The extent of any hazardous area depends upon a number of factors - it can be much more than 1.5m, this distance doesn't apply to all conditions. It is incorrect (dangerous?) to simply apply a blanket 1.5m distance to your Zone 2. The the principle factors to take into account are: The physio-chemical properties and classification of the flammable material, as given in the relevant standard - see later. The quality of the ventilation in the area The reliability of the ventilation The grade of likely release The pressure of the system in which the flammable liquid is held - open vessels at atmospheric temperature / pressure or in a pressurised/pumped system? There are 2 principle standards used in the hazardous area classification exercise, either BS60079-10 Electrical apparatus for explosive gas atmospheres and Pt 20 OR IP15 Area Classification Code for Installations Handling Flammable Liquids 4th Edition