Temperature Classes T1 to T6

T3 vs T4: When to Choose Which

• T3 (200°C max) — Sufficient for the majority of oil & gas and petrochemical environments. Covers H₂S, hexane, gasoline, and most common refinery gases. Wider product selection and lower cost.
• T4 (135°C max) — Required when diethyl ether (175°C AIT) or acetaldehyde (140°C AIT) may be present. Also specified in some pharmaceutical and fine chemical facilities as an extra safety margin.

Rule of thumb: If your hazardous area classification study identifies gases with AITs above 200°C, T3 equipment is sufficient. If any gas has an AIT between 135°C and 200°C, you need T4 or better.

T3 in Equipment Marking (Temperaturklasse T3)

The temperature class T3 appears in the ATEX/IECEx marking string:

Ex db IIC T3 Gb
              ^^
              Temperaturklasse T3 — max 200°C Oberflächentemperatur

In German-speaking markets, the temperature class is referred to as Temperaturklasse (abbreviated T-Klasse). The classification system is identical worldwide per IEC 60079-0 — T3 always means a maximum surface temperature of 200°C regardless of region or language.

T3 and Ambient Temperature

Standard T3 certification assumes an ambient range of −20°C to +40°C. At higher ambient temperatures (e.g., desert installations at +55°C), the equipment may reach its 200°C limit at lower power dissipation. Manufacturers must specify a Ta (ambient temperature) range on the certificate. Equipment installed outside this range requires re-evaluation or a different temperature class.

Temperature Considerations for Dust

Dust equipment does not use the T1–T6 classification. Instead, the maximum surface temperature is determined by two dust-specific properties:

1. Dust Cloud Ignition Temperature (T_CL)

The minimum temperature of a hot surface that will ignite a dust cloud.

Rule: Equipment max surface temp ≤ ⅔ × T_CL (in Kelvin, then convert back to °C)

Example: If T_CL = 450°C (723 K), max surface = ⅔ × 723 = 482 K = 209°C

2. Dust Layer Ignition Temperature (T_5mm)

The minimum temperature of a hot surface that will ignite a 5mm layer of settled dust.

Rule: Equipment max surface temp ≤ T_5mm − 75°C

Example: If T_5mm = 300°C, max surface = 300 − 75 = 225°C

Combined Rule

The maximum allowed surface temperature for dust equipment is the lower of:

• ⅔ × T_CL (Kelvin) converted to °C
• T_5mm − 75°C

For thicker dust layers (>5mm), additional derating applies — every 5mm of additional dust reduces the allowable surface temperature further. This is why housekeeping in dusty environments is critical.

Dust Temperature Examples

Ex db IIC T4 Gb
              ^^
              Temperature class T4 (max 135°C)

Ex tb IIIC T125°C Db
                ^^^^
                Max surface temperature 125°C

Ta = −40°C to +60°C