Products bearing either an ETL mark or a UL mark have been certified by a Nationally Recognized Testing Laboratory, or NRTA, as compliant with the relevant minimum safety standards for that product. NRTLs are certified by the Occupational Safety and Health Administration OSHA to perform independent safety testing, and generally follow the same standards and procedures in conducting tests. Therefore, the only practical differences between them are customer service and the capabilities of the testing laboratory.
The ETL mark stands for Electrical Testing Labs, and has a history dating back to Thomas Edison's early electrical products. The mark is valid only in the United States and Canada, but its parent company, Intertek, has other marks valid around the globe. Intertek is a conglomeration of many different companies, each of which was originally started to certify products in a different market. The principal companies that comprise the modern Intertek are Electrical Testing Laboratories (U.S.), SEMKO (Sweden), Warnock Hershey (Canada), Caleb Brett companies (U.K.) and Inchcape, an international group that ultimately acquired all of the other companies. Intertek continues to use an established mark in each market, rather than setting an international standard, as of 2015.
Underwriter's Laboratories, or UL, is even larger in scope, with marks in 113 different countries and employees in 44. The company advises clients on trade challenges and market access in addition to compliance with governmental regulations. UL helps government regulators write guidelines in addition to testing for them. UL also offers a range of different marks, generally tied to an industry rather than a geographic location. UL traces its history back to 1894, when founder Henry Merrill tested a non-combustible insulation material for a Mr. Shields. Since then, the company has expanded its testing to include a range of products outside of its original focus on potential fire hazards.