Maximum Experimental Safe Gap

Maximum Experimental Safe Gap (MESG)

Definition: (Ref: IEC 60079-1 ) Maximum Experimental Safe Gap (for an explosive mixture ) of a joint 25mm in width which prevents any transmission of an explosion during 10 tests made under the conditions specified in IEC 60079-1-1

Flameproof Joint:

Place where the corresponding surfaces of two parts of an enclosure, or the conjunction of enclosures, come together, and which prevents the transmission of an internal explosion to the explosive atmosphere surrounding the enclosure. (IEC 60079-1- 7th edition)

Width of flameproof joint (L)

Shortest path through a flameproof joint from the inside to the outside of an enclosure (Not applicable to the threaded joints -IEC 60079-1)

Gap of flameproof joint (i)

Distance between the corresponding surfaces of a flameproof joint when the electrical apparatus enclosure has been assembled.

For cylindrical surfaces , forming cylindrical joints , the GAP is the difference between the diameters of the BORE and the CYLINDRICAL component – ( IEC 60079-1)

Use of MESG

This parameter (MESG) is one of the ways, used to quantify the integrity of the BASIC Property of a FLAMEPROOF enclosure. The Basic Property is :

During an internal explosion , the over-pressure which is developed is safely contained and the transmission of the explosion to the outer explosive atmosphere is prevented by releasing over-pressure through specially designed gaps.

It is quite obvious that each explosive gas or vapours behave differently under same conditions during an experiment. This fact is utilized to classify the different Explosive Gases & Vapours in relation to MESG.

In fact MESG forms the basis of Classifying the gases & vapours in GAS GROUPS

Group I : Mine (Involving Methane and Coal Dust )

Group II: Places where an explosive gas atmosphere exists other than mines

Gas Sub Group IIC

Gas Sub Group IIB

Gas Sub Group IIA

If MESG is less than 0.5mm then it is Gas sub-Group IIC

If MESG is between 0.5mm & 0.9 mm then it is Gas sub-Group IIB

If MESG is greater than or equal to 0.9mm then it is Gas sub-Group IIA

An interesting question arises at this point .

What about the IP rating or IP index of a flameproof enclosure?

IEC 60079-1does not mention IP rating of the Flameproof enclosures. In fact Flameproof enclosures have specially designed gaps and pathways to let the exploded gases & soot, out safely, so as not to ignite the hazardous atmosphere outside of the enclosure.

However most manufacturers of Ex d enclosures state the IP rating of the Ex d enclosures in the certificates as IP6X