 ⁴ needs at least Dc (or higher like Db/Da).

So a Db phone can be used in Zone 22, and a Dc beacon can be used in Zone 22, but that Dc beacon should not be used in Zone 21.

IP is tied to Ex t and dust group, not to marketing

Ex t has minimum dust ingress protection based on the dust group ⁵:

• IIIC typically drives IP6X for dust ingress protection.

• IIIB/IIIA can allow IP5X in certain Ex t levels.

In real purchasing, I prefer a conservative approach in powder plants: if IIIC is possible, standardize to IP6X-capable products and lock glands and plugs tightly.

How to choose “harsh site” suitability

Harsh sites are not only “more dust.” They include:

• washdown + dust,

• vibration that loosens glands,

• public access (impact on lenses),

• sticky powders that build layers and trap heat.

In these sites, a beacon with a higher IP and higher mechanical rating can be smart even if the zone only requires Dc.

A quick decision table

Installation zone	Minimum dust EPL	Typical Ex t level	Practical note
Zone 21	Db	Ex tb	Stronger sealing expectations
Zone 22	Dc	Ex tc	Still must keep IP via glands
Zone 22 (standardization)	Db	Ex tb	Often reduces wrong installs

The key takeaway: matching is about suitability, not about identical letters. The next question is temperature marking, because warning devices often create heat, and dust marking must show surface temperature clearly.

How do I mark temperature for external warning devices?

Many teams correctly mark the phone, then forget that the beacon has its own temperature limits and its own Ta assumptions.

External beacons/sounders in dust zones should be marked with Ex t and a maximum surface temperature value in °C (Tmax). If gas is also relevant, they also need a gas T-class on the gas marking line.

Dust uses Tmax (°C) and may include dust-layer assumptions

For dust, the marking uses a maximum surface temperature like:

• T85°C, T90°C, T120°C, etc.

Some standards guidance also discusses how marking can differ depending on whether the equipment was tested with a dust layer or without a layer. This is exactly why the safest habit is to copy the marking from the certificate “Marking” section word-for-word. The certificate schedule is where special temperature conditions are documented.

Gas-and-dust combined devices need two temperature languages

If your beacon is certified for both gas and dust, it will often show two lines:

• Gas line: includes T-class (T1–T6)

• Dust line: includes Tmax in °C

This avoids mixing systems. It also helps commissioning teams check both hazards in mixed plants.

Ta still matters for warning devices

A beacon can run hotter when:

• ambient is high,

• sun hits the lens directly,

• the sounder runs continuously,

• the device is driven at max duty cycle.

So Ta belongs on the device nameplate or in its certificate marking text. If Ta is narrower for the beacon than for the phone, the system is only as strong as the weakest Ta in the chain.

How to verify “maximum surface temperature vs process heat”

A practical method on site:

1) Confirm the certified Ta range for both phone and beacon.

2) Check proximity to hot pipes, steam tracing, or hot walls.

3) Check sun exposure and airflow.

4) If the point is high-risk, do a simple surface temperature spot check during worst-case operation.

If process heat adds significant radiant heating, it effectively raises the “local ambient” seen by the device. In that case, relocating the beacon a short distance often solves the problem without changing product category.

A marking clarity table

Device	Temperature marking to show	Common mistake
Gas-only phone	T-class (T1–T6) + Ta range	Treating T-class as ambient
Dust-only beacon	Tmax (°C) + Ta range	Printing gas T-class on dust line
Dual G/D beacon	T-class on gas line + Tmax on dust line	Combining into one confusing value

Once temperature marking is clean, wiring and cable entry become the next main failure point. Ex t depends heavily on preserving enclosure integrity.

What wiring and cable-gland rules apply for Ex t circuits?

Most “Ex t failures” in the field are not electronics failures. They are entry failures: wrong gland, wrong seal, wrong torque, or wrong plug.

For Ex t systems, wiring must maintain enclosure integrity and required IP. Cable glands and seals must match the entry thread, cable OD, temperature range, and be suitable for hazardous areas. Unused entries must be correctly plugged to preserve dust ingress protection.

Treat the cable entry as part of the protection concept

Ex t is “protection by enclosure.” That means the entry system is part of the protection, even if the internal circuit is simple. In procurement, I lock these items:

• thread type and size,

• gland part number and certificate,

• sealing method (compression seal, washer, O-ring),

• permitted cable OD range,

• stopping plug part number for unused holes.

