The Fallacy of “Intrinsically Safe” Cases for Tablets

  

There are various products on the market called “intrinsically safe” cases for tablets. These cases are intended to make a typical or rugged tablet safe for use in hazardous locations where explosive atmospheres exist. No case, however, can make a tablet “intrinsically safe.” Intrinsically safe equipment must be engineered to meet specific criteria and then certified by official certifying bodies to be deemed truly “intrinsically safe.” Intrinsically safe devices, by definition, are designed to be incapable of releasing sufficient electrical or thermal energy under normal and abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration. To achieve the Intrinsic Safety (IS) marking, the device design must pass the appropriate requirements described in UL913, IEC Ex60079 and ATEX standards. For example, the UL913 Class I Division 1 is defined as areas of hazardous locations where flammable material is present at least 10 hours per year. To be used in these areas, a device must demonstrate:

1. Control over the energy level of circuits
2. Control over thermals/temperature of circuits or components
3. Inability to cause an explosion even with two faults in the product
4. Device can pass a drop test, plus an Ingress Protection test for liquid such as water and a solid such as dust

Only if a device meets all of the above criteria and is certified by a certifying body can it be considered “intrinsically safe.” Therefore, it is incorrect to call a tablet “intrinsically safe” if it is not designed to meet the prescribed certification requirements. Covering a tablet with a protective case does not meet the “intrinsically safe” requirements. The protective case does not prevent the tablet from generating sufficient electrical or thermal energy to cause ignition. In short, a case does not change the way the device is designed. A tablet encapsulated in a case may be able to gain certification for hazardous locations of lesser volatility, like Class 1 Division 2 areas, for example, where ignitable concentrations of flammable vapors and gases are handled, processed or used, but which are normally in closed containers or closed systems from which they can only escape through accidental rupture or breakdown of those containers or systems. In this situation, the protective case is designed and tested for situations where a hazardous substance enters the case and is ignited by an electrical spark or hot surface. The explosion is confined within the enclosure, and the case is meant to prevent the ignition of gas or vapor in the surrounding atmosphere. This type of case can be certified as an “Explosion-Proof Enclosure,” in which an apparatus is encased in a Type 7 enclosure that meets explosion-proof requirements for indoor use in locations classified as Class I, Groups A, B, C, or D. A case can also be deemed a “Dust Ignition-Proof Enclosure,” or Type 9 enclosure, that is designed to prevent the entrance of dust and meets dust ignition-proof requirements for indoor use in Class II, Groups E, F, or G locations. The device enclosed in this type of case must be designed not to cause external surfaces to reach temperatures capable of igniting or discoloring dust on the case nor igniting dust-air mixtures in the surrounding atmosphere. Neither explosion-proof enclosures nor dust ignition-proof enclosures make a device “intrinsically safe.” Encapsulating a tablet in a case minimizes the impact of an explosion, but it does not address the thermal and fault management characteristics needed to prevent an explosion, nor to classify the device as “intrinsically safe.”