Understanding Enclosure Ratings: A Guide to Hazardous Area Classification, Explosion Proof Ratings, and Ingress Protection Ratings.
My understanding is that they are not interchangeable, and we are going to delve deep into each of them. before we begin take a look at the image below to give you a broad understanding.
Hazardous Area Classification:
This is focused on clearly defining the risk zones. A hazardous area is a location within a petrochemical /gas processing plant or some other equally volatile plant an explosive atmosphere may form due to the presence of flammable gases, vapors, or dusts in the air.
These areas are defined/classified by a competent person and to ensure safety, electrical equipment must be designed and certified for use in these areas. Hazardous area classification is based on the probability and frequency of an explosive atmosphere being present. The most common classification system is the Class/Division system used by National Electric Code (NEC 500), National Fire Protection Association (NPFA 497) and Underwriters' Laboratories (UL).
Currently there are two systems used to classify these hazardous areas: the Class/Division system and the Zone system. The Class/Division system is used predominately in North America and is guided by the NEC article 500 and NPFA 497 standard, whereas the rest of the world generally uses the Zone system guided by IEC 60079 and ATEX Directive 2014/34/EU. Parts of North America tending towards the zone system follows NEC Article 505.
Class/Division System
Class - The class defines the nature of the flammable or combustible material that could be present in the surrounding atmosphere
- Class I: Flammable gases or vapors
- Class II: Combustible dusts
- Class III: Ignitable fibers or flyings
Division - The division defines the probability of the material been present in the atmosphere
- Division 1: the hazardous material could be present in the air continuously or intermittently under normal circumstances
- Division 2: the material could be present in the environment only during abnormal circumstances
Group - the group defined the exact type of hazardous material in the atmosphere. Groups A, B,C, and D are gases while groups E, F, and G are for dusts and fibers
- Group A—Atmospheres containing acetylene.
Group B—Atmospheres containing hydrogen, butadiene, ethylene oxide, propylene oxide, and acrolein.
Group C—Atmospheres containing ethyl either, ethylene, acetaldehyde, and cyclopropan
Group D—Atmospheres containing acetone, ammonia, benzene, butane, ethanol, gasoline, methane, natural gas, naphtha, and propane.
Group E—Atmospheres containing combustible metal dusts such as aluminum, magnesium, and their commercial alloys.
Group F—Atmospheres containing combustible carbonaceous dusts such as carbon black, coal, or coke dust.
Group G—Atmospheres containing combustible grain dusts these include flour, starch, grain, wood, plastic, and chemicals
Zone System
In the zone system hazardous areas are classified into gas and dust zones. Single digit zones being gas, mists and vapors while double digit zones being dust. Electrical equipment for use in these zones is further divided into Groups and Subgroups.
Zone—The Zone defines the likelihood of the of the hazardous material (gas or dust) to be present in explosive or ignitable concentrations.
Gas Zones
1) Zone 0— Is an area where ignitable concentrations of flammable gases or vapors are present in the air continuously, frequently, or for long periods.
2) Zone 1— Is an area where ignitable concentrations of flammable gases or vapors are likely to be occasionally present in the air under normal operating conditions.
3) Zone 2— Is an area where ignitable concentrations of flammable gases or vapors are not likely to be present in the air occur under normal operating conditions but could be present in the air under abnormal conditions for short periods of time
Dust Zones
1) Zone 20— Is an area where ignitable concentrations of combustible dust are present in the air continuously, frequently, or for long periods.
2) Zone 21— Is an area where ignitable concentrations of combustible dust are likely to be occasionally present in the air under normal operating conditions.
3) Zone 22— Is an area where ignitable concentrations of combustible dust are not likely to be present in the air occur under normal operating conditions but could be present in the air under abnormal conditions for short periods of time.
Group —Electrical equipment is divided into three groups. Like in the Class/Division system the group defines the types of ignitable material that could be present in the hazardous area. Like with Zone we also have Gas group and Dust group. Group II equipment's are used in gas zones while Group II equipment are used in dust zones.
1) Group I—Equipment intended for use in mining application (flammable mixture of gases and dust naturally occurring in a mine).
2) Group II—Equipment intended for use in places with an explosive gas atmosphere other than mines susceptible to firedamp. Group II equipment or gas groups is subdivided into three subgroups based on the amount of energy it needs to ignite.
- Group IIA—Atmospheres containing methane, propane and vapors of equivalent hazard.
- Group IIB—Atmospheres containing ethylene and vapors of equivalent hazard.
- Group IIC—Atmospheres containing acetylene or hydrogen vapors of equivalent hazard.
3) Group III—Equipment intended for use in places with an explosive dust atmosphere. Group III equipment is subdivided into three subgroups.
- Group IIIA—Atmospheres containing combustible flyings.
- Group IIIB—Atmospheres containing non-conductive dust.
- Group IIIC—Atmospheres containing conductive dust.
Temperature Classification for a rated group. (T Code)
Every hazardous material has an auto-ignition temperature. The auto-ignition temperature is the lowest temperature at which a combustible or flammable material spontaneously self-ignite without an external source of ignition such as a flame or spark. the lowest temperature at which the fuel will spontaneously ignite in a normal atmosphere without an external source of ignition such as a flame or spark.
