In an oil refinery, a grain silo, a paint shop or a pharmaceutical solvent room, a single spark is not a maintenance nuisance — it is an ignition source that can level a facility. The humble limit switch, chosen carelessly, is exactly the kind of device that produces that spark: contact arcs, surface heat, or a cable gland that leaks flammable vapour into an energized enclosure. This guide explains how to select an explosion-proof switch correctly, walking through certification, area classification, protection methods and gas groups, using the Schmersal EX range as the reference series.
1. Why a Standard Switch Cannot Go Into a Hazardous Area
A hazardous area is any location where flammable gas, vapour, mist or combustible dust can form an explosive atmosphere. A standard switch carries two built-in ignition risks. First, its electrical contacts arc every time they open and close; inside a sealed switch body that arc is harmless, but if the enclosure is not designed to contain it, the energy escapes. Second, the switch surface can heat up — and if that temperature exceeds the auto-ignition temperature of the surrounding gas or dust, the gas ignites on contact.
Explosion-proof switches are engineered to break both of these ignition chains. Their housings either contain an internal explosion and cool the escaping gases below the ignition point, or they limit the energy so that no ignition-capable spark can exist in the first place. The specific method used is defined by the protection concept stamped on the nameplate — Ex d, Ex e, Ex ia and the like — which we decode in Section 4.
Choosing one is not a matter of picking the most expensive model. It is a matter of matching the switch's certification markings to the formal area classification that a site risk assessment has already determined. Get that matching wrong and the switch is not merely non-compliant — it is a hazard, and the certification is void.
2. Step 1 — Read the Certification: ATEX, IECEx and NEC/UL
The first thing to confirm is which certification regime applies to your region and your end customer's market. The three dominant systems overlap but are not interchangeable.
| System | Region | Marking example | What it tells you |
|---|---|---|---|
| ATEX (Directive 2014/34/EU) | Europe | II 2G Ex db IIC T6 Gb | Equipment group, category, protection method, gas group, temperature class |
| IECEx | International | Ex db IIC T6 Gb | Same coding as ATEX, recognised in many countries for global trade |
| NEC / UL (NFPA 70, UL 1203 / UL 60947-5-1) | North America | Class I, Div 1, Groups C, D; T6 | Class/Division system instead of zones |
In practice, a switch often carries dual certification (ATEX + IECEx), which lets one part number serve European and many international markets. A North American installation may additionally demand UL listing. Verify the marking against the equipment's official certificate before you buy — a printed label is not a substitute for the certificate number. The Schmersal EX family is documented for ATEX and IECEx use, with the Explosion-proof Switches category listing the certified models.
3. Step 2 — Classify the Area: Zones, Classes and Divisions
The site hazard assessment produces a classification that tells you how likely an explosive atmosphere is to be present, and for how long. Under ATEX/IECEx this is expressed as zones; under NEC it is classes and divisions.
For gases and vapours: Zone 0 means an explosive atmosphere is present continuously or for long periods; Zone 1 is likely in normal operation; Zone 2 is present only rarely and briefly. For combustible dusts the equivalent zones are 20, 21 and 22. The tighter the zone, the more demanding the protection method must be — a Zone 0 application typically requires intrinsic safety (Ex ia) because any contained-explosion approach is inappropriate where gas is always present.
The North American Class/Division system runs parallel: Class I (gases/vapours), Class II (dusts), Class III (fibres), each divided into Division 1 (hazard present in normal operation) and Division 2 (hazard only under abnormal conditions). When exporting, do not assume a Zone 2 ATEX switch automatically meets a Class I Division 2 requirement — confirm the specific marking.
4. Step 3 — Pick the Protection Method: Ex d, Ex e and Ex ia
The protection concept is how the switch actually stops an explosion from propagating. The three you will meet most often in position switches are compared below.
| Method | Principle | Typical zone | Best for |
|---|---|---|---|
| Ex d (flameproof) | Robust enclosure contains an internal explosion; flame paths cool escaping gases | Zone 1 / Zone 2 | Limit switches and mechanical position switches carrying full switching energy |
| Ex e (increased safety) | No sparking parts in normal operation plus extra creepage/clearance | Zone 1 / Zone 2 | Junction boxes and terminals rather than arcing contacts |
| Ex ia (intrinsic safety) | Circuit energy limited below ignition threshold, even under fault | Zone 0 / Zone 1 / Zone 2 | Non-contact and sensor circuits wired through an IS barrier |
For a mechanical position switch with normal switching contacts, Ex d (flameproof) is the workhorse: the cast-metal enclosure is machined so that any internal flash is cooled as it passes through the precisely machined flame path between body and cover. Intrinsically safe variants (Ex ia) are common where a non-contact or coded magnetic switch feeds a low-energy circuit through a barrier — the energy is limited so that no spark can occur at all. Some Schmersal EX units, such as the EX-BNS 33-12Z-2187-3GD coded magnetic sensor, are built for exactly this low-energy duty.
