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Is an ungrounded electric receptacle outlet dangerous?
Friday, August 31, 2018
Most homes built mid-century and earlier have two-slot receptacles, like the one in the photo above. The third, round slot that we are used to seeing today, and which has been required by the building code since the early 1960s for connecting a grounded three-prong electric appliance cord, is missing. Two-slot receptacles are acceptable to continue using where a ground connection does not exist at the receptacle, per National Electrical Code [NEC 406.4(D)(2)], as long as only two-prong cords for lamps, and other appliances that do not require grounding, are plugged into them. The code also allows a two-slot receptacle to be replaced with another one.
But here's the rub: lots of electric appliances used today have cords with three prongs. Using an adapter, like the one shown connected to the top half of the receptacle shown above, is not a safe solution to the dilemma. Theoretically, if the metal ring at at the two-slot side of the adapter is connected to the securing screw for the receptacle cover plate, then the ground slot is connected to a ground. But that is almost never the case and most people don’t even bother connecting it—with the result that an appliance that requires grounding to be safe, such as a refrigerator or washing machine, is ungrounded.
Even worse is a defect we sometimes see in older homes, where the old ungrounded receptacle is changed out for a three-slot receptacle but there is no ground connection made. A three-light circuit tester, like the one shown below, is used by many home inspectors to check for this safety defect. Two orange lights means the circuit is grounded, and one orange lights indicates lack of ground. Unfortunately, this little tester does not detect what is called a “false ground,” a condition in which the ground slot is connected to the neutral wire in the circuit—which is also an unsafe. Detecting a false ground requires a more sophisticated electronic testing device.
There are three safe and code-approved ways to repair an ungrounded three-slot receptacle:
- Run a ground wire to the new three-slot receptacle. This is often the most difficult and expensive solution.
- Install a GFCI (Ground Fault Circuit Interrupter) receptacle. It’s the kind with two small push-buttons in the center. A GFCI shock-protection device is considered an acceptable alternative to a ground connection, but the receptacle cover plate must have a small sticker (provided in the box with the receptacle) attached that says “NO GROUND PROVIDED.”
- Replace it with a two-slot receptacle, which is still manufactured and available on the shelf at many hardware stores. If not, they can special-order it for you. Since having just one three-slot receptacle in each room is adequate for most homes, this solution is satisfactory for many of the ungrounded three-slot receptacles.
Many appliances require a ground connection to operate properly and have a manufacturer’s sticker, like the one shown below on side of a fluorescent ceiling fixture, that states “FIXTURE MUST BE GROUNDED for safety and proper operation.”
One final comment: while two-slot receptacles are reasonably safe when used correctly, three-slot (grounded) receptacles are ultimately safer and better for avoiding a shock or electrical fire. If you are able to upgrade to a grounded or GFCI receptacle, it’s definitely a good idea. That’s why the code was changed 50 years ago to require them for new construction.
Also, see our blog posts Can you plug a surge protector into a two prong outlet? and How can I tell if the electric receptacle outlets are grounded?
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To learn more about electrical wiring, devices, and receptacles, see these other blog posts:
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