What does "UL" in a circle mean, when found on electrical equipment?

The “UL” mark in a circle on electrical equipment refers to Underwriters Laboratories. UL is an independent safety organization that develops standards, tests products, and certifies that samples meet specific safety requirements.

The mark is especially common on appliances, power supplies, lighting, wiring devices, computers, and fire-safety equipment. A manufacturer generally must submit products for testing and agree to follow UL’s production and inspection requirements before using an applicable UL mark. UL may also conduct follow-up inspections after certification.

A common mix-up is treating the symbol as a universal guarantee that a product is safe in every situation. The mark applies to the particular product, construction, components, and conditions covered by the relevant certification. Different versions—such as UL Listed, UL Classified, and UL Recognized Component—carry different meanings. “UL” is also the modern short name used by the organization, whose historical name was Underwriters Laboratories Inc.