5. Step 4 — Match Gas Group and Temperature Class
Two further markings decide whether the switch is safe for the specific substance present.
The gas group ranks how easily a gas ignites and how violently it burns. Group I covers mining (firedamp/methane); Group IIA covers propane and similar; IIB covers ethylene; and IIC covers the most easily ignited gases — hydrogen and acetylene. A switch rated for IIC can be used for IIA and IIB, but not the reverse. Always select for the most demanding gas present in the area.
The temperature class (T1 to T6) caps the maximum surface temperature of the equipment. The classes run from T1 (450 °C) down to T6 (85 °C), with each step being safer. A T6-rated switch can go anywhere; a T1 switch can only go where the auto-ignition temperature of the gas is comfortably above 450 °C. The T-class you need is dictated by the gas, not by preference — hydrogen and carbon disulphide demand T6-class equipment.
6. Step 5 — Specify the Switch Function and Enclosure
With the certification settled, the remaining decisions are the same as any position switch: actuator type, contact configuration, ingress protection and material.
Actuator. Roller lever, plunger and roller plunger heads cover most end-of-travel and position duties; rope-pull variants extend emergency-stop coverage along conveyors in classified areas. The EX-RS655-T22 is a flameproof rope-pull switch for long conveyors in hazardous zones, while the EX-BS655-T22 covers fixed position monitoring.
Contact configuration. For safety-related functions, specify positive-opening (positive-break) contacts so the circuit is mechanically forced open even if a contact welds. For solenoid-interlocked guards in hazardous areas, a device such as the EX-AZM300Z-I2-ST-1P2P-A-CL-3GD combines position monitoring with guard locking.
Ingress protection and material. Explosion protection and water/dust protection are separate qualifications. A switch rated Ex d may still need IP66/IP67 for outdoor or washdown duty. Check the material — stainless steel or coated cast aluminium — against the corrosive agents in the environment.
7. The Schmersal EX Range at a Glance
Schmersal builds a complete EX portfolio covering position monitoring, safety interlocks and non-contact sensing for classified areas. Representative certified models available through Glorient:
- EX-BS655-T22 — flameproof position/limit switch
- EX-RS655-T22 — flameproof rope-pull emergency-stop switch
- EX-I-RS655-Z22 — intrinsically safe rope-pull variant
- EX-AZM300Z-I2-ST-1P2P-A-CL-3GD — solenoid safety interlock
- EX-BNS 33-12Z-2187-3GD — coded magnetic non-contact sensor
- EX-MAF 330-11Y-3D — explosion-proof switch for dust (3D) applications
Browse the full Explosion-proof Switches category to filter by protection method, gas group and temperature class.
Frequently Asked Questions
What is the difference between ATEX and IECEx?
ATEX is the European Union directive (2014/34/EU) governing equipment for explosive atmospheres; IECEx is the international IEC certification system. The technical coding is essentially identical — the same Ex d IIC T6 marking means the same thing — but IECEx is accepted in a wider set of countries for global trade. Many switches carry both.
Can I use a Zone 2 switch in a Zone 1 area?
No. A switch certified for Zone 2 assumes an explosive atmosphere is only rarely present, whereas Zone 1 assumes it is likely in normal operation. A Zone 2 device does not provide the protection level required for Zone 1, so using one there is a compliance violation and a safety hazard.
What is the difference between Ex d and Ex ia protection?
Ex d (flameproof) contains an internal explosion inside a heavy enclosure and cools the escaping gases. Ex ia (intrinsic safety) limits circuit energy so that no ignition-capable spark can exist in the first place. Ex d is typical for mechanical switches with normal contacts; Ex ia is used for low-energy sensor circuits wired through an IS barrier and is the only approach allowed in Zone 0.
How do I choose the correct temperature class?
The temperature class (T1–T6) must keep the switch's maximum surface temperature below the auto-ignition temperature of the gas or dust present. Determine the gas group and ignition temperature from the area classification, then select a T-class with adequate margin — T6 (85 °C) is the safest and is required for hydrogen and other easily ignited substances.
Conclusion
An explosion-proof switch is a compliance component before it is a control component. The correct selection is a chain: confirm the certification system, classify the area, pick the protection method, match the gas group and temperature class, and only then specify the mechanical function. Cut any link and the certification — and the protection — collapses. Schmersal's EX range covers the full chain from flameproof position switches to intrinsically safe sensors, and matching the right model to your area classification is the difference between a compliant installation and an avoidable hazard.