The IEC Ex scheme is the first international certification scheme in the world and certificates issued under this scheme will be accepted by all member countries which include UK, France, Germany, Australia, Canada and USA.

Australian Standards are following IEC Standards with Australia being a member country of the IEC Ex certification scheme.

Selection of Electrical equipment

In order to select electrical apparatus for use in hazardous areas it is necessary to have the following information:

1. The Area classification
2. The Apparatus or Gas group
3. The “T” rating or the ignition temperature of the hazard
4. The required IP rating of the apparatus

1. Area Classification

The areas are classified into ZONES, and these zones are based on the frequency of the appearance of an explosive atmosphere and the duration for which it can last.

Summarized as follows:

Gases and vapours

2. Apparatus or Gas Group

Gases and vapours are divided into two groups:

3. Temperature or “T” rating

The source of ignition in a hazardous area can be an arc or a spark or even a hot surface.

Since electrical equipment generates heat in normal operation we need to ensure that a hot surface on electrical apparatus cannot ignite a surrounding explosive gas atmosphere or a dust cloud or layer.

All hazardous materials have what is known as an “Ignition Temperature” , This is the minimum temperature at which the hazardous material when mixed with air will ignite and sustain combustion, without an ignition source (auto-ignition or spontaneous ignition).

“T” rating for T6 rated product will not exceed 85ºC and a T3 will not exceed 200ºC etc while in service in hazardous areas.

Selection of T Rating for Hazardous Dusts

Dusts pose additional problems when dealing with temperature. Dust can settle on equipment and ledges to form layers.

Because of this we have to consider dusts in two forms:

1) As a cloud mixed with air
2) As layers settled on surfaces

Dust as a cloud and Dust in layers will have different Ignition Temperatures and we need to consider the lowest of these two.

Dust in the form of a layer on electrical equipment can impair heat dissipation and cause increased temperatures to the equipment.

Temperature increase as a result of layers could also cause the dust to dry out and give rise to spontaneous combustion of the dust in the layers.

On the other hand a disturbance of the dust would cause it rise and mix with air to form an explosive cloud. The pressure wave from a relatively small disturbance and resultant explosion would give rise to a much larger disturbance and probably a much larger and more damaging explosion.

a) for Dust Clouds
You must select equipment with a T rating that is 2/3rds that of the ignition temperature of the dust hazard.

eg if the ignition temperature of the hazardous dust is 150ºC then the equipment T rating must not exceed 100ºC or T5

b) for Dust Layers
You must select equipment with a T rating that is 75ºK less than the ignition temperature of a layer on the surface of the equipment.

eg if the ignition temperature of the dust layer is 275 C then the equipment T rating must not exceed 200ºC or T3.

The thickness of dust layers is critical to the whole operation and this is the main reason for good housekeeping. If the layers are going to be more than 5mm then testing must be carried out on the individual hazardous dust at the specified thickness to obtain the layer ignition temperature at that layer thickness. There are standards available to carry out these tests.

4. The Ingress Protection or “IP” rating

Ingress of moisture or other material could affect electrical equipment and cause it to break down electrically and possibly cause arcs and sparks which could be possible sources of ignition.

For example Ex d or flameproof enclosures are not weatherproof in their own right and need to be rated for Ingress protection if they are used outdoors. Other techniques such as Ex e increased safety must have a minimum of IP54 but the ingress of moisture could cause insulation breakdown which would be dangerous in hazardous areas.

Types of Protection

In areas where explosive atmospheres can occur despite the explosion protection measures employed, only explosion-protected electrical equipment may be used.

Explosion protected electrical equipment can be manufactured to IEC protection type levels.

Types of protection for electrical apparatus in hazardous areas