Installation Guide for Hazardous Areas - 2010
Published by Jack Hunter – March 18, 2011
Categories: Hazardous Area
The concept of intrinsic safety in wiring recognizes that a sufficient concentration of ignitable, flammable or combustible materials will be present, with air or another oxidizer, to represent a fire or explosion hazard. These mixtures could easily be ignited by a match or other open flame, or by a high-energy spark. The wiring used in areas where these mixtures are present can be implemented in a manner which absolutely precludes any possibility of igniting these mixtures. That is the essence of intrinsic safety. Intrinsically safe wiring will never have enough energy available within the defined hazardous area to ignite any explosive or combustible mixture of gasses, dusts, or metals.Where it is impossible to reduce the electrical circuit energy (as with electric motor power) the circuits must be physically isolated from the hazardous atmosphere, dust or metals. This is the principle behind explosion-proof wiring. Even if the circuit did ignite a quantity of hazardous mixture, the wiring container, can "contain" the resulting explosion and cool any escaping hot gasses so that they would be incapable of igniting the hazardous mixture outside of the explosion-proof container.
1.1 The fire triangle
The fire triangle is a tool to illustrate the three elements which must be present to have a fire or explosion: fuel, oxidizer, and energy. Fuel and oxidizer must be present in a concentration appropriate to form a combustible mixture. The ignition source must supply enough energy to initiate combustion. If any one of the elements of the triangle is not present in sufficient amount, then combustion cannot occur.
The availability of energy, by either thermal or electrical means, can cause the ignition of a combustible mixture. It need not be a spark or a flame; temperature alone can supply the energy of initiation. The energy required to ignite various groups of combustible substances have been proven by experimentation. Graphs of this data have been produced, and can be used to indicate safe levels of energy. Only a very small amount of energy may be required to cause ignition, such as the mixture of hydrogen and air, which requires only 2O .Joule of energy to ignite. In electrical circuits the mechanism for the release of this ignition energy is often a spark from a circuit wiring fault that creates a gap in the wire allowing a spark to form. Electrical components and equipment with hot surfaces also can cause ignition.
Fire is simply an oxidation process. Some oxidation processes proceed at a slow pace while fire is a rapid oxidation process. While the oxidizer in most fires is oxygen, other chemicals may be oxidizers. For example, elemental magnesium will react violently with water to release heat. The magnesium is the fuel and water provides the oxidizer.
The fuel component can be almost any substance. Most materials will burn under the right temperature and pressure conditions. If steel is finely granulated, placed in a pure oxygen environment, and then exposed to high temperatures, it will burn rapidly; almost like it was coal. Flour in a bakery can do the same thing as can the dust produced from grain stored in a grain elevator.
Certain fuels, when combined with air, can form an explosive mixture. The main difference between a rapid fire and an explosion is that an explosion creates a pressure wave due to the rapid production of hot gas volume. That pressure wave is what is responsible for the "bang" associated with an explosion. The explosive pressure wave can cause serious damage to facilities and humans.