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Recent Posts

Maximum Experimental Safe Gap Test Process

Posted by Michael Lim on 08.07.19

Michael Lim, Flammability and Dust Operations, Fauske & Associates, LLC

Industrial Background with Blue BarrelsProcess industries that handle flammable gases and vapors may involve concentrations that can potentially form an explosive atmosphere. In a situation where the right precautions are not taken into consideration, a flammable mixture may be exposed to an ignition source such as a static electric discharge, an electrical arc or a spark, which may ignite and potentially cause an explosion which can then result in personnel injury or damage to property.

Various regulations and standards have been developed to ensure a high level of safety in these hazardous conditions. The testing standards that have been established are aimed to provide information that can be used in engineering designs. An important piece of data that is valuable in designing explosion protection is the safe gap value or the maximum experimental safe gap.

The Maximum Experimental Safe Gap (MESG) is defined as the maximum gap between two flat surfaces, under specified test conditions, that prevents an ignition of a flammable gas/air mixture propagating from an inner chamber through a 25-mm long path into a secondary (outer) chamber. The data generated from MESG testing is commonly used as a guideline in installing properly sized flame arresters on process equipment. Additional information regarding flame arresters is found in NFPA 69 and ISO 16852.

The gases and vapors are classified into different explosion groups. Per NFPA 70, National Electrical Code, Class I locations are those in which flammable gases, flammable liquid-produced vapors or combustible liquid-produced vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixtures. Class I locations are divided into divisions and zones depending on current or expected conditions. Material Groups, based on the MESG, are also used to further classify the explosive characteristics of specific gas/vapor air mixtures.

The Material Groups are as follows:

MESG Material Group Chart

a. The standard did not provide an MESG value

Per the European standard IEC 60079-20-1, the equipment is classified into groups in accordance with the properties of explosive atmospheres for which it is intended. The groups for equipment for explosive gas atmospheres are as follows:

Group I: Equipment for mines susceptible to firedamp

Group II: Equipment for locations with explosive gas atmospheres other than what was stated for Group I

Group II equipment is then subdivided into three sub-groups. For the purpose of classification of gases and vapors, the MESG limits are:

MESG Group II Chart

FAI has added testing capability to determine the MESG values of flammable gases or vapors. The test is performed in accordance with IEC 60079-20-1 and the setup is shown in Figure 1.

MESG Testing Setup

Figure 1: Setup for MESG Testing

The test equipment was verified by performing tests using propane and methane. The MESG values of the reference samples were determined in accordance to the IEC 60079-20-1 standard. Results obtained for MESG are compared with other reported literature values.

MESG Results of Propane and Methane

MESG Results of Propane and Methane
a. Data was obtained from IEC 60079-20-1 (2010)


For more information about MESG or other flammability testing, please contact us.

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Topics: flammable vapor, flammable gas, flammable liquids, flammablity

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