Ex protected equipment for hazardous locations

While national regulations exist in some countries, there is an urgent need for an international approach to curb explosion-related industrial accidents wherever workers and communities are exposed explosion risk

All workers in hazardous locations must be declared competent to express their opinions in compliance with the Electrical Machinery Regulation (EMR). Human error or incompetency has been found to be at least one part of the domino effect in major disasters.

Fig. 1: Fire triangle.

Fig. 1: Fire triangle.

All hazardous locations must comply with the Occupational Health and Safety (OHS) Act, as well as with the relevant codes of practice and standards. Section 9 of the Electrical Machinery Regulation (EMR) requires that all hazardous areas be classified and equipment used in such areas be certified. All explosion protective equipment (EPE) must be certified by a locally-approved test laboratory (ATL), as required by regulation ARP 0108. Workers in hazardous locations must receive proper Ex training and be declared competent.

A fire or explosion can only occur if three things are present: fuel, oxygen and an ignition spark. The required ratio of fuel and oxygen varies with each fuel (gas or vapour). Suppressing or separating one or more of these three components can avoid a fire or explosion.

Step 1

Establish if it is a hazardous area due to the presence of explosive gas or dust and divide the area into zones of risk-of-release of gas, vapour or dust. The process of determining the type and size of these hazardous zones is called area classification. The guidance on assessing the extent of the hazard is given in the current edition of IEC/SANS 60079-10-1 for gas zones. The applicable standard for dust zones is IEC/SANS 60079-10-2 and the dust zones consist of zones 20; 21 and 22. Area classification takes into account gas groups and temperature classes.

Table 1: EPL’s and Zones

Table 1: EPLs and zones.

Equipment protection level (EPL):

Fig. 2: Typical zone classification. Note: Zone 0 is the most severe zone and requires the highest level of protection – EPL Ga.

Fig. 2: Typical zone classification. Note that zone 0 is the most severe zone and requires the highest level of protection – EPL Ga.

An explosion occurs when an ignition source is in contact with the explosive atmosphere. In very high-risk areas like zones 0 and 20 (see table 1), in which the risk of explosion is caused by the presence of explosive gas or dust, the equipment installed must have very high levels of protection. In low risk areas such as zone 2 for gas and zone 22 for dust, you can use equipment with lower levels of protection (see Table 1). Risk analysis can be done by means of a risk assessment according to SANS 60079-10-1 and SANS 60079-10-2.

 

 

 

Step 2

Gas groups

Once it has been established which gas or dust is present, different instructions must be followed, depending in whether it is underground or above ground.

Hazardous areas are defined by the type of hazard (e.g. gas, vapour or dust); the likelihood of the hazard being present in flammable concentrations and determining the lower explosion limits (LELs) and upper explosion limits of the flammable substance (UEL). If the mixture between the LEL and UEL is explosive, it is called the explosion range. The (auto) ignition temperature of the hazardous material must not exceed the ignition temperature of the flammable gas or vapour. It is important to note that each gas has its own LEL and UEL (See Table 2).

 

Table 2: Flammable Properties.

Table 2: Flammable properties.

Gas groups

Electrical equipment used in gas atmospheres is divided into two groups; group I is equipment used in mines with atmospheres containing methane or gases and vapours of equivalent hazard; group II is all other equipment. This group is further subdivided into three subgroups: group IIA for atmospheres containing propane, or gases and vapours of equivalent hazard; group IIB for atmospheres containing ethylene, or gases and vapours of equivalent hazard and group IIC, which includes atmospheres containing hydrogen or gases and vapours of equivalent hazard.

Table 3: Gas groups.

Table 3: Gas groups.

Dusts and fibres (group III)

Group III is subdivided into three sub-groups (see Table 4).

Table 4: Dust groups.

Table 4: Dust groups.

Step 3

Temperature classification – group II (gas)

Temperature class of the selected equipment may not exceed the ignition temperature of any gas or explosive atmosphere present. It is easier to decide on the appropriate type of electrical apparatus required when you the know ignition temperature of the explosive atmosphere.

The maximum surface temperature of electrical or mechanical apparatus must always be lower than the ignition temperature of the surrounding explosive atmosphere. Temperature class is not the operating temperature range of the equipment, but the maximum permissible surface temperature, in relation to an ambient temperature of 40°C or higher. All flammable gases and vapours are placed in a gas group and a temperature class as per Table 7.

Table 5: Temperature classification.

Table 5: Temperature classification.

 

Table 6: Combustible Gas Ignition temperatures.

Table 6: Combustible gas ignition temperatures.

Group III (dust)

Dust and fibres are also defined in terms of their ignition properties including dust cloud ignition properties. The user must take both the cloud and layer temperatures into consideration when selecting Ex equipment. Selection will always follow the area classification.

After assessment of the installation, the user should consider the following criteria when selecting electrical equipment:

  • Maximum permissible surface temperature of the equipment, taking into consideration the type of dust.
  • The ignition temperature of the dust cloud.
  • If dust deposits cannot be excluded, the ignition temperature of the dust layer.

Table 7: Ignition temperatures for common flammable dusts and fibers.

Table 7: Ignition temperatures for common flammable dusts and fibres.

Step 4

EMR9(2) selection of explosion protected equipment (EPE)

Selection of EPE will always follow the area classification. The type of explosion protection of the equipment is to be suitable for the zone of use. All EPE must be covered by an approved inspection authority (AIA) certificate issued by an accredited ATL to comply with the applicable standards. The selection of suitable apparatus plays an important role when setting up a plant in hazardous zones. A greater effort must be made to avoid hot surfaces to or prevent equipment from generating ignitable sparks.

Typical example and interpretation of an Ex marking:

Ex d I/IIC T3 Gb

  • Ex – Explosion-proof electrical equipment.
  • d – Type of protection: “flameproof enclosure”.
  • I – Gas group I, only for mines.
  • II – Gas group II, surface industries.
  • C – Gas sub-group = C.
  • T3 – Temperature class T3 (300C surface temperature).
  • Gb – Equipment protection level (EPL).

Table 8: Protection Techniques and Methods.

Table 8: Protection techniques and methods.

Step 5

Ingress protection (IP) ratings

Ingress protection of the equipment must be suitable for prevailing conditions at the point of use. The IP rating of Ex equipment is of utmost importance as we do not want any foreign objects or a flammable substance to penetrate the Ex enclosure. IP rating is the resistance offered by the fixture to the penetration of solids and liquids. It is indicated by the IP rating, which is a two-digit number:

  • The first number identifies the degree of protection against the ingress of solids.
  • The second number against liquids.
  • IP65 indicates total protection against dust and protection against low-pressure jets of water.

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Step 6

Ambient temperature rating

Ambient temperature rating of the equipment must cover the full range of possible temperatures in the area of use. If the marking (or the certificate) of the equipment does not include an ambient temperature range, this equipment should only be used within the ambient range from –20°to +40°C.

People are the most valuable asset and the industry must ensure a safe work environment in accordance with Section 8 and EMR 9 as stipulated in the latest issue of the OHS Act and Regulations (85 0f 1993).

Acknowledgement

This paper was presented at the IDC Safety Control Systems & Hazardous Areas Conference 2015, and is published here with permission.