Electrical apparatus in gaseous potentially explosive atmospheres
Electronics Engineering
Electrical apparatus can be installed in potentially explosive atmospheres only if they have been designed and manufactured in accordance with the ATEX types of protection required by the standards IEC 60079-xx, and if they have been issued a compliance certificate by a notified laboratory (Two laboratories in France : INERIS, LCIE).
The ATEX types of protection aim at mastering the risks in normal use and in the event of faults, so to prevent the ignition of an explosive atmosphere because of a high temperature surface (risk due to heating effects), an electrical spark (risk due to sparking), an electrostatic discharge, and all the conditions and faulty situations being able to lead to these events.
Four areas are defined depending on the probability of presence of a potentially explosive atmosphere : from "Zone 0" (strong probability or constant presence) to "Non-hazardous Area" (normally no probability), and the two intermediate areas "Zone 1" and "Zone 2". In the "Non-hazardous Area", no ATEX type of protection is required ; it is for instance our neutral and daily space of life, described as "not ATEX".
The applicable ATEX types of protection depend on the Zone where the apparatus is installed, the most restrictive being required for "Zone 0". An apparatus which is certified for a given Zone can never be installed in a more restrictive Zone ; the opposite possibility being implicit. A part of an electrical system can be installed in one Zone while another part is in another Zone, as for example a sensor in "Zone 0" and the acquisition system in "Zone 1", or even in "not ATEX". In this situation, the different parts of the ATEX system use the ATEX types of protection applicable to the Zones where they are installed, and those which are required for the electrical connections between the Zones.
According to the gases or vapours likely to be encountered, the apparatus is assessed for a Group of gas. The standards define three Groups of gas with increasing sensitivity to the energy involved by a spark : Group IIA, Group IIB, Group IIC. Group IIB is commonly used, while Group IIC includes 4 additional gases (or reaction equivalent) as highly reactive as hydrogen.
As all the gases do not ignite at the same temperature, six temperature classes from T1 to T6 define the maximum permissible surface temperature, with T6 being the most restrictive. The temperature classification determines the maximum temperature being able to be reached by any surface in contact with the explosive atmosphere, case or package of electronic components, under the most critical faults and components failures, at the higher operating temperature the apparatus is specified for. For small-sized components, the standards can however also allow higher surface temperatures than the raw limitations given below.
Group IIA
Group IIB
Group IIC
T1 : 450°C
Methane, Propane, Ammonia, ...
Hydrogen
T2 : 300°C
Butane, ...
Ethylene, ...
Acetylene
T3 : 200°C
Kerosene, ...
Hydrogen sulfide, ...
T4 : 135°C
Diethyl ether, ...
T5 : 100°C
T6 : 85°C
Carbon disulfide, Ethyl nitrate
Table with some common gases and vapours
In the matter of ATEX designing, the principle of "he who can do more can do less" can be a big mistake. For instance, it is indeed far more delicate to design an equipment for Group IIC than for Group IIB. In the same way, concerning the constraints related to the temperature classification, it is far more delicate to design an equipement intended for Class T6 than class T4.
Therefore, it is strongly recommended to very carefully determine from the beginning, the Zone of installation, the operating temperature range, the Group of gas and the temperature class, according to the actual use of the apparatus, to avoid any complication in design and ATEX protection which could be as useless as expensive.
In practice and except specific cases, a development for the context IIB T4 is rather classic, and is suitable for the great majority of electrical equipments intented to operate in an ATEX area.
