1 Scope
This part of GB 12476 gives requirements on the design, construction, testing and marking of electrical apparatus for use in combustible dust atmospheres in which a protective gas (air or inert gas), maintained at a pressure above that of the external atmosphere, is used to prevent the entry of dust which might otherwise lead to the formation of a combustible mixture within enclosures which do not contain a source of combustible dust.
This standard contains the specific requirements for construction and testing, including protective requirements that apply to electrical apparatus with type of protection pressurization "pD" intended for use in potentially combustible dust atmospheres.
This Part includes the requirements for the construction of the enclosure and its associated components, including, if any, the inlet and outlet ducts for the protective gas, and for the safety provisions and devices necessary to ensure that pressurization is established and maintained for type of protection pressurization "pD".
Requirements for pressurized enclosures with an internal source of dust release are not included in this Part.
This Part does not cover the requirements for pressurized rooms with or without internal sources of dust release.
This Part does not apply to dusts of explosives which do not require atmospheric oxygen for combustion or to pyrophoric substances.
This Part does not cover combined gas and dust hazard.
2 Normative references
The following normative documents contain provisions which, through reference in this part, constitute provisions of this part of GB 12476. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this part are encouraged to investigate the possibility of applying the most recent editions of the normative documents listed below. For undated references, the latest edition of the normative document referred to applies.
GB 755-2008 Rotating electrical machines - Rating and performance (IEC 60034-1:2004, IDT)
GB 12476.2-2010 Electrical apparatus for use in the presence of combustible dust - Part 2: Selection and installation (IEC 61241-14: 2004, IDT)
IEC 61241-0: 2004 Electrical apparatus for use in the presence of combustible dust - Part 0: General requirements )
3 Terms and definitions
For the purpose of this part 4, the definitions from of IEC 61241-0:2004 and the following apply.
3.1
“pD”type of protection“pD”
technique of applying a protective gas to an enclosure in order to prevent the formation of an explosive dust atmosphere inside the enclosure by maintaining an overpressure against the surrounding atmosphere
3.2
overpressure
pressure above ambient pressure within a pressurized enclosure
3.3
pressurization
technique of guarding against the ingress of the external dust atmosphere, which may be explosive, into an enclosure by maintaining a protective gas therein at a pressure above that of the external atmosphere
3.4
protective gas
air or inert gas used for maintaining an overpressure
Note: For the purpose of this part inert gas means nitrogen, carbon dioxide, argon or any gas which, when mixed with oxygen in the ratio of 4:1 as found in air, does not make the ignition and combustible properties, such as combustible limits, more onerous.
3.5
enclosure
all the walls which surround the live parts of electrical apparatus including doors, covers, cable entries, rods, spindles and shafts and which ensure the protection of the electrical apparatus
3.6
pressurized enclosure
enclosure in which protective gas is maintained at a pressure greater than that of the external atmosphere
Note: The pressurized enclosure may or may not also be the enclosure surrounding the live parts and ensuring the protection in other respects of the electrical apparatus.
3.7
static pressurization
maintenance of an overpressure within a pressurized enclosure without the addition of protective gas in a hazardous area
3.8
pressurization with leakage compensation
maintenance of an overpressure within a pressurized enclosure so that, when the outlet apertures – if any – are closed, the supply of protective gas is sufficient to compensate for any leakage from the pressurized enclosure and its ducts
3.9
pressurization with continuous flow of the protective gas
maintenance of an overpressure within a pressurized enclosure with continuous flow of the protective gas through the enclosure
3.10
electrical apparatus
items applied as a whole or in part for the utilization of electrical energy. These include, among others, items for the generation, transmission, distribution, storage, measurement, regulation, conversion, and consumption of electrical energy and items for telecommunications
3.11
ignition-capable apparatus
apparatus which, in normal operation, constitutes a source of ignition for a specified combustible dust atmosphere. This includes electrical apparatus not protected by a type of protection
3.12
self-revealing fault
fault which would cause a malfunction of the apparatus necessitating correction before proceeding with further operation of the apparatus and which may be indicated, for example, by an audible or visible signal
3.13
opening
aperture, door, window or non-airtight fixed panel
3.14
protective device
device provided to protect a system against conditions which could result in a fire or explosion
3.15
alarm
piece of apparatus that generates a visual or audible signal that is intended to attract attention
3.16
indicator
piece of apparatus that shows flow or pressure and is monitored periodically, consistent with the requirement of the application
3.17
protected apparatus
electrical apparatus internal to the pressurized enclosure
3.18
pressurization system
grouping of components used to pressurize and monitor a pressurized enclosure
3.19
alternate(or auxiliary)source of supply of protective gas
second source of protective gas that may take over in the event of failure of the primary source
3.20
zones
classified areas are divided into zones based upon the frequency and duration of the occurrence of explosive dust/air mixtures.
3.21
zone 20
area in which combustible dust, as a cloud, is present continuously or frequently, in sufficient quantity to be capable of producing an explosive concentration of combustible dust mixed with air
3.22
zone 21
area in which combustible dust, as a cloud, is likely to occur during normal operation, in sufficient quantities to be capable of producing an explosive concentration of combustible dust mixed with air
3.23
zone 22
areas in which combustible dust clouds may occur infrequently, and persist for only a short period, during normal operation
Foreword I
1 Scope
2 Normative references
3 Terms and definitions
4 Pressurization principle
5 General constructional requirements
6 Temperature limits
7 Safety provisions and safety devices (except for static pressurization)
8 Safety provisions and safety devices for static pressurization
9 Supply of protective gas
10 Verification and tests
11 Marking
Annex A (normative) Ducts for protective gas supply
Figure A.1 Examples of the static overpressure along the ducts and through a pressurized enclosure
Figure A.2 Example of the static overpressure in a pressurized electric rotating machine with fan
Table 1 Requirements on failure of pressurization
