Should I use a grounded or ungrounded extension cord for devices with a metal case?

Use a grounded (3-prong) cord for anything with a metal chassis or three-prong plug—like power tools, appliances, and computers—because the third prong provides a safe path to trip the breaker if something goes wrong. Use an ungrounded (2-prong) cord only for double-insulated devices with two-prong plugs, like phone chargers or lamps. Buying a 2-prong cord for a three-prong device removes a safety feature, so stick with grounded cords unless you’re using an older two-slot outlet.