The Underwriters’ Laboratories, or UL, inspects electric products made in the United States. If a product is proven to be safe from shock and fire, it becomes UL listed. When purchasing indoor or outdoor Christmas lights, you should make sure they are all UL listed and have the tags on the cord to prove it. If not, you could be in danger of shock or a fire.
Indoor Christmas lights will have a green UL listed tag, or a silver tag with the “UL” written in green. Outdoor Christmas lights will have the UL listed tag in red, or silver with red writing. Outdoor Christmas lights are also safe for indoor use. If you use indoor Christmas lights outdoors, they will not be as resistant to any moisture. Moisture could cause an electric short and your lights could be damaged. If an indoor light goes out, usually the rest of the lights will go out as well, making outdoor repairs difficult.