Equipment for use within hazardous areas must be prevented from reaching Auto ignition temperature. This requirement necessitates the need temperature classification for equipment located in a hazardous area to avoid ignition from contact with hot surface.
From the table above, an equipment rated T6 would never generate a surface temperature hotter than 85°C.
Explosion-Proof Ratings:
Focuses on mitigating the risk of ignition. Explosion-proof ratings indicate that an enclosure can be able to withstand and contain any explosion originating within the enclosure housing and prevent the ignition of surrounding flammable dust, fibres, gases or vapors. So once hazardous areas are classified, the next step would be to select electrical enclosures and equipment with appropriate explosion-proof ratings to mitigate the risk of ignition. The most common explosion-proof rating is the "Ex d" rating, which denotes a flameproof enclosure. Other ratings include "Ex e" (increased safety) and "Ex n" (non-sparking).
Protection principles are defined to prevent equipment and components becoming sources of ignition within a hazardous area. Essentially these principles fall under four main methods listed below with some explanation.
1) Manufactured with the intent to prevent a potential ignition arising. The principle is referred to as Increased safety because temperature and sparks are inhibited. (Ex e)
2) Manufactured with the intent to limit the ignition energy of the equipment. The principle is referred to as Intrinsically Safe because the current, voltage, and power are reduced to a level too low to cause ignition. Intrinsically Safe electrical equipment are incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition. (Ex ia, Ex ib, Ex nl)
3) Manufactured with the intent to prevent the explosive atmosphere reaching the ignition source. It achieves this by either pressurizing, purging or immersion. (Ex p, Ex m and Ex o)
4) Manufactured with the intent prevent an ignition from escaping outside the equipment. Eg , Ex d. It achieves this by either pressurizing, purging or immersion.
Ex n or Non-Sparking deserves a special mention it is a manufactured using a protection principle where precautions are taken so that hazardous area electrical equipment that has the potential to arc is not capable of igniting a surrounding explosive atmosphere. This can be further categorised as follows:
Ex nA -Where components used in construction are non-sparking
Ex nC -Where components used in construction are non-incentive (designed to cut the risk of sparks and arcs). In other words, they could be sparking equipment where contacts are protected.
Ex nR – Where components used are tightly enclosed to restrict the breathing and prevent ignition
Ex nL -Where components used in construction do not contain enough energy to cause an ignition
|
Symbol |
Type of protection |
Basic concept of
protection |
|
|
e |
Increased safety |
No arcs, sparks or
hot surfaces, enclosure IP54 or better |
|
|
n |
Type ‘n’
(non-sparking) |
|
|
|
Type ‘n’
(closed-break) |
Containment of the
explosion |
|
|
|
Type ‘n’ (sealed and
hermetically sealed) |
Keep the flammable
substances out |
|
|
|
Type ‘n’ (restricted
breathing) |
|
||
|
d |
Flameproof |
Containment of the
explosion |
|
|
q |
Powder filled |
Quenching of the
flame |
|
|
ia |
Intrinsic safety |
Limitation of spark
energy and surface temperatures |
|
|
ib |
|
||
|
ic |
|
||
|
px |
Pressurized
enclosure |
Keep the flammable
substances out |
|
|
py |
|
||
|
pz |
|
||
|
ma |
Encapsulation |
|
|
|
mb |
|
||
|
mc |
|
||
|
o |
Oil immersion |
|
|
|
Op pr |
Optical radiation |
Inherently safe,
protected by shutdown |
|
|
Op sh |
|
||
|
Op is |
|
||
|
ta |
Dust ignition
protection by enclosure |
Dust-tight enclosure |
|
|
tb |
|
||
|
tc |
|
||
|
pd |
Pressurized
enclosure |
Keep the flammable
substances out |
|
Ingress Protection Ratings:
Focuses on safeguarding against environmental factors. In addition to protecting against explosions, electrical enclosures must also safeguard equipment from ingress of solid foreign objects (dust, dirt) and liquids (water, oil). Ingress could introduce elements that detoriate the internal components, cause failure of the component and ultimately lead to a sparks that could be the genesis of an explosion. Ingress protection (IP) ratings consist of two digits that indicate an enclosure's resistance to solid particles and water. The rating consists of two digits:
- First digit: Solid particle protection (0-6)
- Second digit: Water protection (0-9)
For example, an IP 67 rating indicates an enclosure is dust-tight (6) and can withstand immersion in water up to 1 meter for 30 minutes (7).
Choosing the Right Enclosure:
When selecting an enclosure, consider the environment and hazards present. Ask yourself:
- Is the area hazardous (Class/Division)?
- Does the enclosure require explosion-proofing (Ex rating)?
- What level of ingress protection is needed (IP rating)?
Conclusion:
Understanding enclosure ratings is crucial for ensuring electrical equipment safety. By grasping hazardous area classification, explosion-proof ratings, and ingress protection ratings, you can specify the right enclosure for your application. It is perfectly possible to have IP68 equipment that is not certified for use in a hazardous area. Remember, safety is no accident – it's a deliberate choice. Choose wisely.
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