If there is any question about the safety of the light set, it should be thrown away. Check the labels on the light set for inside or outside use. Never use inside lights outside. Be sure to fasten your outside light sets tightly so they will not be damaged by the wind. The wind can cause the wires to fray and break.
Miniature lights produce less heat and reduce the drying effect on the tree.
Do not overload extension cords, and do not connect more than three sets of lights to one cord. Outlets should be readily accessible for quick disconnection if necessary. Never use lighted candles on or near a tree or other decorations. All lights should be securely fastened to the tree. No bulbs should come in contact with needles or branches.