Providing a path for stray electrical current to follow is an important safety feature. Modern wiring codes require all outlets and fixtures to be grounded, meaning a separate conductor must be provided for current to follow in the event the wiring is compromised. An open ground means the safety path is open, or incomplete. This should be fixed as soon as possible.
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Identify an open ground using the neon lamp tester. Insert one lead into the receptacle's hot side and the other lead into the ground slot. See the illustration to identify the receptacle slots. The light on the tester should illuminate. If it doesn't, check that the receptacle is live by removing the lead from the ground slot and inserting it into the neutral slot. The lamp should illuminate if the power is on.
Turn the power to the receptacle off and verify it is off with the neon lamp tester. Remove the faceplate and the screws that attach the receptacle to the box. Gently pull the receptacle out and observe the wires attached to it and also the wires in the box. There should be two wires attached to the lugs on the sides of the receptacle, and one to the ground lug.
Ground the receptacle by attaching a ground wire from the receptacle ground lug to the ground wire in the box. Some metal boxes are grounded by conduit and don't have ground wires. In this case, attach the ground wire to the metal box with a green ground screw.
Turn the power on and carefully check that the receptacle is now grounded. Touch the leads of the contact tester to the hot receptacle lug and to the ground lug. The lamp should illuminate if the receptacle is grounded. If not, perform Step 5.
Run a ground wire or cable from the box to a properly grounded box or to the circuit panel. Do this only if the ground cannot be made by attaching the receptacle ground lug to the box or grounded wire in Step 4. In the circuit panel, attach the cable or wire to the grounded bus bar. When running the ground wire to another box with a working ground, attach it to the grounded wire in the box or to the box itself if the box is properly grounded.
Turn the power off and verify it is off using the non-contact voltage tester. Run a ground wire or cable of the proper gauge from the outlet box to the circuit panel. Attach the wire or cable to the grounded bus in the circuit panel.
Attach the ground wire or cable to the metal box and to the receptacle ground lug. Briefly turn the power on and test that the receptacle is grounded by touching the leads of the neon tester to the ground lug and to the hot lug of the receptacle. Use a new receptacle if the old one does not have a ground slot or ground lug.
Turn the power off. Carefully push the wires back into the box and install the receptacle. Replace the cover plate and turn the power back on. Double-check the ground one more time, and the job is finished.