Hazardous (Classified) Locations and Special Occupancies
Classification drives equipment choice
Hazardous (Classified) Locations and Special Occupancies tests whether you can read a zone classification, select equipment with the correct Ex marking, and apply the additional rules for bathrooms, medical locations and similar areas. The classification is based on the likelihood and duration of a flammable atmosphere, not on whether the process smells of gas.
Core rules
- Gas and vapour zones: Zone 0 is continuous or long-period exposure, Zone 1 is likely in normal operation, and Zone 2 is unlikely and short-lived. Dust zones use 20 for continuous, 21 for likely and 22 for unlikely but possible.
- Equipment markings include the Ex protection concept, group and temperature class. Group IIC covers hydrogen and acetylene and is the most demanding gas group. Temperature class T1 allows 450 degrees C, T4 allows 135 degrees C, and T6 allows 85 degrees C.
- Intrinsic safety Ex ia permits Zone 0; Ex d flameproof, Ex e increased safety and Ex p pressurisation are common in Zone 1 and 2. Ex n is generally for Zone 2. Dust equipment must suit the dust group and maximum surface temperature, including dust layer effects.
- Cables, glands and barriers must maintain the protection concept. An ordinary junction box in a classified area defeats the Ex rating of the equipment around it.
- Special occupancies add their own rules. Bathroom zones 0, 1 and 2 permit only suitable equipment, with 30 mA RCD additional protection and supplementary bonding where required. Medical locations require medical IT systems or RCDs selected for the group, and equipotential bonding.
Worked example
A Zone 1 gas atmosphere contains a heater whose maximum surface temperature is 125 degrees C at 40 degrees C ambient. The gas has an ignition temperature of 200 degrees C, and T3 permits 200 degrees C while T4 permits 135 degrees C. A T4 rating is acceptable because 125 degrees C is below 135 degrees C. A T5 rating would permit only 100 degrees C and would be unacceptable. The reasoning must use the maximum surface temperature under fault conditions, not the normal operating temperature alone.
Traps that cost marks
Confusing gas zones 0, 1 and 2 with dust zones 20, 21 and 22; using T5 equipment at 125 degrees C; installing an Ex d enclosure with an ordinary gland; and treating bathroom zone numbers as if they were gas zones.