Abstract :
Two techniques¿flameproofing and intrinsic safety¿have been regularly employed to meet safety regulations for dangerous atmospheres. Certification has been based on representative tests on sample equipment. In the case of instrumentation and control equipment, the complexity of modern electronic equipment makes testing onerous, complicated and time consuming. An entirely new method is based on the use of an electrical barrier placed at the boundary between the safe and hazardous areas to ensure that the energy entering the hazardous area cannot approach, by a large margin of safety, the level at which ignition occurs. A barrier device applicable to electrical process-control circuits using d.c. signals is described. The philosophy behind the design of the unit, its physical construction and method of application, and the limitations it imposes are detailed. Further designs for similar purposes are outlined, but the method is limited to circuits where the power does not exceed a few watts. The barrier comprises a network of resistors and semiconductor diodes chosen to have negligible effect at normal signal levels. The diodes conduct during fault conditions, and we need only consider the worst case, namely, the direct application of the power mains to the protected circuit. The maximum energy which can enter the hazardous area is defined, and the degree of safety can be determined by inspection of the components in the danger area.
