Hazardous Area Guidelines for Compliant SCADA Systems

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The suitability of equipment for specific hazardous areas in the North American regulated market is required to be tested by a NRTL (Nationally Recognized Testing Laboratory). This is a testing facility, recognized by OSHA (Occupational Safety & Health Administration), as primarily private sector organizations that provide product safety testing and certification services to manufacturers.
The testing and certification are done, for purposes of the program, to U.S. consensus-based product safety test standards. These test standards are not developed or issued by OSHA, but are issued by U.S. standards organizations, such as the American National Standards Institute (ANSI). Some well-known organizations currently recognized by OSHA as NRTL's include (but are not limited to):
Canadian Standards Association (CSA); FM Approvals LLC (FM); and Underwriters Labs Inc (UL)

Refer to Article 500 of the National Electrical Code, publication NFPA 70, or Section 18 of the Canadian Electrical Code, publication C22.1 for complete definitions of hazardous areas.  For our purposes, a brief recap follows:

Hazardous Location (classified) is a North American term for locations where the potential for fire or explosion exists because of gases, dust, or easily ignitable fibers or filings in the atmosphere.
Such an area is categorized according to the nature of the risk and the relative risk level.

To define the level of safety required for equipment installed in these locations, Classes, Divisions, and Groups separate the different hazardous areas:
Classes define the general form of the flammable materials in the atmosphere.
Divisions define the probability of the presence of flammable materials.
Groups classify the exact flammable nature of the material.

A Class I Hazardous Location  is one in which material may be present in the air in sufficient quantities to be explosive or ignitable. Some typical Class 1 locations are utility gas plants and operations involving storage and handling of liquefied petroleum gas or natural gas petroleum refineries gasoline storage and dispensing areas.

When the hazard would be expected to be present in everyday production operations or during frequent repair and maintenance activity, this is called the normal condition.

When the hazardous material is expected to be confined within closed containers or closed systems and will be present only through accidental rupture, breakage or unusual faulty operation, the situation could be called abnormal.

The Code writers designate these two kinds of conditions very simply: normal condition is deemed  Division 1, and abnormal condition is known as Division 2. Thus:
Class I, Division 1 - Where ignitable concentrations of flammable gases, vapors or liquids are present within the atmosphere under normal (day to day) operating conditions.
Class I, Division 2 - Where ignitable concentrations of flammable gases, vapors, or liquids are present within the atmosphere under abnormal (system failure) operating conditions.
Groups A,B,C, & D designate the various types of flammability, and for the purposes of most SCADA applications, are quite secondary to the Class/Division rating.

With respect to SCADA systems, radio modems require certification ratings to operate in a hazardous area.  Typically, commercially available gain antennas and virtually all surge suppressors are not rated.  Due to increased demand for intrinsically safe wireless equipment, however, a growing number of unity gain omni-directional models certified for use in Class I Div 1 areas are more readily available.

Field services personnel seeking to meet the compliance requirements using (non-rated) directional antennas may consider the following:
*  connect the cable and antenna in place on the mast before it is deployed in the restricted area, and
*  install the antenna at a height that is deemed to be out of the Class I Division 2 zone:
typically this zone is understood to extend 25 feet outside the adjacent Division 1 zone.  
As height is one of the most important considerations with respect to optimizing the operating characteristics of an antenna, this strategy works to the benefit of the site anyway.

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Hazardous Area Guidelines for Compliant SCADA Systems

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This article was published on 2010/09/28