Explosive Atmospheres – ANSI/ISA/UL/NFPA

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

About the Author

Ver Pangonilo
A Filipino Engineer, Registered Professional Engineer of Queensland (RPEQ) - Australia and Professional Electrical Engineer (PEE) - Philippines with extensive experience in concept select, front-end engineering, HV & LV detail design, construction and commissioning of Hazardous and Non-Hazardous Area electrical installations in water and waste water pipeline and pumping facilities, offshore platforms, hydrocarbon process plants and pipelines including related facilities. Hazardous area classification and design certification (UEENEEM015B, UEENEEM016B, UEENEEM017B).
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