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. Not every leak creates an explosive atmosphere — the concentration must fall within the flammable range (between the LEL and UEL of the substance). But when it does, any ignition source — a spark from an electrical contact, static discharge, a hot surface, or an open flame — can trigger an explosion.
Common Singapore industrial locations where explosive atmospheres may form include:
- Jurong Island oil refineries, petrochemical plants, and chemical terminals
- Gas compressor stations and LNG/LPG storage facilities
- Pharmaceutical manufacturing areas handling solvent-based processes
- Paint spray booths and solvent cleaning areas
- Flour mills, wood processing, and other facilities handling combustible dusts
- Battery charging rooms (hydrogen release during charging)
- Fuel dispensing areas 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
ATEX (from the French Atmospheres Explosibles) refers to two European Union directives: ATEX 114 (equipment for use in explosive atmospheres) and ATEX 153 (minimum health and safety requirements for workers). Equipment bearing ATEX certification has been assessed and certified by a Notified Body for use in explosive atmospheres within the EU. The ATEX mark shows the Ex symbol, category (1G/2G/3G for gas; 1D/2D/3D for dust), temperature class, gas group, and equipment protection level (EPL).
IECEx
IECEx is the international certification scheme administered by the International Electrotechnical Commission. It is technically equivalent to ATEX but has global scope. IECEx certificates are accepted in Singapore, Australia, Gulf states, and many other markets outside the EU. For Singapore applications, IECEx is generally preferred over ATEX for imported equipment because it has direct international recognition without requiring additional national approval.
Both schemes use the same underlying technical standards (IEC 60079 series) and the same zone and EPL framework, so equipment certified to either scheme 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 |
Types of Equipment Protection Techniques
IEC 60079 defines multiple protection techniques, each achieving the EPL through a different engineering approach:
- Ex d — Flameproof enclosure: the enclosure is strong enough to contain an internal explosion without igniting the external atmosphere. Common for motors, switches, and junction boxes.
- Ex e — Increased safety: additional measures to prevent sparks and high temperatures in normal operation. Used for terminal boxes, motors (squirrel cage only), and luminaires.
- Ex i — Intrinsic safety: the electrical energy in the circuit is limited so that even in fault conditions, it cannot ignite an explosive atmosphere. Used for instrumentation, sensors, and transmitters — the most common technique for process measurement instruments.
- Ex n — Non-sparking (Zone 2): normal operation does not produce arcs, sparks, or hot surfaces; simplified requirements for Zone 2 only.
- Ex p — Pressurisation: enclosure is maintained at positive pressure with a protective gas (instrument air or inert gas) to exclude the explosive atmosphere. Used for large control panels and analyser shelters.
- Ex m — Encapsulation: components are embedded in a compound that prevents contact with explosive atmosphere. Used for sensors and electronic components.
- Ex o — Oil immersion: components immersed in oil that prevents contact with explosive atmosphere.
Gas Groups and Temperature Classes
Flammable gases and vapours are classified into gas groups based on their ignition characteristics (minimum ignition energy and maximum experimental safe gap):
- Group IIA — propane, most hydrocarbons
- Group IIB — ethylene, some chemical gases; more demanding than IIA
- Group IIC — hydrogen, acetylene; most demanding, highest risk
Equipment certified for a higher group covers lower groups (IIC covers IIA and IIB). Temperature classes (T1 to T6) specify the maximum surface temperature the equipment may reach, which must be below the auto-ignition temperature of the hazardous gas or vapour present.
Selecting Ex-Certified Instruments for Process Measurement
For instrumentation in hazardous areas, intrinsically safe (Ex i) transmitters and sensors are the most common choice because they can be maintained and replaced without gas-freeing the area, and the associated apparatus (safety barriers or galvanic isolators) sits in the safe area control room. When specifying Ex instrumentation:
- Determine the zone classification from the area classification drawing
- Identify the gas group for the substances present
- Identify the auto-ignition temperature to determine the required temperature class
- Select equipment with EPL appropriate for the zone (Ga for Zone 0, Gb for Zone 1, Gc for Zone 2)
- Verify that the certificate covers the specific gas group and temperature class
- Confirm the associated apparatus (barriers, isolators) is certified and matched to the field device
Calibration of Ex-Certified Instruments
Certified equipment must be maintained and calibrated in a manner that preserves its certification. Opening an Ex d flameproof enclosure in a live hazardous area to adjust a transmitter is prohibited. For Ex i (intrinsic safety) instruments, calibration can typically be performed with the instrument in the hazardous area while connected to its safety barrier, because the circuit is inherently safe. For any maintenance or calibration that requires opening the enclosure, the area must be gas-freed and all ignition sources controlled.
Unitest Instruments provides calibration services for process instruments including those used in hazardous area applications. Our SAC-SINGLAS accreditation (LA-2023-0845-C) covers multiple measurement disciplines. Contact our team for calibration scheduling and to discuss hazardous area compliance requirements for your facility.
For gas detection instruments used in hazardous areas, see our guide on industrial gas leak detection methods and equipment.