ATEX-certified equipment is designed and tested to prevent ignition of explosive atmospheres. Selecting instruments, sensors, or electrical equipment without appropriate hazardous area certification in a Zone 1 or Zone 2 location is a serious WSHA breach that can cause catastrophic fires and explosions. In Singapore, facilities handling flammable liquids, gases, or combustible dusts must classify their hazardous areas and select equipment accordingly. The Ministry of Manpower (MOM) enforces these requirements under the Workplace Safety and Health Act, while the Singapore Civil Defence Force (SCDF) regulates flammable materials storage under the Fire Safety Act.
What Is an Explosive Atmosphere?
An explosive atmosphere is a mixture of flammable gas, vapour, mist or dust with air in which, after ignition, combustion spreads to the entire unburned mixture — the concentration must fall within the flammable range (between the LEL and UEL), but when it does, any ignition source (an electrical spark, static discharge, hot surface, open flame) can trigger an explosion. Common Singapore locations include Jurong Island refineries and petrochemical plants, gas compressor stations and LNG/LPG storage, pharmaceutical solvent-based processes, paint spray booths, flour mills and wood processing handling combustible dusts, battery charging rooms (hydrogen release), and fuel dispensing at petrol stations and marine bunkering facilities.
Hazardous Area Classification: Zones
The IEC 60079 series of standards (adopted by Singapore as SS EN 60079) defines hazardous zones based on the frequency and duration of explosive atmosphere presence:
Gas and Vapour Zones
| Zone | Definition | Example Location |
|---|---|---|
| Zone 0 | Explosive atmosphere present continuously or for long periods | Inside a fuel tank or closed vessel |
| Zone 1 | Explosive atmosphere likely to occur in normal operation | Around pump seals, valve bonnets, compressor rooms |
| Zone 2 | Explosive atmosphere unlikely in normal operation; may occur in abnormal conditions | General area around Zone 1, open-air process areas |
Dust Zones
| Zone | Definition |
|---|---|
| Zone 20 | Combustible dust cloud present continuously or frequently inside equipment |
| Zone 21 | Combustible dust cloud likely in normal operation |
| Zone 22 | Combustible dust cloud unlikely in normal operation |
Hazardous area classification must be documented in an area classification drawing (hazardous area drawing or HAD). Singapore's MOM and SCDF require these drawings for Dangerous Goods premises and Major Hazard Installations (MHIs) under the WSHA (Major Hazard Installations) Regulations.
ATEX and IECEx: What Do They Mean?
ATEX (from the French Atmosphères Explosibles) refers to two EU directives: ATEX 114 (equipment for explosive atmospheres) and ATEX 153 (minimum health and safety requirements for workers). Equipment bearing ATEX certification has been assessed by a Notified Body for use in explosive atmospheres within the EU, with the mark showing the Ex symbol, category (1G/2G/3G for gas; 1D/2D/3D for dust), temperature class, gas group and equipment protection level (EPL). IECEx is the international scheme administered by the IEC, technically equivalent to ATEX but with global scope, accepted in Singapore, Australia, Gulf states and many other markets — generally preferred for imported equipment in Singapore since it has direct international recognition without needing additional national approval. Both schemes share the same IEC 60079 technical standards and zone/EPL framework, so equipment certified to either is technically equivalent.
Equipment Protection Levels (EPL) and Categories
The Equipment Protection Level (EPL) indicates the level of protection and which zone the equipment may be used in:
| EPL | Zone Permitted | Protection Standard |
|---|---|---|
| Ga (gas) | Zone 0, 1, 2 | Very high protection. Two independent protection types or equivalent |
| Gb (gas) | Zone 1, 2 | High protection. One protection type remains effective under foreseeable faults |
| Gc (gas) | Zone 2 only | Enhanced protection. Suitable for normal operation conditions |
| Da (dust) | Zone 20, 21, 22 | Very high protection for dust atmospheres |
| Db (dust) | Zone 21, 22 | High protection for dust atmospheres |
| Dc (dust) | Zone 22 only | Enhanced protection for dust atmospheres |
Protection Techniques, Gas Groups and Instrument Selection
IEC 60079 defines multiple protection techniques: Ex d (flameproof enclosure) contains an internal explosion without igniting the external atmosphere, common for motors, switches and junction boxes; Ex e (increased safety) adds measures preventing sparks and high temperatures in normal operation, used for terminal boxes and luminaires; Ex i (intrinsic safety) limits circuit electrical energy so even fault conditions can't ignite an atmosphere, the most common technique for instrumentation, sensors and transmitters; Ex n (non-sparking) suits Zone 2 only with simplified requirements; Ex p (pressurisation) keeps the enclosure at positive pressure with protective gas, used for control panels and analyser shelters; Ex m (encapsulation) embeds components in a protective compound; and Ex o (oil immersion) submerges components in oil. Flammable gases are classified into groups by ignition characteristics: Group IIA (propane, most hydrocarbons), Group IIB (ethylene and other chemical gases, more demanding), and Group IIC (hydrogen, acetylene, most demanding). Equipment certified for a higher group covers lower groups (IIC covers IIA and IIB). Temperature classes (T1–T6) specify the maximum surface temperature, which must stay below the auto-ignition temperature of the gas present.
For process instrumentation, intrinsically safe (Ex i) transmitters and sensors are the most common choice since they can be maintained without gas-freeing the area, with associated safety barriers or galvanic isolators sitting in the safe-area control room. Specifying Ex instrumentation means determining the zone classification from the area drawing, identifying the gas group and auto-ignition temperature for the required temperature class, selecting equipment with the EPL appropriate for the zone (Ga for Zone 0, Gb for Zone 1, Gc for Zone 2), verifying the certificate covers the specific gas group and temperature class, and confirming associated apparatus is certified and matched to the field device.
Calibration of Ex-Certified Instruments
Certified equipment must be maintained and calibrated in a way that preserves its certification — opening an Ex d flameproof enclosure in a live hazardous area to adjust a transmitter is prohibited. For Ex i instruments, calibration can typically be performed in the hazardous area while connected to the safety barrier, since the circuit is inherently safe; any maintenance requiring opening the enclosure needs the area gas-freed with ignition sources controlled. Unitest Instruments provides calibration services for process instruments including hazardous area applications, under SAC-SINGLAS accreditation (LA-2023-0845-C) covering multiple measurement disciplines. For gas detection instruments, see our guide on industrial gas leak detection.