It can test earth resistance of various earth systems including power systems, electric equipments, lightning conductors as well as resistance value of low resistance conductor and AC voltage test. Take one of the wires and place it in the earthing port, which is the third hole at the top or bottom of your outlet. Place the second wire in the live port again to see if the lightbulb illuminates. If the lightbulb has the same intensity as your first test, then the outlet is properly earthed.