There are degrees of rugged computing environments. Rugged systems can be designed to survive shock, vibration, altitude, and temperature extremes, but there are times when a computer or display may need to operate in a considerably more hazardous location. The National Electrical Code (NEC) defines hazardous locations as being where a fire or explosive hazard may exist due to flammable gases or vapors, flammable liquids, and combustible dust.
There are three types of hazardous locations:
- Class I – Flammable gases or vapors in the air
- Class II – Presence of combustible dust
- Class III – Ignitable fibers and flyings
There are two classifications for when the hazardous conditions exist:
- Division 1 – Conditions exist under normal circumstances.
- Division 2 – Conditions exist under abnormal circumstances.
The next classification that is used is the nature of the substance present. The nature of the substance present is classified by its ignition temperature/conductivity. These hazardous substances are put into groups by their properties.
Class I – Gases, vapors and liquids
- Group A – Acetylene
- Group B – Hydrogen, etc.
- Group C – Ether, etc.
- Group D – Hydrocarbons, fuels, solvents, etc.
Class II – Dusts
- Group E – Metal dusts (conductive and explosive)
- Group F – Carbon dusts (some are conductive and all are explosive)
- Group G – Flour, starch, grain, combustible plastic or chemical dust, etc.
Class II – Fibers and flyings
- Textiles, wood-working, etc., (easily ignitable but not likely to be explosive)
Designing Rugged Computers for Hazardous (Classified) Locations
If you need a computer to operate in a location that has been classified as hazardous, you will need to define these requirements and provide the Class, Division, and Group to which the unit will be subjected. Typically, these units require some extensive customization to operate safely in these environments. Chassis Plans can discuss your requirements and what solutions are possible for your computing environment.
The design criteria for a device to be installed into a Class I hazardous location is two-fold. It is assumed the gas, vapors, etc., can leak into an enclosure and ignite. Therefore, the enclosure must be strong enough to contain an internal explosion without rupturing and releasing flames into the surrounding environment. Second, escape paths for the hot combustion gases must be provided that allow equalization of internal and external pressure but only after these hot gases have been cooled sufficiently to prevent external flame propagation. Often, enclosures in Class I environments are purged with pressurized air to prevent incursion of external gases. That is, the pressure inside the enclosure is always higher than outside.
In the case of Class II (dust, etc.), the design must seal out the dust, it must operate below the ignition temperature of the dust, and it must allow an insulating layer of dust to accumulate without the enclosure rising in temperature above the ignition temperature of the dust. Thus, enclosures for Class I are typically very heavy with thick walls, flanges, etc., while Class II designs can be much of much lighter construction as internal explosions do not need to be accounted for.
Class III designs are similar to Class II in that the flammable fibers must be excluded from the enclosure, any sparks or hot metal from internal failure must be contained inside the enclosure, and the device must operate below the ignition temperature of the flammable fibers.
The National Electrical Code is the “Bible” of the Electrical Industry, and the primary source of reference for hazardous locations. The NEC is also the basis for OSHA standard 1926.407, Hazardous (Classified) Locations. There are several OSHA standards that require the installation of electrical wiring and equipment in hazardous (classified) locations according to the requirements of Subpart K, Electrical. The NEC should be consulted as a supplement to the OSHA standards for additional background information concerning hazardous locations.