An explosive atmosphere can be one in the form of gases, vapours, mists or dusts, which can ignite under certain operating conditions. Potentially explosive atmospheres are found in many areas of industry, from mines and the chemical, oil and gas, and pharmaceuticals industries, to plants handling cereal, animal feed, paper, wood and coal. All these have the potential to produce gas, dust or fumes which can be ignited by a spark or flame. The classification of the potentially explosive atmosphere depends on the frequency with which potentially explosive atmosphere may exist and the capability of the gas or dust to create an explosion. Explosive gas and dust is classified according to its likelihood to be ignited, according to its characteristics: Minimum ignition energy; Minimum ignition temperature; Auto-ignition temperature; and Layer ignition temperature.
This week we look at EEx definitions used within ATEX specifications for fans and motors:
Flameproof, for Zone 1. The enclosure of this motor type will prevent an internal explosion, or flame, from being transmitted to the explosive atmosphere surrounding the machine, hence the name flameproof. The enclosure must withstand any pressure levels caused by such an internal explosion. Its design includes a flamepath, a route for exploding gas to escape by whilst cooling off. The motor also has flameproof joints with long spigots that form part of the flamepath. Every cast iron part must be pressure tested to ensure explosions will be contained. The designation under ATEX for this type of motor is EExd, which supersedes the earlier designation Exd.
A flameproof motor with the terminal box of an increased safety motor, combining the superior safety of a “d” protection with the less stringent electrical connection requirements of Eexe motors.
Increased safety motors, for Zone 1. Prevent sparks, arcs or hot spots during service, including starting, by a number of constructional or dimensional provisions, and by the use of special protection devices, designed to trip within a specified time. To reduce the temperature rise, this type of motor typically has a special winding that effectively de-rates the motor; for instance a motor that normally delivers 5.5 kW output may only deliver 5 kW in its increased safety design. This type of motor has never been widely used in the UK, although it is very popular on the continent, where it is used both as an alternative to non-sparking, as well as an economical alternative to flame proof. The type designation under ATEX for this type of motor is EExe, which supersedes the designation Exe. Increased safety motors are designed for Zone 1, but can also be used in Zone 2.
The lowest level of Ex protection for motors is non-sparking, so called because the design aims to prevent an internal malfunction from creating sparks in normal operation; it also aims to prevent excessive heating during starting, and the EHSR (Essential Health & Safety Requirements) introduced by ATEX now make this design safer against the risk of sparks during starting. These motors can also be fitted with thermistors or other thermal devices that cut the power if the temperature rises above certain limits. This level of protection provides an economical compromise between high safety requirements and normal industrial standards. They are for use in zone 2 only, i.e. for areas where explosive atmospheres occur occasionally, but not during normal duty. The designation for this type of motor under ATEX is EExnA, which supersedes the previous descriptions used, ExnA and ExN. The letter “n” stands for non-sparking equipment according to EN50021.
Large AC machines are available with a pressurised enclosure that can be purged and filled with an inert gas to prevent an explosion. They meet the most stringent safety requirements for hazardous areas in the chemical and mineral oil industries.
Axair Fans supply a wide range of ATEX fans for use in hazardous environments, for more information view our technical page on ATEX Fans.
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