Application of electrical fire restrictive protector in places where combustible dust is hazardous

Abstract: The article elaborates the characteristics of combustible dust and the hazards of dust explosion in combustible dust environment, combined with the requirements of the current national electrical product specifications, through the electrical design example of the dust explosion hazard area inside the extraction workshop, the system expounds the zoning principle of combustible dust hazardous environment, the selection of electrical equipment, the design of line short-circuit arc extinguishing, the lightning protection design, the design of lightning protection and anti-static electricity, etc.
, and finally puts forward the requirements for continuously using new technologies and new products to improve the design level of combustible dust dangerous places
。
Keywords: flammable dust; Electrical equipment; High surface temperature; Lightning protection and anti-static design; Electrical fire restrictive protector; 1 Overview The so-called combustible dust environment refers to an environment in which dust or a mixture of fibrous flammable substances and air is ignited under atmospheric conditions, and combustion will be transmitted to all unburned mixtures
.

With the development of modern industrial technology, the dangerous places of combustible dust are increasing, and its hazards have become inevitable, and corresponding dust explosion accidents have occurred
from time to time.

On March 15, 1987, an extra-large flax dust explosion accident occurred in the Harbin Linen Textile Factory in China, causing huge personal injuries and property losses
.

Therefore, the electrical design in this combustible dust environment, not only in strict accordance with the national design standards, specifications requirements, the correct selection of dust explosion-proof electrical equipment, but also combined with the current new technology, the use of new products for line arc protection, of which the electrical fire restrictive protector is a more reasonable line short-circuit arc extinguishing products
.

2 Relevant standards and specifications In the electrical design of combustible dust hazardous environments, the national norms and industry specifications that can be referred to are: (1) "Electrical equipment for combustible dust environments (Part (1)): Electrical equipment protected with enclosures and limited surface temperature" (GB 12476.
1-2000) Section (1): Technical requirements for
electrical equipment.

(2) "Electrical Equipment for Combustible Dust Environment (Part 2): Selection and Installation" (GB 12476.
2-2010).


(3) "Dust explosion-proof safety regulations" (GB 15577-2007
).

(4) Safety production industry standard "Electrical Explosion Protection Safety Code for Hazardous Places" (AQ 3009-2007
).

(5) Design Code "Design Code for Power Installations in Explosion and Fire Hazardous Environments" (GB 50058-92
).

After research, it can be found that compared with the current standard GB 12476.
1-2000 and the current national design code GB 50058-92, there are many inconsistencies
in the type of electrical equipment, the type of protection of the shell, the division of dust danger areas, and the combustible dust ignition temperature grouping.

In view of the fact that most of the products of explosion-proof electrical manufacturers on the market are designed, manufactured and inspected according to the new product specification GB 12476.
1-2000, and in terms of the selection of electrical equipment for combustible dust hazardous environments and the division of dust hazard areas, GB 12476 is consistent with the international standard IEC 61241 and the industry standard AQ 3009-2007, and the selection of electrical equipment in this design will be in accordance with the product specification GB 12476.
1-2000 was conducted
.

3 According to the national standard GB 12476 and the industry standard AQ3009-2007 article 4.
2.
2.
3, according to the frequency and duration of the combustible dust/air mixture and the thickness of the dust layer, the combustible dust hazardous environment can be divided into three regional levels: 20 zones, 21 zones and 22 zones
.

The characteristics of each zone are as follows: Zone 20 occurs continuously or frequently during normal operation of combustible dust, in quantities sufficient to form a mixture of combustible dust and air and/or may form an uncontrollable and thicker dust layer in the place and inside
the vessel.

Zone 21 is a hazardous environment zone for dust explosion, and in normal operation, Area 21 may have a sufficient amount of dust to form a combustible dust-air mixture but is not classified as a place
in Area 20.

This area includes: sites directly adjacent to the point of charge or discharge of dust, places where dust layers occur and combustible concentrations of combustible dust and air mixtures that may occur under normal operating conditions
.

Zone 22 Under abnormal conditions, combustible dust occasionally appears and only exists for a short time or combustible dust occasionally appears to accumulate or may exist in a dust layer and produce a combustible dust air mixture place
.

If it is not guaranteed to exclude combustible dust accumulation or dust layers, it should be divided into zones
21.

Procedures and examples of zoning in flammable dust environments can be found in Appendix A of GB 12476.
3-2007/IEC 61241-10:2004 or Appendix C
of AQ 3009-2007.

