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What constitutes an area’s IEC Zone or NEC Division classification?  

If I have an empty building. It obviously would be a non-classified area.  However, say I install a natural gas fired furnace.  Does the mere potential of a flammable gas (from the combustion system) raise the classification to Zone 1.  “Area in which an explosive gas-air mixture is likely to occur for short periods in normal operation.” Or,  Zone 2  Area in which an explosive gas-air mixture is not likely to occur, and if it occurs it will only exist for a very short time due to an abnormal condition.

Or is the classification determined by the process.

 

 

The classification to a zone is covered in IEC EN 60079-10-1. There is a section that covers the procedures for determining the zone classification:

5 Area classification procedure
5.1 General
5.2 Sources of release
5.3 Type of zone
5.4 Extent of zone
5.4.1 Release rate of gas or vapour
5.4.2 Lower explosive limit (LEL)
5.4.3 Ventilation
5.4.4 Relative density of the gas or vapour when it is released
5.4.5 Other parameters to be considered
5.4.6 Illustrative examples

The document is not public and is available for sale at:

http://webstore.iec.ch/webstore/webstore.nsf/artnum/028806

You may be able to get guidance on how your particular installation would be classified from your company's UL contact. You would likely need to be able to provide some information on the release rate of the natural gas, space dimensions of the area of release and presence of ventilation to determine if there is sufficient gas to create an explosive hazard.

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