Three-prong adaptor

[1]: 112  They are needed to allow appliances with 3-wire power cords to plug into legacy ungrounded (two slot) receptacles found in older buildings.

Cheater plugs were previously available with a short flexible grounding wire rather than a flat metal screw tab.

However, this ground-wire style of cheater plug was discontinued when it was noted that a loose unattached grounding wire could accidentally become connected to the "hot" blade of a nearby outlet, potentially leading to electric shock.

As an additional failure mode, the thin flexible wire could break unnoticed inside the insulation.

The flat parallel plug blades are polarized to prevent the hot and the neutral connections from being reversed.

In addition, many versions have a molded obstruction bump on top of the adapter, to block the grounding prong and thus physically prevent forcible insertion of a 3-prong plug in the wrong orientation.

In 1971, the US National Electrical Code (NEC) required grounded receptacles in all locations of the home (effective January 1, 1974).

Its casual use as a method for avoiding ground loops in analog audio and video signals (to eliminate hums and buzzes) is dangerous.

[5] Bill Whitlock, president of Jensen Transformers, writes, "never, ever use devices such as 3 to 2-prong AC plug adapters, a.k.a.

Even worse, if the ground screw of the receptacle is connected to the neutral side, electric shock is possible even when the appliance is properly functioning.