Can I Ground a Three Prong Plug to the Metal Receptacle Box?

Adaptor

• A three-prong plug cannot be grounded to a two-prong ungrounded receptacle box. The little gray three-prong to two-prong adapter with an additional wire coming from the bottom that is to be screwed behind the connecting screw on the receptacle faceplate does not by itself ground the appliance in an ungrounded circuit box. Attaching an ungrounded, two-prong adapter to a three-prong cord means you can no longer depend upon the manufacturer's safety features, according to the Occupational Safety & Health Administration

Testing

• It is possible for the two prong circuit box to be grounded to the main breaker box if the conduit used in the wiring is the proper metal. A multimeter or a circuit tester provides a quick answer. Set the multimeter on AC and place one of the prongs of it or the circuit tester in the smaller of the two slots in the receptacle. Touch the other prong to the middle or corner screw of the outlet cover.

Grounded Box

• If the meter reads 110 or 120, or the light on the tester comes on, you know the box is grounded and the three-prong adapter can be screwed into the plate and provide a safe plug. If the older style plug does not have distinguishable slots, and the meter or tester does not read positive, redo the test with the probe in the other slot. If it still does not read positive then the circuit box itself is not grounded.

Possible Tragedy

• Never cut the round prong off from a three-prong cord to make it fit a two-prong outlet. The convenience you save is not worth the tragedy that can happen if something goes awry in the circuit and either starts a fire or hurts someone. The installation of electrical systems in homes and other buildings is strictly governed by codes that are there to protect you from harm. Going around the codes may work for a while but eventually electricity has a way of escaping a too-loose leash.