1 Scope
This part of GB 12476 specifies general requirements, additional to those required for basic electrical safety, for the selection of electrical apparatus and instruments and associated equipment, and for the installation of electrical apparatus to ensure safe use in areas where combustible dust may be present in quantities which could lead to a fire or explosion hazard.
Note: Various parts of the GB 12476 series specify requirements for the design, construction and testing of electrical apparatus. Apparatus within the scope of this standard may also be subjected to additional requirements in other standards.
The application of electrical apparatus in atmospheres which may contain explosive gas as well as combustible dust, whether simultaneously or separately, requires additional protective measures which are not within the scope of this standard.
This standard includes several types of protection that protect either from ingress of dust or have insufficient energy to cause ignition and provide surface temperature limitation.
The principles of this standard may also be followed when combustible fibres or flyings cause a hazard.
Where the apparatus is required to meet other environmental conditions, for example, protection against ingress of water and resistance to corrosion, additional methods of protection may be necessary. The method used should not adversely affect the integrity of the enclosure. The requirements of this standard apply only to the use of electrical apparatus under normal or near normal atmospheric conditions. For other conditions, additional precautions may be necessary. For example, most flammable materials and many materials which are normally regarded as non-flammable might burn vigorously under conditions of oxygen enrichment. Other precautions might also be necessary in the use of electrical apparatus under conditions of extreme temperature and pressure. Such precautions are beyond the scope of this standard.
This standard does not apply to dust from explosives which do not require atmospheric oxygen for combustion; neither does it apply to pyrophoric substances.
This standard is not applicable to electrical apparatus intended for use in underground parts of mines, nor those parts of surface installations of such mines endangered by firedamp and/or combustible dust.
This standard does not take account of any risk due to an emission of flammable or toxic gas from the dust.
The requirements specified in this standard are supplementary to and not alternative to any requirements that would apply to apparatus and installations in non-hazardous areas.
Contents
Foreword
Introduction
1 Scope
2 Normative references
3 Terms and definitions
4 General requirements for all types of protection
4.1 Access for inspection
4.2 Associated equipment located in non-hazardous areas
4.3 Segregation
4.4 Electrical rating
4.5 Protection from mechanical damage and environmental injury
4.6 Documentation
4.7 Qualifications of personnel
4.8 Additional considerations
4.9 Prevention of explosion
5 Area classification
6 Selection of electrical apparatus
6.1 Permitted apparatus
6.2 Selection according to dust characteristics and zones
6.3 Selection according to temperature
6.4 Selection of apparatus
7 Protection from dangerous (incendive) sparking
7.1 Danger from live parts
7.2 Danger from exposed and extraneous conductive parts
7.3 Potential equalization
8 Electrical protection
8.1 General
8.2 Location of protection and control equipment
8.3 Resetting of short-circuit and earth-fault protection devices
8.4 Protection of rotating electrical machinery
8.5 Loss of supply phase
8.6 Transformer protection
9 Emergency switch-off and electrical isolation
9.1 Emergency switch-off
9.2 Electrical isolation
10 Wiring systems
10.1 General
10.2 Entries for cable and conduit
10.3 Accessories
10.4 Course of wiring
10.5 Barriers
10.6 Flexible connections
10.7 Cable entries
10.8 Unauthorized methods
10.9 Telecommunications circuits
10.10 Unused openings
11 Plugs and socket outlets
11.1 General
11.3 Location
12 Additional requirements for type of protection "Ex tD"
12.1 Practices A and B
12.2 Practice A
12.3 Practice B
12.4 Motors supplied at varying frequency and voltages
13 Additional requirements for type of protection "pD"
13.1 Sources of protective gas
13.2 Automatic switch-off
13.3 Alarm
13.4 Common source of protective gas
13.5 Switching on electrical supply
13.6 Motors supplied at varying frequency and voltages
14 Additional requirements for type of protection "iD"
15 Additional requirements for type of protection "mD"
16 Inspection of installation
16.1 General
16.2 Testing
Appendix A (Informative) Examples Of Dust Layers Of Excessive Thickness
Appendix B (Normative)Frictional Sparking Risks With Light Metals And Their Alloys
