Components that create an explosion: combustible materials, oxygen, ignition source
An explosion is an exothermic reaction with a sudden release of a large amount of energy, resulting in a strong blast wave.
An explosion occurs when a combustible material, oxygen, and an ignition source are present. If any one component is missing, no exothermic reaction occurs.
Various explosion protection measures exist to safely prevent explosions. While primary explosion protection prevents the presence of the explosive atmosphere, with secondary explosion protection an explosion is safely prevented by avoiding effective ignition sources by design. For example, effective ignition sources include electric sparks and hot surfaces of electrical equipment or electric arcs, static electricity, etc. Depending on the application, various types of protection with different protection principles exist for secondary explosion protection. There is also tertiary explosion protection. This involves design measures that reduce the consequences of an explosion to an acceptable level.
The following overview focuses on secondary explosion protection. When selecting devices, according to the types of protection, the user must analyze the type of danger and the duration and frequency of the danger. The type of danger is determined by examining the ignition source based on the ignition temperature and ignition energy. In the case of the duration and frequency of the danger, the relationship between zone classification and the equipment category and equipment protection level is established.
Devices for use in Group II (gases) potentially explosive atmospheres are divided into temperature classes (T1 - T6), which indicate the maximum device surface temperature that may occur. The surface temperature is valid for all parts of an item of electrical
equipment that can come into contact with the explosive gas mixture.
A device may only be used if the temperature assigned to the temperature class is lower than the ignition temperature of the gas mixture. Devices marked with a higher temperature class, i.e., lower surface temperature, may also be used with gas mixtures with a higher ignition temperature. A device with T6 marking may therefore be used for all gases.
For dusts, no distinction is made between temperature classes (T1 - T6). Devices for Group III (dusts) are marked with a maximum surface temperature. Additional dust deposits must be taken into account. You will find more information about temperature limits in the principles of explosion protection (see download).
| Maximum surface temperature in °C | Ignition temperature range in °C | Examples | |
|---|---|---|---|
| Temperature class | |||
| T1 | 450 | >450 | Carbon monoxide, methane, hydrogen |
| T2 | 300 | >300 ... ≤450 | Acetylene, butane, ethylene |
| T3 | 200 | >200 ... ≤300 | Gasoline, hydrogen sulfide, cyclohexane |
| T4 | 135 | >135 ... ≤200 | Acetaldehyde, diethyl ether (no other substances) |
| T5 | 100 | >100 ... ≤135 | No substances |
| T6 | 85 | >85 ... ≤100 | Carbon disulfide |
Devices for use in Group II (gases) and III (dust) potentially explosive atmospheres are divided into groups (IIA - IIC, IIIA - IIIC), which indicate the maximum ignition energy of the device.
A device may only be used if the maximum ignition energy of the device is lower than the minimum ignition energy of the substance (gas or dust mixture). A device with IIC marking may be used for all gases and a device with IIIC marking may be used for all dusts.
Gases are more explosive than dusts, e.g., devices with IIB are suitable for IIIC.
| Required ignition energy | Danger from substances | Examples of substances | |
|---|---|---|---|
| Group | |||
| IIA | +++ | + | Acetone, ethane, ammonia, carbon monoxide. Propane, butane, gasoline, diesel fuel, acetaldehyde |
| IIB | ++ | ++ | Methane, ethylene, ethyl ether, ethyl alcohol, hydrogen sulfide |
| IIC | + | +++ | Hydrogen, acetylene, carbon disulfide |
| IIIA | +++ | + | Combustible flyings, e.g., cotton fibers |
| IIIB | ++ | ++ | Non-conductive dust, e.g., flour, wood |
| IIIC | + | +++ | Conductive dust, e.g., aluminum dust |
Zone classification is the responsibility of the plant operator. It is regulated in Europe via the ATEX Directive 1999/92/EC (Workplace Directive). Furthermore, the operator must also comply with the requirements of IEC/EN 60079-10.
Equipment categories and equipment protection levels describe the safety level and the protection level of devices. Which devices are suitable for which zone is shown in ATEX Directive 2014/34/EU via the equipment category (categories 1, 2, 3). In standard IEC/EN 60079-0, the applicability of the devices for the respective zones is described via the equipment protection level (EPL: a, b, c). For both the equipment category marking and the equipment protection level, the G stands for “gas” and the D for “dust”. For example, in gas applications, a device marked 1G and Ga, i.e., with a very high level of safety and protection, is suitable for hazardous zone 0 and therefore also for all other gas zones. Structurally, this device is designed to still be safe in the event of two independent faults.
| Safety level or protection level |
Equipment category
ATEX Directive 2014/34/EU |
Equipment protection level (EPL)
Standard IEC/EN 60079-0 |
|
|---|---|---|---|
| Zone classification | |||
| Directive 1999/92/EC | |||
| Gas zones | |||
| Zone 0 | Very high – the device is still safe in the event of two independent faults | 1G | Ga |
| Zone 1 | High – the device is still safe in the event of a single independent fault | 2G | Gb |
| Zone 2 | Normal – not failsafe | 3G | Gc |
| Dust zones | |||
| Zone 20 | Very high – the device is still safe in the event of two independent faults | 1D | Da |
| Zone 21 | High – the device is still safe in the event of a single independent fault | 2D | Db |
| Zone 22 | Normal – not failsafe | 3D | Dc |
In Europe, in addition to the marking in accordance with standard IEC/EN 60079-0, there must also be a marking in accordance with ATEX Directive 2014/34/EU.
