In late 2009, the International Society of Automation (ISA) and Underwriters Laboratories Incorporated (UL) jointly issued ANSI/ISA 60079-0 Explosive atmospheres – Part 0: Equipment – General Requirements which is a modification of the 5th edition of IEC 60079-0 Explosive atmospheres – Part 0: Equipment – General requirements. The purpose of this adaptation is to have a similar or common standard for ANSI and IEC as the preface of the ANSI, to quote
It is the intention of the ISA12 Committee to develop an ANSI Standard that is harmonized with IEC 60079-0 to the fullest extent possible.
In NFPA® 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, hazardous areas are classified as follows:
Class I – Flammable Gases or Vapors
- Class I Divison 1
- Flammable vapors or gases can exist under normal conditions, repair or maintenance, or a breakdown of equipment may cause simultaneous failure of electrical equipment becoming a source of igntion.
- Class I Division 2
- Flammable gases or vapors are present but contained or ventilated that only a breakdown of the containment or mechanical ventilation may cause the area to become hazardous. These are also areas adjacent to Class I Division 1.
- Class I Zone 0
- Ignitible concentrations of flammable gases or vapors are present continuously, or for long periods of time.
- Class I Zone 1
- Ignitible concentrations of flammable gases or vapors are likely to exist under normal operating conditions, repair or maintenance, or equipment breakdown or faulty operations may cause simultaneous failure of electrical equipment becoming a source of ignition. Also locations adjacent to a Class I, Zone 0.
- Class I Zone 2
- Ignitible concentrations of flammable gases or vapors are not likely to occur in normal operation and, if they do occur, will exist only for a short period; confined or provided with mechanical ventilation. May become hazardous with the breakdown of containment or ventilation. It includes locations adjacent to a Class I, Zone 1.
It is noticeable that the definitions for Class I Zone 0, Class I Zone 1 and Class I Zone 2 for the ANSI/ISA is very similar to the IEC definitions:
- zone 0
- an area in which an explosive gas atmosphere is present continuously or for long periods or frequently
- zone 1
- an area in which an explosive gas atmosphere is likely to occur in normal operation occasionally
- zone 2
- area in which an explosive gas atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only
References:
1. ANSI/ISA 60079-0 Explosive atmospheres – Part 0: Equipment – General Requirements
2. IEC 60079-0 Explosive atmospheres – Part 0: Equipment – General requirements.
3. NFPA® 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas