Explosion-proofing

Technical information is provided here explaining the explosion-proof construction required of gas detectors.

What explosion-proof means

In (hazardous) locations where gas, vapor, or powder posing explosion risks is present or is assumed to be present in the air—for example, oil refineries and chemical plants—the technical measures (explosion-proof construction) implemented to prevent explosions are collectively referred to as explosion proofing, and devices incorporating such measures are referred to as explosion-proof devices.

The conditions required to ignite an explosion

An ignition source, combustible material (e.g., combustible gas), and combustion-supporting gas (e.g., oxygen) are referred to as the three factors required for combustion and explosion. No explosion can occur if even one of these factors is missing.

* Ignition source: fire, static electricity, impact, temperature, sparks, or electromagnetic radiation, etc.
* Combustible materials: dust (solid), vapor (liquid), gas, etc.

Hazardous locations and non-hazardous locations

Locations where an explosive atmosphere is present at levels requiring special preventive measures for the construction, installation, and use of electrical equipment or not present:

① Present (or potentially present)
⇒ Hazardous location (explosion-proof area)
② Not present
⇒ Non-hazardous location (non-explosion-proof area)

Types of explosion-proof construction

Riken Keiki gas detectors incorporate one of the following three explosion-proof design approaches.

・ Intrinsically safe explosion-proof construction “i”: Design intended to eliminate the risk of arcing or sparks caused by electrical circuits igniting gas or vapor when the device is in a normal state or when it is in a specified failure state. The electricity used is restricted to prevent flames. “ia” for two assumed failures and “ib” for one.
・Flame-proof enclosure “d”: This refers to a design intended to withstand internal explosions without sustaining damage in the event of explosive atmospheres entering the device and to prevent the ignition of external gas or vapor.
・Non-explosion-proof construction “n”: A construction designed to ensure that there is no risk of igniting surrounding explosive atmospheres when the device is in a normal state.
Other types of explosion-proof construction include the following:
Internal pressure explosion-proof construction “p”, oil ingress explosion-proof construction “o”, increased-safety explosion-proof construction “e”, resin-filled explosion-proof construction “m”, dust explosion-proof construction using container “t”, and special explosion-proof construction “s”

Explosion-proof certification in different countries

  • International ⇒ IECEx
  • European ⇒ ATEX
  • North America ⇒ UL/cUL, FM/cFM * “c” corresponds to Canadian standard
  • Brazil ⇒ INMETRO
  • Japan ⇒ Certificate of conformity for electrical equipment used in potentially explosive atmospheres (Japan Ex)
  • China ⇒China Ex
  • Taiwan ⇒ TS
  • Republic of Korea ⇒ KCs

Classification of hazardous locations

Special hazardous location (Zone 0)

Locations in which explosive atmospheres are present continuously or for extended periods
Example: ① Inside combustible gas containers or tanks
② Spaces above liquid surfaces in inflammable liquid containers or tanks

Type 1 hazardous location (Zone 1)

Locations in which explosive atmospheres are produced under normal conditions
Example: ① Locations where combustible gas or vapor may be emitted under normal operating conditions
② Locations where combustible gas or vapor is prone to leaking during maintenance or repairs
③ Indoor or poorly ventilated locations where combustible gas or vapor accumulates around openings

Type 2 hazardous location (Zone 2)

Locations free of explosive atmospheres or where explosive atmospheres are present under normal conditions only for short periods
Examples: ① Locations where combustible gas or vapor may leak if a container or plant is damaged by corrosion, deterioration, or damage
   ② Locations where combustible gas or vapor may be released in the event of an operating error or an abnormal reaction
   ③ Indoor locations surrounding or adjoining Type 1 hazardous locations associated with a remote possibility of combustible gas or vapor ingress

Explosion-proof construction types suitable for each type of hazardous location
Hazardous area Explosion-proof construction Remarks
Special hazardous location (Zone 0) Intrinsically safe explosion-proof construction (ia)
Resin-filled explosion-proof construction (ma)
Type 1 hazardous location (Zone 1) Flame-proof enclosure (d) Explosion-proof construction items used in special hazardous locations (Zone 0) may also be used.
Internal-pressure explosion-proof construction (px, py))
Increased safety explosion-proof construction (e)
Oil-filled explosion-proof construction (o)
Intrinsically safe explosion-proof construction (ib)
Resin-filled explosion-proof construction (mb)
Special explosion-proof construction (s)
Type 2 hazardous location (Zone 2) Intrinsically safe explosion-proof construction (ic) Explosion-proof construction items used in special hazardous locations (Zone 0) or Type 1 hazardous locations (Zone 1) may also be used.
Non-incendive explosion-proof construction (nA, nC, nR)
Resin-filled explosion-proof construction (mc)
Internal-pressure explosion-proof construction (pz)
Explosion-proof class
Gas groups

Gas groups are defined as Group I or Group II, depending on the usage environment. (See Table 1)

Group I
Electrical devices used in mines where explosive mine gas may be present

Group II
Electrical devices used in explosive gas atmospheres other than mines where explosive mine gas may be present
Category

These are categorized 1G to 3G depending on the presence of an explosive atmosphere. (See Table 1)

Table 1 Correlation between group and category

Group Category
(94/4/EC)
Zone Presence of explosive atmosphere
Group Ⅰ
(coal mines)
M1 Present (methane, dust)
M2 Potentially present (methane, dust)
Group II 1G Zone 0 Present continuously, frequently, or over extended periods
2G Zone 1 Present intermittently during normal operation (potentially)
3G Zone 2 Present occasionally or only for short periods
(Not present during normal operation)

Explosion-proof symbol
Symbol indicating explosion-proof construction.

