How to Create an Electrical Area Classification Diagrams

Creating an Electrical Area Classification Diagram involves the following basic steps, done by electrical and mechanical/process/chemical engineers:

1. Find all systems in the plant using flammable materials like hydrogen, natural gas, propane, butane, gas condensates, ammonia, flammable dusts, etc.
2. Along the systems carrying flammables, identify on the plot plan the location of all flammable release sources such as: Open process points (like open tanks), Control valves, Pump seals, Rotating equipment, Seal pots, Drains, Metering points, Sampling points, Vents, PSVs, Rupture Disks. In general, closed metal piping or tubing without valves, fittings or flanges does not need to be considered as a potential source of release
3. Get the correct electrical code, e.g. National Electrical Code (NEC) in U.S., or the Canadian Electrical Code (CEC), whatever applies. Classify the release sources identified in step #2 by the standard. For example, possible abnormal operation leak points along a Hydrogen system may be Class 1 Divison 2, Group B. (Meaning Flammable Gases, only occasionally present, gas is hydrogen). In many cases, the rating can be modified based on the quality of ventilation present.
4. Create an area classification drawing, clouding the area around the hazards with the appropriate area classification. In some cases elevation drawings may also be required (especially multi-level structures). The area you must classify is often not specified in the electrical code, but depends on industry practice and site-specific tests of concentrations of flammable gases in simulated release scenarios. API 500 & 505 are good resources for determining the area to cloud around a potential hazard.
5. If designing a new plant, ensure all equipment in the cloud is built to the standard specified by the cloud. If you are reviewing an existing plant, do a check to ensure that equipment inside the clouds are built for the classification. In some cases it may be possible to get an exception from the regulators for borderline cases

Good reference article: http://www.roe.com/pdfs/technical/practical%20guidelines%20for%20ctg%20power%20plants.pdf

You can hire consulting companies who are masters at steps 3 & 4, and do it easily, but steps 1, 2, and 5 still need the touch of people intimately knowledgeable about the system being drawn.