UL Listing-Significance to the User and Manufacturer

Some local codes require electrical equipment to be investigated by a recognized inspection agency. Listing by Underwriters' Laboratories is a widely accepted solution in the United States. This paper relates some of the procedures, tests, and restrictions that are placed upon the equipment manufacturer to receive and maintain the UL listing for a power supply. Underwriters' Laboratories Inc. (UL) is a nonprofit organization that investigates products with respect to basic safety standards. These safety standards are written by UL engineers who first seek inputs from concerned groups. These groups include industry, consumers, public service groups, government, and other standards agencies. The standards are correlated with the National Electric Code. The UL mark on a product is not internationally accepted, but within the U.S. the UL mark is well known and accepted. The significance of UL listing to the user is that it demonstrates product compliance with basic safety requirements as judged by an independent agency. The UL mark does not indicate that the product meets any particular performance or reliability standards - the focus is on safety. A user of UL listed equipment can, expect: (1) acceptance by most inspection agencies in the U.S. (2) materials and components are used within their maximum ratings (3) continuing compliance at the assembly line by virtue of periodic UL inspections of the factory.