Explosion-proof construction type
Indicates the type of explosion-proof construction. (See Table 2)

Table 2 Type of explosion-proof construction

Intrinsically safe explosion-proof construction i (ia, ib)
Flame-proof enclosure d (da, db)
Internal-pressure explosion-proof construction p
Increased safety explosion-proof construction e
Resin-filled explosion-proof construction m
Non-incendive explosion-proof construction n
Oil-filled explosion-proof construction o
Special explosion-proof construction s

Explosion-proof electrical device groups
Depending on whether they feature flame-proof enclosures or intrinsically safe explosion-proof construction, respectively, explosion-proof electrical device groups (IIA, IIB, IIC) are classified as maximum safety gap or minimum ignition current. (See Table 3)

Table 3 Explosion-proof devices and explosion-proof class

Explosion-proof class Flame-proof enclosure Intrinsically safe explosion-proof construction
Maximum safety gap range for gas or vapor Minimum ignition current ratio range for gas or vapor
II A 0.9 mm or more Over 0.8
II B Over 0.5 mm and less than 0.9 mm 0.45 or higher and 0.8 or less
II C 0.5 millimeters or less Less than 0.45

* The minimum ignition current ratio is indicated with reference to the minimum ignition current for methane.

Maximum safety gap
This is the maximum joint gap that prevents the propagation of a flame to an external gas mixture through a 25 mm long joint in a flame-proof test container when the gas mixture with the highest flame propagation is ignited inside the specified test container.

Minimum ignition current
This is the minimum current for an inductive or resistive circuit used to ignite the most easily ignitable gas mixture in a spark ignition test.

Temperature grade

This is categorized based on the maximum surface temperature of the electrical device and the ignition temperature of the combustible gas or vapor. Combustible gases and vapor have their own specific ignition temperatures. The methods for measuring ignition temperature are internationally stipulated in IEC 60079-20-1 “Gas and vapour classification - Test methods and data”. Values measured by these measuring methods are often used. (See Table 4)

Table 4 Temperature grade and ignition temperature

Electrical device maximum surface temperature Temperature class Combustible gas vapor ignition temperature
450 °C or less T1 Over 450 °C
300 °C or less T2 Over 300 ℃ and 450 ℃ or less
200 °C or less T3 Over 200 ℃ and 300 ℃ or less
135 °C or less T4 Over 135 ℃ and 200 ℃ or less
100 °C or less T5 Over 100 ℃ and 135 ℃ or less
85 °C or less T6 Over 85 ℃ and 100 ℃ or less

Ignition temperature
This refers to the minimum temperature of a heated surface leading to ignition when the heated surface is in contact with a mixture of combustible gas or vapor and air under specified conditions.

EPL (Equipment Protection Level)

EPL (IEC 60079-26:2006) conceptualizes explosion-proof performance in terms of protection level by identifying and indicating the specific ignition risks posed by that device. This facilitates the selection of devices suitable for specific hazardous locations. EPL is classified Ga, Gb, or Gc. (See Table 5)

Table 5 Explanation of EPL

EPL Protection Zone for normal operation
Ga “Extremely high protection”
Protected by two independent explosion-proof functions or capable of maintaining explosion-proof performance even when two failures occur independently.
・ Equipment that becomes Ga by itself: Intrinsically safe explosion-proof construction “ia”
・ Two independent explosion-proof structures: Combining equipment with Gb structures
Zone 0
Gb “High protection”
Maintains explosion-proof performance even under normal operation, and maintains explosion-proof performance even in the presence of frequent severe disturbances to explosion-proof performance or in cases where faults may occur during normal use.
・ Flame-proof and intrinsically safe explosion-proof construction “ib”, flame-proof construction “d”, etc.
Zone 1
Gc “Improved protection”
Maintains explosion-proof performance only during normal operation.
・ Type n devices
Zone 2

Gas detector groups and temperature grades for typical combustible gases and vapors

Gas detector groups and temperature grades for typical combustible gases and vapors

Temperature
class

Group
T1 T2 T3 T4 T5 T6
II A Acetone
Ammonia
Isobutane
Ethane
Acetate
Ethyl acetate
Toluene
Benzene
Methane
Isopentyl acetate
Acetic anhydride
Butane
Propane
Methanol
Hexane Acetaldehyde
II B Carbon monoxide Ethanol
Ethylene
Ethylene oxide
II C Hydrogen gas
Hydrogen
Acetylene Carbon disulfide

Reference:
Technical recommendations of the National Institute of Occupational Safety and Health JNIOSH-TR-No.44: 2012 (published by National Institute of Occupational Safety and Health, Japan)