Publisher Summary This chapter presents various design philosophies used for electrical apparatus used in explosive atmospheres. The construction of electrical equipment specifically for use in areas where explosion risks might occur due to clouds of gas, vapor, mist or dust was first addressed in the early part of the twentieth century and initially was aimed at the coal-mining industry. This initial addressing of the problem concentrated upon the problems associated with methane (firedamp) and the problems of coal dust. The types of protection envisaged for coal mining were those associated with protection against gas clouds and the levels of overall protection envisaged were those, which were suitable for underground areas where men normally worked. There are four basic approaches, which can be adopted to prevent uncontrolled ignitions of gas, vapor or mist and air clouds. First of all, the gas, vapor, or mist/air cloud can be excluded by mechanical barriers or other equivalent means. Secondly, the components of the electrical circuits can be prevented from sparking or becoming sufficiently hot to ignite the gas, vapor, or mist/air cloud. Thirdly, the gas, vapor, or mist/air cloud can be permitted to enter the equipment enclosure and ignited, but the ensuing propagation of the explosion outside the enclosure can be prevented by some form of quenching. Finally, the electrical components within the enclosure can be permitted to spark, but the energy fed to the enclosure will be limited to a level, which is not capable of igniting the gas, vapor, or mist/air cloud.