Intrinsic Safety at Underwriters´ Laboratories

The Underwriters´ Laboratories has been investigating intrinsically safe equipment for use in hazardous locations for many years. It has recently published a Standard for Intrinsically Safe Circuits and Equipment, UL913. This presentation traces the development of the intrinsically safe requirements from the initial outlines to the printed Standard. The contents of the Standard are briefly discussed as they would apply to process-control equipment and, also, to other types of electrical equipment having intrinsically safe circuits. A typical investigation of an intrinsically safe process-control system is presented. Construction and performance requirements are discussed. A sample circuit is selected to illustrate fault analysis and test conditions in the explosive atmosphere.