Safety Tester Model ST-05 is a test set for testing small equipment powered by 120V 60 Hz, from an approved U-ground source. Test also can be run on double-insulated tools and on appliances with two-pin parallel plugs. Extension cords can also be tested. The test set is powered from the 120V 60 Hz power line and tests the wiring of the input supply, if the grounding is defective or the power wires are reversed, the instrument cannot be operated.