EPLs and T-codes: reading an explosion-proof nameplate

Two labels, one decision

When you pick up an explosion-proof luminaire, the nameplate tells you three things that matter for installation:

EPL — how reliably the equipment avoids becoming an ignition source
Equipment group — the gas or dust family it's certified for
T-code — the maximum surface temperature it can reach

Get any of these wrong and the fixture is not code-compliant, regardless of whether it's "explosion-proof."

EPL — Equipment Protection Level

EPL is the modern way CSA and IEC describe how hard the equipment works to stay inert. Section 18 defines six EPLs, split by hazard family.

Gas EPLs

EPL	Protection level	Ignition source during…
Ga	Very high	Normal, expected malfunctions, AND rare malfunctions
Gb	High	Normal, AND expected malfunctions
Gc	Enhanced	Normal operation only

Dust EPLs

EPL	Protection level	Ignition source during…
Da	Very high	Normal, expected, and rare malfunctions
Db	High	Normal and expected malfunctions
Dc	Enhanced	Normal operation only

Definitions paraphrased from Rule 18-002.

Matching EPL to zone

This is the one-to-one relationship you memorize:

Zone	Minimum EPL
Zone 0	Ga
Zone 1	Gb (or Ga)
Zone 2	Gc (or Gb or Ga)
Zone 20	Da
Zone 21	Db (or Da)
Zone 22	Dc (or Db or Da)

Higher-EPL equipment is always acceptable in a less severe zone. Lower-EPL equipment is never acceptable in a more severe zone.

T-codes — surface temperature ratings

The T-code caps the maximum external surface temperature the equipment is allowed to reach under rated conditions. Rule 18-054 says the T-rating shall not exceed the minimum ignition temperature determined for the atmosphere in which the equipment is installed.

The standard T-code ladder:

T-code	Max surface temp
T1	450 °C
T2	300 °C
T3	200 °C
T4	135 °C
T5	100 °C
T6	85 °C

Lower numbers = hotter surfaces allowed. Higher numbers = safer for low-ignition-temperature atmospheres.

Under Rule 18-054(2), if equipment installed in a Zone 0/1/2 location has no marked T-code at all, the maximum surface temperature is deemed to be 100 °C — regardless of what the equipment actually runs at. If the atmosphere's minimum ignition temperature is lower than 100 °C (e.g. carbon disulphide at 90 °C), you can't install that unmarked equipment there. Don't assume "probably fine" — verify the marking or the ignition temperature.

[!pitfall] Under Rule 18-054(2), if equipment installed in a Zone 0/1/2 location has no marked T-code at all, the maximum surface temperature is deemed to be 100 °C — regardless of what the equipment actually runs at. If the atmosphere's minimum ignition temperature is lower than 100 °C (e.g. carbon disulphide at 90 °C), you can't install that unmarked equipment there. Don't assume "probably fine" — verify the marking or the ignition temperature.

For dust zones (Rule 18-054(3)), the T-rating must stay below the lower of the dust cloud ignition temperature or the dust layer ignition temperature — a tighter constraint than gas zones, because dust layers on hot surfaces smoulder at temperatures well below the airborne cloud's ignition point.

Equipment groups

The Group tells you which atmospheric families the equipment is tested for.

Gas groups (Rule 18-050(2))

Group	Representative gases
IIA	Propane, gasoline, natural gas, ammonia, butane, most lacquer solvents
IIB	Ethylene, ethylene oxide, hydrogen sulphide, diethyl ether
IIC	Acetylene, hydrogen, carbon disulphide

IIC-marked equipment works in IIA or IIB locations (Rule 18-050(3)). IIB works in IIA (18-050(4)). IIA is the narrowest.

Dust groups (Rule 18-050(6))

Group	Atmosphere
IIIA	Combustible flyings (fibres > 500 µm)
IIIB	Non-conductive combustible dust
IIIC	Conductive metal dust

Putting it together

A nameplate reading Ex db IIB T4 Gb means:

Ex db — flameproof "db" type of protection
IIB — approved for Group IIB atmospheres (and IIA)
T4 — max surface temp 135 °C
Gb — EPL Gb, suitable for Zone 1 or Zone 2

Before you buy, check that every piece of that string is appropriate for your Zone, your Group, and your atmosphere's minimum ignition temperature. If any one of them is a mismatch, the fixture doesn't belong in your project.