Depending on the country of use, power cords are usually certified by a national group who verifies the design and safety of the power cord. Shown here are the certification marks and the associated countries. For North America, it is the Underwriters Laboratory (UL). Each power cord carries a traceable UL file number for verification and reference. To be approved through UL, a power cord must pass a series of tests to determine its safe and effective use under normal working conditions. Such tests may include pull force, bending, blade tests, insulation tests, cord push-back, strain relief, impact resistance, crushing resistance, and jacket retention.