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I NTRODUCT I ON HAZARDOUS LOCATION DATA

Comparing Class/Division & Zone System Gas groups The first definitions of flammable gases in North American Standards appeared in 1935 and were based on theoretical calculations. In the 1960s an engineer at UL developed an instrument called the Westerberg Explosion Test Vessel that could vary gap and joint width dimensions of a chamber to perform an actual test. In the early 1970s, the IEC® developed a different test vessel that could perform the same test. Although most of the results were similar, they were not identical. Both Systems grouped materials based on the test results. In the 1997 Edition of NFPA 497 a new method to estimate the group classification of a mixture was introduced. While some materials, mostly Groups C and D, no longer met the new definitions exactly, based on the safety of historical practices, the standard committee decided not to reclassify them.

This results in slight differences in how gases are identified in the Zone system versus the same gas in the Division System. For purposes of increasing equipment selection in North America, the area classification should identify both the Zone and Division gas group. GAS GROUPS DIVISION ZONE A IIC B B* (IIB + H 2 ) C IIB D IIA * Added to accommodate equipment that excluded acetylene atmospheres

COMPARISON OF DIVISION AND ZONE SYSTEM GAS GROUPS EVALUATION

DIVISION SYSTEM

ZONE SYSTEM

Group

MESG (mm)

MIC Ratio

Group

MESG (mm)

MIC

Not Classified

< 0.076 (e.g. Carbon Disulphide)

Acetylene (Has same MESG and MIC Ratio as group B, but generates much higher explosive pressures)

A

IIC

≤ 0.50

≤ 0.45

> 0.076 ≤ 0.45 > 0.45 ≤ 0.75 > 0.75

B

≤ 0.40

> 0.40 ≤ 0.80 > 0.80

> 0.50 ≤ 0.90 > 0.90

> 0.45 ≤ 0.80 > 0.80

C

IIB

D

IIA

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