For electrical designers, choosing a reasonable dust explosion-proof and flameproof appliance is a reliable way
to eliminate the EDM caused by electrical equipment.

The surface temperature of Class II electrical equipment can be divided into 6 groups: T1-T6 surface temperature 450 °C, T2 surface temperature 300 °C, T3 surface temperature 200 °C, T4 surface temperature 135 °C, T5 surface temperature 100 °C, T6 surface temperature 85 °C
.

At present, there are two design types of dust enclosures in IEC standards, A and B, which represent two standard systems
in Europe and North America respectively.

Although the two design types have different requirements for the selection/installation of equipment, they have the same level of
protection and can be universal.

At present, most of the explosion-proof electrical manufacturers in the domestic market are designed and manufactured
according to the A-type.

For type A electrical equipment, its high surface temperature should not exceed 2/3 of the low ignition temperature of the relevant dust cloud, that is, Tmax≤2/3Tcl
.

When there is a dust layer thickness of 5mm, the surface temperature should not exceed the ignition temperature of the relevant dust layer thickness of 5mm minus 75K, that is, Tmax≤ T5mm-75K, taking the small value of
both.

For B-type electrical equipment, the surface temperature should not exceed 2/3 of the low ignition temperature of the relevant dust cloud, that is, Tmax≤2/3Tcl
.

When the thickness of the dust layer is 12.
5mm, the high surface temperature should not exceed the low ignition temperature of the relevant dust layer thickness of 12.
5mm minus 25K, that is, Tmax≤T12.
5mm-25K, taking the smaller value
of the two.

In summary, we can conclude that the selection requirements of type A electrical equipment and type B electrical equipment are completely consistent in the presence of only relevant dust clouds, and the selection requirements will only differ
when the thickness of the dust layer exceeds a certain amount.

Specific to the design project, the building in this project adopts a fully cast-in-place frame structure, the production category is Class C, and the fire resistance grade is (1
).

The building area is 10871m2, the main body of the building is two-storey, and the building height is 19.
3m
.

The workshop process is mainly through the microbial fermentation liquid through centrifugal spray drying, mixed packaging to obtain Enramycin finished products
.

Dry hyphae pneumatic conveying equipment occupies a room
on the first and second floors of the building.

According to the process conditions, we divided the two rooms into flammable dust explosion hazard environments
.

According to the above principles, we can analyze the explosion-proof level
of electrical equipment needed to be selected in the combustible dust hazard area inside the extraction workshop.

Since the dry mycelium pneumatic conveying system inside the extraction workshop only appears occasionally under abnormal conditions and produces a combustible dust air mixture, the room where the dry mycelium pneumatic conveying system is located can be divided into dust explosion-proof zone
22.

As a chemical feed, the dry hyphae has a low ignition temperature of 130 °C
at high temperatures.

Since the electrical equipment has a certain distance from the dust release source, dust removal measures have been taken at the site, so that the accumulation thickness of the dust layer can reach 5mm or 12.
5mm
.

Tmax≤2/3Tcl=86.
7 °C can be known to select the electrical equipment for combustible dust environment (including lamps, motors, on-site operationscolumn, etc.
) only needs to be higher than the following standards: DIP A22 TA 85 °C (or TA, T6) IP6X, to meet the electrical design requirements
of the hazardous environment of combustible dust in the extraction workshop.

4 Lightning protection design is in accordance with Article 3.
0.
3 of the Code for the Design of Lightning Protection of Buildings (GB 50057-2010), Article 7: Buildings with explosion hazard sites in Zone 2 or Zone 22 shall be classified as Class (2) lightning protection buildings
.

The room occupied by the dry hyphal pneumatic conveying system of this extraction workshop shall be divided into the class (2) lightning protection building part
.

According to point 3 of Article 4.
5.
1 of this Code, when the area of the part of a type I and II lightning protection building is less than 30% of the total area of the building and it is impossible to be directly struck by lightning, the building may be determined as a type III lightning protection building
.

Since the area of the combustible dust hazard area does not reach 30% of the extraction workshop, the extraction workshop can be divided into the third type of lightning protection building, and the measures for direct lightning protection are designed in accordance with the third type of lightning protection building standards, but for the (2) type of lightning protection building part of the combustible dust danger area, anti-lightning induction and anti-lightning surge intrusion measures, the protection measures of the type (2) lightning protection building should be adopted
.

5 Anti-static grounding design for flammable dust dangerous environment equipment and pipelines, should be anti-static grounding
.