This avoids the field substitution problem where the contractor uses what is on the truck.

Wiring approach depends on the product architecture

Two common architectures exist:

1) Separate certified beacon mounted externally

The beacon has its own Ex marking. The wiring between phone and beacon is a normal power/control circuit, but it must be installed using hazardous-area wiring rules and suitable glands for each enclosure.

2) Beacon included as a certified option on the phone

The phone certificate schedule will often list the beacon option and any limits (duty cycle, Ta impact, wiring method). In this case, internal wiring and terminal chamber rules matter a lot. Configuration control is critical.

If the phone provides only a relay contact output, the beacon power circuit still needs to be safe for the area. Ex t does not magically make the external circuit “safe.” The beacon must be certified, or the circuit must use an appropriate protection concept (like Ex i ⁶) if that is the chosen method.

Gland and sealing priorities for powdery environments

Powdery plants need:

• tight compression seals (no loose cable fit),

• stable materials (seals compatible with chemicals and cleaning),

• strain relief that prevents cable movement from working the seal loose.

For conductive dust group IIIC, I push for IP6X-capable sealing performance and strict installation workmanship.

A field checklist for entries

Check item	What “good” looks like	What fails audits
Gland selection	Correct Ex/IP gland PN for the enclosure	Random industrial gland
Thread fit	Full engagement, correct thread type	Metric/NPT mismatch
Seals	Cable OD inside gland range, seal not twisted	Under/over compression
Unused entries	Correct certified plug and seal	Open hole or generic plug
Strain relief	Cable is supported and not pulling	Cable weight on gland

After wiring and glands, the last piece is documentation. Auditors do not approve “a system story.” They approve traceable certificates and controlled configurations.

How do I document combined phone-plus-beacon certification?

Many procurement teams accept a phone certificate and a beacon certificate, then discover later that the configuration or mounting is not covered.

Document combined phone-plus-beacon compliance by providing certificates for each Ex product (or one combined certificate if the beacon is part of the phone scope), plus the certificate schedules, approved accessory lists, nameplate photos, installation drawings, and change-control records for OEM/ODM builds.

Two acceptable certification models

1) Separate certificates (most common)

• Phone has its own IECEx CoC / ATEX certificate.

• Beacon/sounder has its own IECEx CoC / ATEX certificate.

• The system is acceptable when both are suitable for the same zone/dust group and installed per their manuals.

2) Single certificate covering the phone with beacon option

• The phone certificate schedule lists the beacon option as part of the certified configuration.

• The marking section may include the same for the configured assembly.

• This is often the cleanest path for OEM/ODM projects because configuration is controlled.

What auditors typically ask for

A complete procurement pack usually includes:

• IECEx CoC and/or ATEX EU-type examination certificates for each device

• Certificate schedules/annexes showing:

  • marking text,

  • Ta range,

  • Tmax for dust,

  • IP statements where relevant,

  • any “X” conditions

• Installation manuals showing approved glands, plugs, and wiring limits

• Nameplate photos of phone and beacon (so the marking can be matched to certificates)

• Factory quality documents:

  • IECEx QAR ⁷ (or equivalent quality surveillance evidence),

  • ATEX QAN ⁸ production QA evidence (where applicable)

• OEM/ODM change-control records:

  • drawing revisions,

  • BOM control for glands, seals, lenses,

  • ECO history for any enclosure or PCB changes.

A procurement-ready table

Document	Phone-only	Beacon-only	Phone + beacon system file
Certificate	Required	Required	Required, matched to nameplates
Schedule/annex	Required	Required	Required, with options and “X” notes
Manual	Required	Required	Combined install method statement
Gland list	Required	Required	Locked part numbers and threads
Configuration control	Optional	Optional	Mandatory for OEM/ODM assemblies

When the project is global, I also keep the same naming convention across phone and beacon documents. It saves time in audits because inspectors can map each enclosure to its certificate fast.

Conclusion

Ex t warning add-ons are safe when each enclosure is correctly certified, Tmax and Ta are clear, entries keep the required IP, and procurement includes certificates, schedules, manuals, and OEM/ODM change control.

Footnotes