This specification applies to the design, construction and testing of electrical equipment with intrinsically safe circuits for use in hazardous atmospheres in which explosive gases and vapours may occur. It defines intrinsic safety in terms of electrical sparking which may occur within the equipment or cirucits, and does not apply to other means or source of ignition, e.g. the exposure of the glowing filament of an electric lamp, frictional sparking, and the like. These other hazards, where they exist, are controlled, as far as possible, by suitable inherent arrangements. The principle of intrinsic safety, as defined in this specification, is restricted to electrical equipment and circuits in which the output or consumption of energy is small, and in which suitable means can be used to control the energy which may be liberated in the sparks produced when the circuits are interrupted or short-circuited, normally or under any fault conditions.