All metal equipment, device housings, metal pipes, brackets, components, components, etc.
, should generally be used anti-static direct grounding; If it is inconvenient or the process does not allow direct grounding, it can be indirectly grounded through conductive materials or products
.

In the specific design, the ring grounding trunk line in the combustible dust hazard environment adopts 25×4 hot-dip galvanized flat steel 0.
3m from the ground along the wall or steel platform, and the grounding trunk line at the door and aisle is buried and dark
.

The grounding branch line connecting the equipment is buried with 25×4 galvanized flat steel buried to the equipment
.

6 Line short-circuit arc extinguishing protection design electrical fireproof current-limiting protector is specially used for short-circuit arc extinguishing protection and overload protection in low-voltage distribution lines, which can reasonably overcome the large short-circuit current existing in traditional circuit breakers, air switches and monitoring equipment, the long time of cutting off the short-circuit current, the large arc spark generated when the short circuit is short, and the short service life is short, when a short-circuit fault occurs, the short-circuit current can be quickly limited at a microsecond speed to achieve arc extinguishing protection, which can significantly reduce electrical fire accidents.
Guarantee the safety of personnel and property in the place of
use.

This engineering design uses the Acquaray ASCP series of electrical fireproof current-limiting protectors, its products have rich functions, a variety of models and other characteristics, as shown in the following table: in this design, according to the different current specifications of different end loops, a total of 67 ASCP200-20D products with a larger rated current of 20A, and 46 ASCP200-40D products with a larger rated current of 40A, As well as 35 products with a larger rated current of 63A
.

In order to improve the intelligence of monitoring, in this design, all the current-limiting protectors are connected to the Acrel-6000/B electrical fire monitoring host of Acrel-6000/B through their own RS485 communication network, and the host is set in the electrician's duty room to realize the remote monitoring
of all the current-limiting protector monitoring data.

7 Attention points in the design (1) Due to the lack of sufficient understanding of combustible dust explosion-proofing, some electrical design engineers use gas-proof electrical equipment
, whether it is a gas explosion hazard place or a combustible dust hazard place.

This practice is wrong, combustible dust hazardous environment electrical equipment and gas explosion-proof electrical equipment shell protection requirements are completely different, gas explosion-proof electrical equipment in the combustible dust hazard place is completely useless, and vice versa
.

Therefore, the two are not used
interchangeably.

(2) Combustible dust hazardous environment electrical equipment according to the shell grade can be divided into dustproof type and dust-tight type, but does not mean that the electrical equipment has a waterproof function, the vast majority of combustible dust dangerous environment with electrical equipment can not prevent water from entering its interior, so these equipment can not be directly installed in outdoor open space, can not be washed with water shell
.

If you want the electrical equipment in combustible dust hazard places to have the above functions, it must be pointed out in the design that the equipment must meet the requirements
of IPX5 or above enclosure protection class.

(3) The electrical fireproof current-limit protector should pay attention to its rated current in the design should be consistent with
the rated current of the circuit breaker in its predecessor.

In addition, because it uses a solid-state switch as a switching device inside, heat will be generated during use, so it should be paid attention to ensure its good ventilation and heat dissipation conditions when installing, and it is not recommended to install in a closed distribution box
.

8 Conclusion In the actual design process, to do a good job in the design of a combustible dust hazardous environment project in all aspects and perfectly, it requires multiple coordination and cooperation such as process, ventilation, and construction, and each professional has corresponding design requirements in its own design field
.

At the same time, we electrical designers are also required to constantly understand new technologies and new products, for the design of this combustible dust hazardous environment engineering, you can try to use short-circuit arc extinguishing products to reduce the arc light caused by the circuit short-circuit caused by line aging, insulation loss and other reasons, which has a good effect
on reducing the on-site electrical fire accident.

It is hoped that through the collaborative efforts of designers, and with the continuous emergence and application of new technologies and new products in the country, the design level of combustible dust hazardous environment will surely reach a new level
.

Reference[1] Ma Zhongfei.
Industrial ventilation and dust protection[M].
Beijing:Chemical Industry Press,2011.
[2] Electrical equipment for combustible dust environments (Part (1)): Electrical equipment protected with enclosures and limited surface temperature (GB 12476.
1-2000)[S].
[3] Electrical equipment for combustible dust environment (Part (2)): selection and installation (GB 12476.
2-2010)[S].
[4] ASCP series electrical fire restrictive protector.
Acqua Electric Co.
, Ltd.
product sample 202109