01Three layers, three decisions
Zone (1 / 2)
Zone 1: explosive atmosphere likely in normal operation. Zone 2: only abnormal, short-lived occurrences. Flameproof Ex d suits both; some increased-safety options fit Zone 2 only.
Gas group (IIA/IIB/IIC)
Based on the gases present, IIC being the strictest (hydrogen, acetylene). Equipment group must equal or exceed the site gas group: IIB gas accepts IIB or IIC equipment.
Temperature class (T1–T6)
Caps the equipment's maximum surface temperature: T6 = 85°C (strictest) up to T1 = 450°C. The site gas ignition temperature must sit above your equipment's T-class limit.
02What makes heaters different: T-class is usually the bottleneck
Motors and junction boxes control surface temperature easily — T4 is routine. A heater's job is to be hot; if the process runs at 260°C, exposed surfaces will exceed any T4 (135°C) or T3 (200°C) limit by physics alone. Simply 'specifying a higher Ex rating' does not solve this.
The three proven paths: first, verify the actual ignition temperature of the gases on site — many common hydrocarbons ignite above 400°C, so T2 or T1 equipment is compliant and T4 was never required. Second, lock the surface temperature with a validated over-temperature interlock. Third, decouple surface temperature from process temperature by placing elements inside an insulated pressure envelope. Every path must be confirmed in writing with the design institute and the owner's safety department — never on a vendor's verbal assurance.
03Common gases quick reference
Group IIB, higher ignition temp
Ethylene, propane, butane, diesel-range vapours — most oil & gas sites land on IIB T2–T3.
Group IIA
Methane, ammonia, benzene family. Relatively relaxed; IIA T2 is common.
Group IIC
Hydrogen, acetylene, carbon disulphide. Equipment must be IIC with T-class verified point by point; flameproof construction plus interlocks as standard.
04The interlock checklist
No Ex certificate protects a heater whose interlocks were never wired: over-temperature protection on an independent sensor that directly cuts heater power, flow or pressure protection that de-energizes heating when the medium stops moving, and a start sequence interlock (pump/fan first, heaters second). The certificate proves the construction; the site safety lives in these interlocks. Function-test each one at commissioning and keep the records.