The ATEX Directive requires that equipment be classified into groups and categories. Equipment group I includes mining equipment liable to be endangered by firedamp and/or dust. Equipment group II includes all other Ex areas not related to mining, such as the chemical industry, oil and gas processing, potentially explosive dusts in mills and silos, etc.
The marking for the IEC/EN 60079 series of standards includes information about the type of protection, the gas and dust groups, the temperature class, and the equipment protection level (EPL).
II – equipment group: I = mining | II = all non-mining Ex areas
1 – equipment category: 1 = suitable for zone 0/zone 20
G – atmosphere: G = gas
i – type of protection: i = intrinsic safety
a – protection level: a = suitable for zone 0/zone 20
II – groups: II = gas
C – subgroups: C = hydrogen, acetylene, carbon disulfide
T6 – temperature class: T6 = 85°C
Ga – equipment protection level: G = gas | a = suitable for zone 0
II – equipment group: I = mining | II = all non-mining Ex areas
1 – equipment category: 1 = suitable for zone 0/zone 20
D – atmosphere: D = dust
i – type of protection: i = intrinsic safety
a – protection level: a = suitable for zone 0/zone 20
III – groups: III = dust
C – subgroups: C = conductive dust, e.g., aluminum dust
125°C – max. surface temperature: 125°C = max. surface temperature of 125°C
Da – equipment protection level: D = dust, a = suitable for zone 20
II – equipment group: I = mining | II = all non-mining Ex areas
1 – equipment category: 1 = suitable for zone 0/zone 20
( ) – brackets: Associated equipment (e.g., an intrinsically safe signal conditioner – located in the safe area, but the Ex i circuit is routed into the Ex area)
G – atmosphere: G = gas
i – type of protection: i = intrinsic safety
a – protection level: a = suitable for zone 0/zone 20
Ga – equipment protection level: G = gas, a = suitable for zone 0
[ ] – brackets: Associated equipment (e.g., an intrinsically safe signal conditioner – located in the safe area, but the Ex i circuit is routed into the Ex area)
II – groups: II = gas
C – subgroups: C = hydrogen, acetylene, carbon disulfide
II – equipment group: I = mining | II = all non-mining Ex areas
1 – equipment category: 1 = suitable for zone 0/zone 20
( ) – brackets: Associated equipment (e.g., an intrinsically safe signal conditioner – located in the safe area, but the Ex i circuit is routed into the Ex area)
D – atmosphere: D = dust
i – type of protection: i = intrinsic safety
a – protection level: a = suitable for zone 0/zone 20
Da – equipment protection level: D = dust, a = suitable for zone 20
[ ] – brackets: Associated equipment (e.g., an intrinsically safe signal conditioner – located in the safe area, but the Ex i circuit is routed into the Ex area)
III – groups: III = dust
C – subgroups: C = conductive dust, e.g., aluminum dust
Principle:
Ignition source possible, explosion safely controlled
Protection concept:
Solid housing construction, type tests
Area of application:
Motors, switching devices, power electronics
Principle:
Safe exclusion of ignition sources
Protection concept:
No operational ignition sources, exclusion of malfunction by special measures
Area of application:
Motors, transformers, luminaires, terminal blocks, busbar boxes
Principle:
Safe exclusion of an explosive atmosphere
Protection concept:
Keep housing below overpressure with ignition protection gas
Area of application:
Machines, motors, control cabinets (electronics)
Principle:
Ignition source present, but safely controlled
Protection concept:
Embedding the equipment in molding compound, mineral oil, or sand
Area of application:
Electronics, transformers, capacitors, relays
Principle:
Sparks or thermal effects possible, but not ignitable
Protection concept:
Safe energy limitation taking into account faults and energy storage effects
Area of application:
Measurement and control technology, sensors, actuators, instrumentation
Simplification of all types of protection for the reduced requirements of zone 2/zone 22. No EU type test certificate is required. The following types of protection are available:
Intrinsic safety (Ex i) type of protection has become established worldwide in the field of measurement and control technology in systems with potentially explosive areas.
Intrinsically safe circuits are generally composed of the following elements:
The protection principle behind the Ex i type of protection is based on limiting the energy that is conducted to the potentially explosive area and is stored there. This means that the energy from any potential spark must always be less than the minimum ignition energy of the surrounding potentially explosive atmosphere.
To ensure that the respective connection cannot produce incendive sparks or hot surfaces, the user or system operator has to demonstrate and document “proof of intrinsic safety” in accordance with EN/IEC 60079-11 and construction standard EN/IEC 60079-14. This process offers the user the advantage of being able to select and combine Ex i field devices and Ex i isolators from different manufacturers in accordance with the specific requirements.
As opposed to all the other types of protection, Ex i refers not only to a single item of equipment but also to the entire intrinsically safe circuit, in accordance with IEC/EN 60079-11.
Advantages of “Ex i – intrinsic safety” type of protection:
