Cheater plug

A cheater plug, AC ground lifter or three-prong/two-prong adapter is an adapter that allows a NEMA 5-15P grounding-type plug (three prongs) to connect to a NEMA 1-15R non-grounding receptacle (two slots). They are needed to allow appliances with 3-wire power cords to plug into legacy ungrounded (two slot) receptacles found in older buildings. The use of such an adapter avoids the need to replace receptacles, but is potentially hazardous if the grounding tab is not connected to electrical ground. A safer and more reliable alternative identified in the US and Canadian electrical codes is to replace the outlet with a Ground Fault Circuit Interrupter (GFCI) breaker outlet. These adapters are illegal in some jurisdictions, in particular throughout Canada.

Cheater plugs are also used to break ground loops in audio systems. This practice has been condemned as disregarding electrical safety. A safer and more reliable alternative is to use an isolation transformer made specifically for this purpose.

Cheater plugs were previously available with a short flexible grounding wire rather than a screw tab. This allows use of the second duplex outlet that does not have a faceplate screw in the correct location for the screw tab. The grounding wire would be wrapped around the adapter to reach the faceplate screw. However, this ground-wire style of cheater plug was discontinued when it was noted that a loose unattached grounding wire could by accident become inserted into the "hot" blade socket of a nearby outlet, potentially leading to electrocution.

Three-prong plugs do not fit into the older, two-prong receptacles. When used as intended, the ground pin of the 3-wire receptacle is to be connected to the grounded cover screw, or to an external ground. In 1969 Underwriters Laboratories mandated three-prong plugs on major appliances for safety. At that time, only half of the receptacles in US homes were three-prong. Wiring in most homes did not include a grounding wire. The screws and outlet boxes were either connected to the neutral, or connected to nothing. Only in rare jurisdictions where non-metallic cable was prohibited and armored cable required (and still in good condition), do cheater plugs work as intended. In 1971, the US National Electrical Code (NEC) required grounded receptacles in all locations of the home (effective January 1, 1974).

...
Wikipedia