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Hazardous Area Classifications

The migration of personal computers onto the plant floor is continuing at a record pace. The price/performance of a standard PC when coupled with new plant floor software packages and networking capabilities have moved PC's onto the plant floor for functions such as supervisory control and data acquisition. However, manufacturing plants and process areas are traditionally not the cleanest nor the most environmentally controlled environments for PC's to operate in. As a result, NEMA rated workstations with built-in cooling are available for protection of the computer system from the environment.

There is another level of protection: however, that needs to be considered when computer systems go into what is classified as a hazardous (explosive) environment. Basically, this involves not only protecting the computer system from the environment but protecting the environment from the computer system.
Defines the form of hazardous substance present

Class I - Area where ignitable concentrations of flammable gases or liquid vapors are present.
Class II - Area where ignitable concentrations of combustible dusts are present.
Defines the level of hazardous substance concentration

Division 1 - Hazardous substances are present during normal operation.
Division 2 - Hazardous substances are present only during abnormal conditions (such as a leak).
Defines the type of hazardous substance

Class I Substances
Group A - Acetylene (Most Volatile)
Group B - Hydrogen
Group C - Ethylene
Group D - Methane

Class II Substances
Group E - Conductive (Metal) Dust (Automatically classified as Division 1)
Group F - Carbonaceous (Carbon or Coal) Dust
Group G - Agricultural (Flour or Grain) Dust
Types of Protection
As these industries look to move their PC's into hazardous environments there are currently three forms of protection available, that are recognized by the NFPA, for the placement of electronic equipment in a hazardous environment. The overall intent is to prevent/contain an explosion that might be caused by an electrical malfunction of the computer system. Towards this point the (3) types of protection are:

Intrinsic Safety - This is a means of limiting the amount of energy that a particular piece of electrical apparatus generates or stores. These voltage levels are typically in the 5 to 24 Volt range and are associated with annunciators, instrumentation and other "light load" electrical equipment. Typically, computer systems do not fall into this category.
Explosion Proof Enclosures - These enclosures are designed to contain an explosion as opposed to preventing one. They're constructed of heavy cast iron material and have limited openings; therefore, limited access. Heavy duty electrical apparatus that requires minimal interface are many times housed in these enclosures which tend to be somewhat bulky and cost prohibitive. Relative to the use of PC's, they are typically not appropriate since there is virtually no operator interface allowed with these enclosures; thereby defeating the purpose of putting a PC into the environment.
Purging Technology - The third form of recognized protection by the NFPA is purging. This concept basically means that a NEMA 4/4X rated enclosure will be pressurized with an inert (non-explosive) gas such that the internal pressure of the enclosure is greater than the external pressure of the environment. This basically means that there will always be a continuous clean air flow out of the enclosure thereby preventing the intrusion of explosive gases or dusts into the enclosure. These "Purge Workstations" contain a pressure switch with a tie back to the purging system to alert and potentially shut down the system if the internal pressure is lost for any reason. This approach is one of the most cost effective ways of placing a PC into a hazardous environment. It provides the normal operator interfaces that are required including: viewing of the monitor screen and use of a specially designed keyboard/pointing device for the environment.
Types of Purging
For computer based systems to be used in a hazardous environment, purging technology is the most logical. Within this technology there are basically two levels of protection and purging systems available based on the environment. The "least critical" is a Class I, Division 2 area in which explosive gases are present only during an abnormal circumstance (i.e. an accidental release of a gas by-product). For this situation a Type Z purge system is utilized which provides for a continuous air flow and pressurization of the enclosure and has a pressure switch with alarm contact to alert the operator and/or control system that pressure has been lost for whatever reason. The operator can then take the necessary steps to shut down the power to the system if a hazardous situation occurs.

The second level of protection is for "critical" applications, Class I, Division 1, in which the explosive or hazardous gases are present during normal operation. For this situation a Type X purge system is utilized which just like the Z purge above, provides a continuous air pressure into the cabinet but also performs one other important function. Basically, the X purge system will control the overall power within the enclosure such that, due to the loss of pressure within the enclosure, for whatever reason, the EPCU (Electronic Power Control Unit) within the X purge system will shut down all power to the system. It will also prevent the power-up of the system until all explosive gases that might have built up within the enclosure during shut down have been effectively flushed out. For a typical PC enclosure this would involve a 10 minute "Rapid Exchange Purge" in which (4) volumes of air are passed through the enclosure before power is allowed to be re-energized.

The basic difference between the Type Z (Div 2) and the Type X (Div 1) is that with Type Z the power is controlled manually, while the Type X power is automatically controlled by the system itself. The codes for the proper configuration and operation of these purge systems are governed by NFPA, Section 496.
A purged OP/Station from ACC contains the safety features described above and will accept the computer/monitor of your choice. Workstations are shipped fully assembled with an integrated purge system whereby the customer only needs to connect up power and air via the connection points provided. OP/Stations are shipped standard with an integrated membrane or elastomer NEMA 4/4X keyboard with optical isolators. The isolators add another layer of protection to prevent any sparks from occurring if the keyboard surface is scratched or punctured. Additionally, with the Type X system, all power is hardwired from the purge control system through a surge suppressor within the enclosure. Therefore, the customer need only attach his power plugs from his computer and monitor into the surge suppressor within the unit.
OP/Station's Offering
A "hazardous environment" is typically classified as one that contains amounts of explosive gases or dust either during normal operations or during an abnormal circumstance. These areas are typically found in petrochem, chemical, pharmaceutical and painting industries where by-products of the process or the end product itself is considered hazardous. The word hazardous is further defined by the NFPA (National Fire Protection Association) under a Class, Division, Group rating system which helps define the severity of the hazardous environment.
Defining Hazardous